Oil crystallization

Oil crystallization DEFAULT

Advances in Lipids Crystallization Technology

Open access peer-reviewed chapter

By Maria Aliciane Fontenele Domingues, Ana Paula Badan Ribeiro, Theo Guenter Kieckbusch, Luiz Antonio Gioielli, Renato Grimaldi, Lisandro Pavie Cardoso and Lireny Aparecida Guaraldo Gonçalves

Submitted: June 16th 2014Reviewed: October 29th 2014Published: May 6th 2015

DOI: 10.5772/59767

chapter and author info

Authors

  • Maria Aliciane Fontenele Domingues*

    • School of Food Engineering, University of Campinas, Campinas, Brazil
  • Ana Paula Badan Ribeiro

    • School of Food Engineering, University of Campinas, Campinas, Brazil
  • Theo Guenter Kieckbusch

    • School of Chemical Engineering, University of Campinas, Campinas, Brazil
  • Luiz Antonio Gioielli

    • Faculty of Pharmaceutical Sciences, University of São Paulo, São Paulo, Brazil
  • Renato Grimaldi

    • School of Food Engineering, University of Campinas, Campinas, Brazil
  • Lisandro Pavie Cardoso

    • Institute of Physics "Gleb Wataghin", University of Campinas, Campinas, Brazil
  • Lireny Aparecida Guaraldo Gonçalves

    • School of Food Engineering, University of Campinas, Campinas, Brazil

*Address all correspondence to: [email protected]



DOI: 10.5772/59767

From the Edited Volume

Advanced Topics in Crystallization

Edited by Yitzhak Mastai

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1. Introduction

In recent years, the industrial sector of oils and fats has become an important area of ​​research and technological development. The number of studies related to the physical properties of oils and fats has been increasing; these properties are broadly the melting and crystallization behavior and the crystalline and oxidative stability of oils and fats.

The crystallization behavior of lipids has important implications in the industrial processing of food products whose physical characteristics depend largely on fat crystals. Such products include chocolates, margarines, spreads, fats for confectionery and bakery, dairy products, and commonly used shortenings [1]. Meanwhile, crystallization is the most important physical problem of oils and fats [2], particularly problems such as unwanted polymorphic transitions, oil exudation, the development of fat bloom, formation of crystalline agglomerates, and fatty bases with a maximum solid fat content or incompatibility of induction periods with certain industrial applications. Thus, recent research has focused on understanding the phenomena involved in the crystallization of lipids in an attempt to achieve effective solutions to stabilize or modify this process, depending on the nature of the raw material and its industrial application. To that effect, the use of emulsifying agents as crystallization modifiers has marked the trend of research in ​​the oils and fats field. In the past, studies were based on the effect of emulsifiers on the crystallization of pure triglycerides or model systems [3, 4, 5], while recent research has focused on the effect of emulsifiers on the crystallization properties of different types of fats such as milk fat [6, 7, 8], low-trans fats [9, 10], palm oil and its fractions [11, 12], cocoa butter [13], in the crystallization of emulsions [14, 15], and production of organogels, which constitute the structuring oils of emulsifiers [16]. While studying the effects of emulsifiers in fatty systems is of great interest for the improvement of industrial bases, particularly with respect to fat for use in chocolate, confectionery, and baking, there is limited research on the role of these compounds as crystallization modifiers of natural and commercial fats [17].

Crystallization of lipids is a serious problem in the food industry with respect to actual industrial processes and post-crystallization events. The crystallization issue presents additional aggravating considerations related to climatic differences between countries and the transport and storage conditions imposed by long distances between producing regions and final distribution regions. Thus, there is a need for appropriate solutions for processes involving crystallization and stabilization of raw materials of significant industrial relevance, such as palm oil and fractionated and interesterified fats, which are now replacing partially hydrogenated fats (or trans fats) in most industrial applications. Therefore, the topic discussed in this chapter is highly relevant to the oils and fats production sector.

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2. Oils and fats

Edible oils and fats are essential nutrients of the human diet, playing a vital role in providing essential fatty acids and energy. Chemically, natural oils and fats consist of multi-component mixtures of triacylglycerols (TAGs), which are glycerol esters and fatty acids. Additionally, polar lipids (minority lipids) such as diacylglycerols (DAGs),monoacylglycerols (MAGs), free fatty acids, phospholipids, glycolipids and sterols are found solubilized in the triacylglycerol matrix. The triacylglycerol composition determines the physical properties of oils and fats, affecting the structure, stability, flavor, aroma, storage quality, and sensory and visual characteristics of foods [18].

The physical properties of an oil or fat are of fundamental importance to determine its use. This is particularly true for a large quantity and variety of oils and fats used in various forms, including foods. The difference between the words “oil” and “fat” refers to a fundamental physical property, the fluidity or consistency at room temperature. The components of fat characterize it as a material composed of an intimate mixture in the liquid and solid phases, and its physical state can vary from a viscous fluid to a solid or brittle plastic [19].

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3. Physical properties of oils and fats

3.1. Crystallization behavior

Plastic fats consist of a lattice network in a continuous oil matrix. The crystallization process is a spontaneous ordering of the system, characterized by the total or partial restriction of movement caused by physical or chemical links between the triacylglycerol molecules. Differences in crystal shapes result from different molecular packings. A crystal, therefore, consists of molecules arranged in a fixed pattern known as a lattice. Its high degree of molecular complexity allows the same set of TAGs be packaged into several different and relatively stable structures [20].

Crystallization of fats determines important properties of foods, including: (i) the consistency and plasticity of fat-rich products such as butter, margarine and chocolate during the stages of production and storage; (ii) sensory properties such as the melting sensation in the mouth; (iii) physical stability with respect to the formation and settling of crystals, oil exudation and coalescence of particles and emulsions; and (iv) visual appearance, for example the shininess of chocolates and toppings [21]. In most foods, isolated crystallization of TAGs is considered the event of greatest importance, although the crystallization of minority lipids such as DAGs, MAGs and phospholipids plays a fundamental role in the quality of various products [22].

3.1.1. Crystallization mechanism of the lipids

Crystallization is generally divided into four distinct phases. Initially, in order to obtain the formation of crystals from the liquid state, the system must reach the supersaturation zone, in which there is a driving force for crystallization. Once the appropriate driving force to overcome the energy barrier for crystallization is reached, nucleation occurs and molecules in the liquid state join together to create a stable nucleus. After the formation of stable nuclei, a rapid transition to the next stage of crystallization occurs, crystal growth, i.e., during which additional molecules (or growth units) are incorporated into the crystal lattice, decreasing the driving force of supersaturation. Unless restricted by a kinetic constraint, growth continues until the system reaches equilibrium, at which the driving force for crystallization approaches zero and the maximum volume of the crystal phase is obtained [23].

3.1.2. Nucleation

According to Boistelle [24], nucleation involves the formation of molecule aggregates that exceed a critical size, and are therefore stable. Once a crystal nucleus has formed, it begins to grow due to the incorporation of other molecules from the adjacent liquid layer that is continuously filled by the supersaturated liquid surrounding the crystal [24].

A crystal nucleus is the smallest crystal that can exist in solution at a given temperature and concentration. The formation of a nucleus from the liquid phase, i.e., the nucleation process, requires the organization of molecules in a crystalline lattice of critical size after overcoming an energy barrier. The mechanisms of nucleation are generally classified as primary nucleation, which can be homogeneous or heterogeneous, and secondary nucleation. It is currently suggested that nucleation occurs via a two-step process. Molecular oscillations in the liquid phase lead to local organization of molecules into amorphous clusters (instead of crystal embryos, as postulated by classical nucleation theory – Gibbs, 1800), which then aggregate to form an amorphous cluster of critical size. This formation of amorphous aggregates is the first step in nucleation. At some point the molecules in the cluster are transformed into a crystalline structure, which is the second step for the formation of a stable nucleus. The combination of these two events characterizes the induction time before the onset of visual nucleation. This type of nucleation, however, rarely occurs under the conditions of industrial processes. In practice, nucleation is usually dominated by the heterogeneous mechanism in the majority of systems, where external surfaces or catalytic sites, such as molecules of different composition, are used to reduce the energy barrier. Although the exact mechanism of heterogeneous nucleation is not yet fully elucidated, the phenomenon can be described as the result of interactions between the solid particle and the supersaturated fluid, causing the local ordering of molecules for formation of the nucleus. Secondary nucleation is the formation of a new nucleus in the presence of existing crystals, which may occur if microscopic crystalline elements are separate from an already formed surface, thus resulting in crystal fracture into small stable nuclei [22, 23, 25, 26].

When the nuclei formed achieve favorable dimensions, these elements become crystallites whose growth depends not only on external factors (supersaturation, solvents, temperature, impurities), but also internal factors (structure, connections, defects). Consequently, the crystal growth rate can vary by several orders of magnitude. Growth occurs by binding of molecules to a crystalline surface. At the same time, molecules are also detached. There is a continuous movement of molecules on the crystal surface, and the result of these processes determines the rate of growth, which is directly proportional to subcooling and varies inversely with the viscosity of the system [21]. Although nucleation and crystal growth are often considered separate events, they are not mutually exclusive. Nucleation also occurs as crystals grow from existing nuclei [27].

3.1.3. Recrystallization

Recrystallization was defined by Fennema [28] as any change in the number, size, shape, orientation or perfection of the crystals after completion of initial solidification.

The basic mechanism of the recrystallization process is size-dependent equilibrium (melting temperature or solubility) documented by the Gibbs-Thomson effect. Small crystals, due to the small radius of curvature of the surface, are slightly more soluble or have a slightly lower melting point than larger crystals. Over time, these differences promote the disappearance of small crystals and growth of larger crystals. These changes generally occur without a change in volume of the crystalline phase, and are driven by the difference in thermodynamic equilibrium based on the size of the crystals. These crystals occur slowly at a constant temperature, but their presence increases with temperature swings as the phenomenon referred to as Melting-Recrystallization becomes dominant. When the temperature rises during a temperature cycle, the crystals melt or dissolve to maintain phase equilibrium. The small crystals, which are less stable, disappear first. When the temperature starts to decrease during the temperature cycle, the volume of the crystal phase increases, but only by growing and without the formation of new nuclei. The mass of small crystals that melted is redispersed among the larger crystals. As the average size of the crystals increases, the number of crystals decreases as a result of these thermodynamic effects. Thus, a dispersion of many small crystals tends to minimize the surface energy (and surface area) by recrystallization [23, 29].

The final stage of crystallization in foods occurs during storage, and a population of crystals undergoes a recrystallization step, reaching a more broad equilibrium state. This phenomenon is of primary concern during storage of foods, and is responsible for changes to the texture of ice cream, fat bloom in chocolates and toppings and exudation of oil in products rich in fat. In lipid systems, the recrystallization process involves changes to the internal arrangement of the crystalline structure via polymorphic transformation [30].

3.1.4. Crystallization kinetics

Crystallization kinetics intensively influences the final structure of fats and shows to be closely related to their rheological and plasticity properties. When monitoring the formation of the solid crystalline material with respect to time it is possible to verify the nature of the crystallization process. Characterization of crystallization kinetics can be performed according to the induction time (τSFC) or the nucleation period (relative to the beginning of crystal formation) and the maximum solid fat content-SFCmax. The induction time reflects the time required for formation of a stable nucleus of critical size in the liquid phase [31]. As a definition, the τSFC is the time required for obtaining one crystalline nucleus per unit volume. The τSFC generally increases with increasing isothermal crystallization temperature and decrease of the sample melting point. Another useful parameter for evaluating isothermal crystallization is the crystallization stability time (tcs), defined as the total time for stabilization of the solid fat content at a given temperature. This parameter consists of the sum of the time characteristics for nucleation and crystal growth [32].

The model most widely used to describe the kinetics of isothermal phase transformation is the Avrami model, developed in 1940, which relates the kinetics determined experimentally with the form of growth and final structure of the crystal lattice [33]. The Avrami equation gives an indication of the nature of the crystal growth process and is given by

E1

where describes the solid fat content (%) as a function of time, is the limit of the solid fat content when time tends to infinity, k is the Avrami constant (min-n), which takes into account both nucleation and growth rate of the crystals and n is the Avrami exponent, which indicates the mechanism of crystal growth [27]. The crystallization half-life (t1/2) reflects the magnitude k and n according to the relationship

E2

Currently, the most common analytical technique for the investigation of crystallization kinetics of fats is nuclear magnetic resonance (NMR). However, various analytical techniques such as differential scanning calorimetry (DSC), polarized light microscopy (PLM), as well as rheological and turbidimetric techniques can be successfully employed. Understanding of the phenomena involved in crystallization kinetics is improved when considering combined use of various instrumental methods [34].

3.1.5. Polymorphism

Long-chain compounds, such as fatty acids and their esters, may exist in different crystal forms. Solids of the same composition which may exist in more than one crystal form are called polymorphs. Polymorphism can be defined in terms of the manifestation ability of different cellular structures, resulting from different molecular packings. The crystal habit is defined as the crystal shape. From a crystallographic perspective, the habit reflects the growth direction within the crystal, while morphology outlines the set of faces determined by the symmetrical components of the crystal. This distinction allows crystals of the same morphology to present different crystal habits [26].In fat, crystals are solids with atoms arranged in a regular three-dimensional pattern. A cell is the repeating unit that makes up the complete structure of a given crystal. A sub-cell, in turn, is the smallest structure in the real unit of the cell, defined as the mode of transverse packing of aliphatic chains in the TAGs. The polymorphic forms of a fat are identified based on their sub-cell structure [24]. In lipids three specific sub-cell types predominate, referring to the polymorphsα, β’ and β, according to current polymorphic nomenclature (Figure 1). The α form is metastable with hexagonal chain packing. The β’ form has intermediate stability and orthorhombic perpendicular packing, while the β form has greater stability and triclinic parallel packing. The melting point increases with increasing stability (α→ β ’ →β), as a result of differences in the molecular packing density [35].

The polymorphic nature of the TAGs is well established. It is also well-known that the mixing of different fatty acid fractions in a TAG produces a more complex polymorphic behavior. Thus, saturated monoacid TAGs present simple polymorphism, followed by TAGs with mixed saturated fatty acids. The mixed saturated/unsaturated fatty acids exhibit more complex polymorphisms [36]. TAGs typically crystallize in the α and β’ forms first, although the β form is most stable. This phenomenon is related to the fact that the β form has a higher free energy of activation for nucleation. Polymorphic transformation is an irreversible transformation process of the less stable form to the more stable form (transformation of the monotrophic stage), depending on the temperature and time involved. At constant temperature, the α and β’ forms can transform, as a function of time, to the β form via the liquid-solid or solid-solid mechanisms [37]. The transformation velocity is dependent on the degree of homogeneity of the TAGs. Fats with low variability of TAGs quickly transform into the stable β form. Fats which consist of a random distribution of TAGs can present the β’ form indefinitely. Additionally, factors such as formulation, cooling rate, heat of crystallization and degree of agitation affect the number and type of crystals formed. However, because fats are complex mixtures of TAGs, at a given temperature the different polymorphic forms and liquid oil can coexist [1].

Fats with a tendency to crystalize in the β’ form include soybean, peanut, canola, corn and olive oil, as well as lard. In contrast, cotton and palm oils, milk fat and suet tend to produce β’ crystals that commonly persist for long periods [21]. In particular, for cocoa butter six polymorphic forms are verified as a result of its unique triacylglycerol composition, wherein symmetrical monounsaturated TAGs predominate. The characteristic nomenclature of cocoa butter polymorphs are based on the roman numeral system (I to VI), where the I form is the least stable and the V form is associated with the desirable crystalline habit in chocolates, which may transform during storage into the VI form, which presents improved stability. However, combinations of this nomenclature with Greek nomenclature are typically encountered, where the forms V and VI are recognized as βV and βVI [38, 39].

The crystal structure of fats is important for the formulation of shortenings, margarines and fat products in general, since each crystal shape has unique properties with respect to plasticity, texture, solubility, and aeration. Fat with crystals in the β’ form present greater functionality, because they are softer and provide good aeration and creaminess properties. Therefore, the β’ form is the polymorph of interest for the production of fat-rich foods such as margarine and confectionary and baking products. For the production of chocolates with good physical and sensory characteristics the βV form is the desirable polymorph, since it is associated with properties such as brightness, uniformity, snap characteristic and improved shelf life [18].

X-ray diffraction is an analytical technique used to identify the polymorphism of crystals by determining the dimensions of the crystalline unit and sub-cells. Due to different geometrical configurations, polymorphs diffract x-rays at different angles. In fats, high diffraction angles correspond to short spacings (distances between parallel acyl groups in the TAG) of sub-cells and allow for verifying the different polymorphs [41].

3.1.6. Microstructure

The lipid composition and crystallization conditions influence the crystal habit, i.e., different crystal morphologies are possible. Crystals aggregate into larger structures forming a lattice, which characterizes the microstructural level of a fat. The microstructure concept includes information regarding the state, quantity, shape, size, and spatial and interaction relationship between all components of the crystal lattice and has tremendous influence on the macroscopic properties of fats [42].

According in [43], the microstructural structure or meso-scale of a crystalline lattice for a fat may be defined as the set of structures with dimensions between 0.5μm and 200μm. Its quantification is achieved primarily by visualization of its geometry. Structural levels in a typical crystal lattice are defined when the fat crystallizes after its complete fusion. Like nanostructural elements (0.4-250nm), TAGs crystallize in specific polymorphic states. Most tags crystallize as spherulites, which implies that crystal growth occurs radially. The formed crystals grow to dimensions of 1 to 4 μm and then combine to form agglomerates (larger than 100μm) in a process governed by mass and heat transfer. The aggregation process continues until a continuous three-dimensional network is formed from the combination of these microstructures, trapped in the liquid fat phase [44]. This structural hierarchy has been recognized by several researchers. However, the arrangement of molecules in the crystalline state also depends on factors such as the cooling rate, crystallization temperature and stirring speed, if necessary [45].

Crystal growth can occur in one, two or three dimensions, characterizing the formation of needle, disk, or spherulite-shaped crystals, respectively [46], and these shapes can be predicted from the results shown by the value of the Avrami exponent (n) (Table 1). According in [47], the application of fats in food products requires that the average diameter of the crystals is less than 30μm to avoid a sensation of grittiness in the mouth.

3+1 = 4growth of spherulitessporadic nucleation
3+0 = 3growth of spherulitesinstantaneous nucleation
2+1 = 3growth of diskssporadic nucleation
2+0 = 2growth of disksinstantaneous nucleation
1+1 = 2growth of rodssporadic nucleation
1+0 = 1growth of rodsinstantaneous nucleation

Table 1.

Values of the Avrami exponent (n) for different types of crystal nucleation and growth.

Another factor that characterizes the formation of the microstructural network of fats is the fractal dimension. The fractal dimension is a parameter that describes the spatial distribution of the mass within the crystal lattice [44]. Fractal geometry was proposed by Benoit Mandelbrot (1982) as a method for quantifying natural objects with a complex geometrical structure which challenged quantification by regular geometric methods (Euclidean geometry). In classical Euclidean geometry, objects have integer dimensions: the reader would be familiar with the reasoning that a line is one-dimensional, a plain a two-dimensional object and the volume of an object is three-dimensional. Thus, Euclidean geometry is suitable for measuring objects that are ideal, or regular. One can imagine that if enough twists are placed on a line or a plane, the resulting object can be classified as an intermediate between a line and a plane. The dimension of such an object is fractional (i.e., between 1 and 2 or between 2 and 3) and such an object can be classified as a fractal object, based on the fact that instead of presenting a Euclidean dimension (integer), it has a fractional dimension [49]. One of the most important characteristics of fractal objects is their similarity, in other words, fractals objects look the same in different magnitudes, at least in a certain range of scales.

Most scientific research on crystallization of fats has been directed towards establishing relationships between lipid composition or polymorphism and macroscopic properties of fats, without in-depth consideration of the microstructure of the crystal lattice, which can lead to failures in predicting the macroscopic properties [50]. In Marangoni and Rousseau [51] investigated the possibility that the solid fat content and/or polymorphic shape of the crystals is not determinant for the mechanical properties of mixtures containing milk fat with canola oil, but instead the macroscopic structure of the crystal lattice in the liquid oil matrix. From the study of fractal dimensions and the application of this theory to the rheological study of milk fat with canola oil moistures, it was observed that the fractal dimension (Db) was the only “indicator” in accordance with the associated changes to the rheology of the product resulting from interesterification. Traditional physical indicators, such as polymorphism and solid fat content, failed to demonstrate the expected changes. Thus, the study confirmed the importance of the fractal dimension, a fundamental indicator of the crystal lattice capable of explaining changes in rheology of fats not attributed to other measurable properties of the network [49]. According in [27], systems with higher fractal dimension values demonstrate higher packing orders of the microstructural elements.

One of the methods most used for calculating the fractal dimension is the box counting method, where grids with length li are placed on the micrographs of the crystalline lattice of a fat obtained by the polarized light microscopy technique. Any lattice containing particles above a threshold value is considered an occupied lattice (solid). The number of occupied grids Ni of side length li is counted. This process is repeated for grids with different lateral lengths. The fractal dimension of box counting, Db, is calculated as the opposite slope of the linear regression curve for the log-log graph of the number of occupied grids Nb versus the lateral length lb, given by

E3

To reduce errors, the grids with extreme sizes should be exempted from the calculation [52]. Polarized light microscopy (PLM) is the most widely used technique for visualization of microstructural network of fats and has been applied so as to explain the differences in texture of fat mixtures, showing crystalline types and morphological alterations in crystal growth [53].

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4. Control of crystallization

Control of crystallization to prevent crystal growth or to achieve the desired crystalline attributes is crucial for obtaining high-quality products with long useful life. Understanding the principles that underlie the crystallization phenomena is necessary to achieve this control [23]. Figure 2 presents a schematic of the crystallization process, storage of fats and associated mechanisms.

The behavior of crystallization, polymorphic transformation and microstructure of a fat is due to a combination of individual physical properties of each TAG and phase behavior of different TAG mixtures. In general, the specific composition of a fat is one of the most important factors for final development of the crystal structure [54].

Crystallization of fats is a critical factor associated with the structure and properties of most foods. The stability of many processed food products is influenced by changes in the physical state of the fats and changes in the crystallization processes, since the events of nucleation and crystal growth occur simultaneously at different rates as they are affected by conditions such as degree and rate of super-cooling, viscosity and agitation [13].

In the initial stages of food processing, the relative rates of nucleation and crystal growth determine the distribution, shape and size of the crystals, parameters that are directly related to the characteristics of consistency and texture. However, during the storage phase, several post-crystallization phenomena may occur, significantly affecting the properties and stability of foods. These include polymorphic transitions to thermodynamically more stable phases, formation of new crystals and crystal growth, and migration of oil or small crystals. It should be noted, however, that such events are not chronological; polymorphic transitions can occur even in the early stages of processing [31].

Additionally, in post-crystallization processes the phenomena known as sintering or bonding of adjacent surfaces can be verified, as well as spontaneous dissolution, also known as Ostwald ripening. The term sintering is described as the formation of solid bridges between fat crystals, with formation of a cohesive network associated with the undesirable increase in the hardness of the fat phase. Ostwald ripening, in turn, is associated with dissolution of previously existing small crystals in the fat phase and development of crystals with undesirable dimensions and weak crystal lattices, which causes loss of consistency of the products [56].

Furthermore, in some specific products the control of crystallization means, above all, avoiding this process, even if it is thermodynamically favored or due to storage or processing conditions [8]. Thus, control of crystallization and polymorphic transitions in fats is a factor of fundamental importance for the food industry.

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5. Fats for industrial use

5.1. Interesterified fats

Interesterification is a technological alternative to the partial hydrogenation process, since it enables the production of oils and fats with specific functionalities. Due to the growing concern of the nutritional impact of fatty acids on health, interesterification has been indicated as the main method for obtaining plastic fats with low levels of isomers or absence of these compounds. In contrast to hydrogenation, this process does not promote the isomerization of double bonds of fatty acids and does not affect the their degree of saturation [57].

In the interesterification process the fatty acids are rearranged in the glycerol molecule. Interesterification is promoted by an alkaline catalyst (chemical interesterification) or by lipases (enzymatic interesterification). The alkaline catalysts most frequently used are sodium methoxide and sodium ethylate [58]. In chemical interesterification the fatty acids are randomly distributed in the glycerol molecule along the three available positions within each molecule. When specific lipases are used to catalyze the interesterification reaction, rearrangement can occur in the and positions of the glycerol molecule, maintaining the position [59].

Chemical interesterification is currently the process most utilized by industry. The random distribution of fatty acids along the glycerol molecules leads to changes in the triacylglycerol composition, which alters the overall solids profile of the fat. In interesterified fats, the random distribution of fatty acids results in great variability of TAGs, with intermediate melting points (S2U and U2S). Such variability in TAGs, associated with the formation of partial acylglycerols, promotes slower crystallization and indefinite maintenance of the polymorphic form β´ [58, 60, 61]. Other observations, such as decreased size of the crystals as well as distribution in the crystal lattice, were also observed in some studies [62].

5.2. Palm oil

Palm oil is obtained from the mesocarp of the fruit . It is semi-solid at room temperature, consisting primarily of TAGs of palmitic and oleic acids. Palm oil is the vegetable oil most used worldwide in the food industry. In June 2013, world production of palm oil reached 58 million tons, surpassing the production of soybean oil [63]. As a result of increased production, many studies are focused on palm oil, especially regarding its crystallization behavior and nutritional aspects. Compared to other vegetable oils, palm oil presents a unique and differentiated fatty acid composition, containing similar percentages of saturated and unsaturated fatty acids. It also presents a significant content of saturated fatty acids (10 to 16%) in the position of the TAGs, as well as significant levels of palmitic acid (44%). In addition to these features, palm oil contains small percentages of MAGs and DAGs as minor components, which are produced during maturation of palm fruits and oil processing. The DAGs, specifically, correspond to 4-8% of the composition of palm oil, with variations according to origin and processing conditions. The removal of these compounds, however, is difficult even under optimal refining conditions [18, 64, 65].

The crystallization behavior of palm oil is extremely important from a commercial point of view, because it is characterized by the crystal habit β’, a fact that, combined with its characteristics of plasticity, ensures its application in margarines, spreads, bakery and confectionery fats, as well as general purpose shortenings. The functional properties of palm oil and its fractions appear to be strongly related to its composition and the quantity and type of crystals formed at the temperature of application. However, the crystals of palm oil require a long time for α→ β ’ transition, a factor considered inadequate from an industrial process standpoint. Resistance to transformation into β’ is mainly attributed to the DAGs. Recent studies on the interactions between TAGs and DAGs in palm oil during crystallization show that the latter have a deleterious effect on the characteristics of crystallization, with intensity proportional to the concentration of these minority lipids in palm oil and its fractions [66, 67]. According in [68], the negative effect of DAGs on the crystallization of palm oil may be related to the low nucleation rate of TAGs in the presence of these compounds.

In addition to the slow crystallization of palm oil, another factor of great concern in industry is its post-processing stability. Palm oil is often associated with hardening problems during storage. In some products based on this raw material, undesired crystal growth occurs which results in gritty texture and poor spreadability [69]. These crystalline shapes may reach dimensions greater than 50 μm after a few weeks of storage, leading to non-uniformity of the processed products [68]. In margarines, specifically, the formation of crystal agglomerates with mean diameter between 0.1 and 3mm is observed, which can easily be observed with the naked eye [70]. In [71] found that the main TAGs of palm oil, 1-palmityl-2-oleoyl-palmitine (POP) and 1-palmityl-diolein (POO), have limited miscibility with each other, which results in formation of large POP crystals surrounded by POO. When these agglomerates are formed, there occurs the joining of other saturated TAGs in a process that promotes β’ →β transition. Therefore, to ensure the stability of the β’ polymorph in palm oil-based products this is a question of great industrial interest, given the great economic importance associated with the use of this raw material.

5.3. Palm Mid Fraction (PMF)

The product of the first fractionation stage of palm olein is termed the soft palm mid fraction (soft PMF), which presents high levels of monounsaturated triacylglycerols, rapid melting and tendency to crystallize in β’, making it an excellent raw material for the production of margarines and shortenings in general [72, 73].

Classically, two methods are proposed for the production of soft PMF: the olein route (most common in Asia) and the stearin route, which is preferentially used in South America because of the need for olein with high iodine index in the first fractionation stage. The best CBE’s are obtained via the olein route, where the second fractionation stage of the triacylglycerols SSU-SUS focuses selectively on soft PMF. In dry fractionation, soft PMF concentrates more than 73% of SSU-SUS triacylglycerols, and the content of SSS triacylglycerols is low. Thus, refractionation of soft PMF produces an excellent hard PMF, particularly enriched in SSU-SUS triglycerides (85%-90%) with low content of SSS triglycerides, and the DAG content can be kept low enough to avoid any adverse effect on the crystallization properties of the fraction [74].

Due to the closely related structural properties, TAGs can produce co-crystals by intersolubility, which frequently present solid solutions, monotectic interactions, eutectic systems and formation of molecular compounds [1]. As a result, the efficiency of fractionation depends not only on the separation efficiency, but is limited by the phase behavior of TAGs in the solid state. Thus, intersolubility of TAGs is a challenge in the dry fractionation process, including the route: olein soft PMF hard PMF.

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6. Crystallization problems in raw materials of great industrial importance

Most natural oils and fats have limited application in their unaltered forms, imposed by their particular composition of fatty acids and TAGs. Thus, oils and fats for various industrial applications are chemically modified by hydrogenation or interesterification, or physically by fractionation or mixture [75]. Although used for a long time, partial hydrogenation results in significant formation of fatty acids, associated with negative health effects [76].

In Brazil, controversial issues surrounding the role of fatty acids in the diet have led to progressive changes in legislation, aiming to include more information for consumers. Resolution RDC No. 360, of December 23, 2003, approved by the MERCOSUR, obligated the declaration of fatty acids on the nutritional label of foods. Companies had until July 31, 2006 to meet regulations, so that the fat content is declared in relation to the standard portion of a certain food, together with statements of total and saturated fats [77]. In response, Brazilian industries opted for the progressive substitution of fat in many products through the development of base fats with functionality and economic viability equivalent to partially hydrogenated fats, but without substantial increase in the content of saturated fatty acids in foods.

In this sense, interesterification was found to be the main alternative for obtaining plastic fats with low levels of isomers or lack thereof. In particular, chemical interesterification of liquid oils with fully hydrogenated oils (hardfats) is currently the alternative of greatest versatility to produce zero fats, producing base fats favorable for the preparation of commonly used shortenings [61]. The use of blends, i.e., mixtures of fats with different physical properties, and fractionation also represent additional alternatives to obtain base fats with appropriate physical and plasticity properties to be used in various products, although with potential limited by the chemical composition of the raw materials [21].

Although the interesterification, fractionation and mixing processes are very functional from a technological point of view, the substitution of partially hydrogenated fats in food products, especially in shortenings and confectionery products, is currently a challenge since appropriate crystallization and texture properties are difficult to obtain in the absence of fatty acids [78].

In particular, adequacy of crystallization kinetics of these base fats is of utmost importance so that their use may be adjusted to the limitations of industrial processes and to improve control of processing steps that involve recrystallization of the fat fraction, ensuring quality of the final product [79]. Contrarily, previously standardized processing times and equipment must be altered according to the characteristics of the fat used. This fact becomes particularly important as new fat fractions began to replace partially hydrogenated fats in most industrial applications, mainly in the production of biscuits and bakery products, where it is noted that fats with the same apparent solids profile present very different crystallization properties [80]. In the specific case of interesterified fats, the formation of partial acylglycerols, such as MAGs and DAGs as a result of chemical interesterification, can influence the crystallization kinetics via alterations to the crystal nucleation process [81]. According in [82], 0.1% of the catalyst sodium methoxide, used for randomization, can produce between 1.2 and 2.4% of MAGs+DAGs. Because the typical catalyst content used industrially ranges from 0.1 to 0.4%, concentrations of these minority lipids may be greater than 9%. Although minority lipids present influence on the crystallization properties of these fats, their complete removal is still difficult and expensive, especially on a large scale [22].

Considering that in the Brazilian industry this substitution process is relatively recent, the problems of crystallization behavior due to the unsuitability of new fat fractions are numerous and aggravated, mainly due to regional differences in climate and conditions of transport and storage. In this context, highlighted problems include unwanted polymorphic transitions, oil exudation, development of fat bloom, formation of crystalline agglomerates, and base fats with a maximum solid fat content or induction periods incompatible with certain industrial applications. Studies on modification, stabilization and control of crystallization of these base fats are therefore of crucial importance for development of the edible oils industry.

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7. General characteristics of emulsifiers

In a classic definition, an emulsifier is an expression applied to molecules which migrate to interfaces between two physical phases, and are therefore more concentrated in the interfacial region than in the solution phase [83]. The main molecular characteristic of an emulsifier is its amphiphilic nature, characterized by an ionic group (polar region) and a hydrocarbon chain (nonpolar region). According to their polar and nonpolar regions, emulsifiers are designated as hydrophilic or lipophilic, which affects their solubility in water or oil [84]. Thus, the term hydrophilic-lipophilic balance (HLB) was suggested, which measures the affinity of an emulsifier for oil or water. Regarding emulsifiers in foods, lipophilic properties are generally the most important, but the hydrophilic-lipophilic balance (HLB) may vary considerably according to the chemical composition of the emulsifier. The dual affinity of emulsifiers results in the formation of a single phase between initially immiscible substances (emulsion). Furthermore, these compounds perform functions that in some products are not related to emulsification, including modification of the crystal habit during crystallization of oils and fats [83].

The concept of HLB makes it possible to characterize the various emulsifiers or mixtures of emulsifiers. In general, the following guidelines are used for applying an emulsifier based on its HLB:

  • HLB of 3-6: a good water/oil emulsifier;

  • HLB of 7-9: a good wetting agent;

  • HLB of 10-18: a good oil/water emulsifier.

Nevertheless, the HLB value is limited because it provides a one-dimensional description of the emulsifier properties, and omits information such as the molecular weight and temperature dependence. It is also difficult to calculate useful HLB values for various important emulsifiers in food applications (eg: phospholipids). Additionally, HLB values do not include the important crystallization properties of emulsifiers [85].

Regarding the crystallization properties, in the crystal structure of emulsifiers, the predominant factor is the hydrophilic portion which is the relatively larger portion of the molecule. The size of the hydrophilic group, along with the extension and spatial distribution of hydrogen bonding between adjacent groups, has a much larger influence on the molecular packing of the crystal than the nature of the fatty acid chain. A simple emulsifier, such as a monoacylglycerol, generally crystallizes in the double chain length (DCL), while those with larger hydrophilic groups more frequently crystallize in the SCL configuration (Figure 3) [83].

Among crystallization properties, an important feature of emulsifiers is their ability to create mesophases. Mixtures of emulsifiers with water form different physical structures, depending on the emulsifier/water ratio and temperature. These mixtures are opalescent dispersions, often called “liquid crystals”, but are better known as mesophases. This term (which means “between stages”) reflects the nature of the mixture. On a micromolecular level, the emulsifier agent and water are separated phases, but at the macro level the mixture becomes uniform and is stable (that is, the phases do not separate) [86]. Liquid crystals are thermodynamic mesophases of the condensed material with a certain degree of ordering between the crystalline solid and liquid states [87]. There are two main families of liquid crystals: thermotropic and lyotropic. Thermotropic liquid crystals are composed of molecules, or mixture of molecules, which exhibit shape anisotropy (also known as anisometry). These molecules may have the shape of rods (most common), disks and arcs, among others. The structural and ordering differences of these individual molecules occur as a function of temperature, and therefore are called thermotropic. On the other hand, lyotropic liquid crystals are mixtures of amphiphilic molecules and polar solvents, which under determined conditions of temperature, pressure and relative concentrations of different components, present the formation of aggregated molecular superstructures, which are organized in space, showing some degree of order [88]. The amphiphilic molecules such as emulsifiers may present both behaviors (thermotropic and lyotropic) in this case, called amphotropic liquid crystals [88, 89]. A simplified schematic of the formation of some thermotropic and lyotropic mesophase structures is shown in Figure 4.

7.1. Use of emulsifiers as crystallization modifiers

In addition to their known functions of emulsification and stabilization of emulsions, emulsifiers can modify the behavior of the continuous phase of a food product, giving it specific benefits. In fat-rich products, emulsifiers may be used to control or modify the crystallization properties of the fat phase. Study of the effects of emulsifiers in fat systems is of great interest to improve industrial products, particularly with respect to fat for use in chocolate, confectionery and baking. However, the role of these compounds as modifiers of crystallization in natural and commercial fats is little exploited in technical literature [17]. To date, the vast majority of studies on the use of emulsifiers as modifiers of the crystallization process in fats were carried out with fully hydrogenated oils, model systems or pure TAGs, and therefore do not reflect the need to control crystallization in fats for industrial application [9, 90].

In general, the effect of emulsifiers appears to be related to different crystalline organizations and the creation of imperfections. Some of them can slow transformations via steric hindrance, while others promote these transformations by favoring molecular displacements [3]. Two different mechanisms have been reported in literature to interpret the effect of emulsifiers on crystallization of fats. The first refers to the action of these additives as hetero-nuclei, accelerating crystallization by direct catalytic action as impurities. During crystal growth, emulsifiers would be adsorbed at the surface of the crystals and would therefore modify the incorporation rate of TAGs and crystal morphology. The second mechanism, of greater consensus among various authors, considers that the TAGs and emulsifiers would be amenable to co-crystallize due to the similarity between their chemical structures. Thus, the structural dissimilarity also entails delays in nucleation and potential inhibition of crystal growth [7, 86].

According to this second mechanism, emulsifiers are associated with triacylglycerol molecules by their hydrophobic groups, especially through acyl-acyl interactions. The acyl group of emulsifiers determines its functionality with respect to the TAGs. The main effects of these additives on the crystallization of fats occur during the stages of nucleation, polymorphic transition and crystal growth, altering physical properties such as crystal size, solid fat content and microstructure. The question of promoting or inhibiting crystallization, however, is still debatable. In general, studies indicate that emulsifiers with acyl groups similar to the fat to be crystallized accelerate this process [12].

Currently, it is known that the behavior of emulsifiers during the crystallization of fats can be divided into three cases: (1) limited miscibility between emulsifier molecules and TAGs: in this situation the emulsifier acts as an impurity and the interaction results in imperfect crystals, which may promote or retard crystal growth and polymorphic transitions, depending on the compatibility of hydrophobic ends in their structures; (2) high degree of miscibility between emulsifiers and TAGs that promotes the formation of molecular compounds; (3) total immiscibility between emulsifiers and TAGs, where emulsifiers can act as crystallization germs and microstructure modifiers [11, 86].

Emulsifiers with high potential for controlling crystallization of base fats include sorbitan esters of fatty acids, fatty esters and polyesters of sucrose, commercial standard lecithin and chemically modified lecithin, and the polyglycerol polyricinoleate [30]. Many studies have confirmed that emulsifier affect the crystallization induction times, the composition of nucleation germs, rates of crystal growth and polymorphic transitions [91]. However, the results are still very incipient, and require greater explanation.

7.2. Sugar-based emulsifiers

While the derivatization of oils and fats to produce a variety of emulsifiers with a wide range of application has shown to be well established for many years [92], the industrial production of emulsifiers based on oils, fats and carbohydrates is relatively new. Such emulsifiers result from a product concept based on the exclusive use of renewable resources, where sucrose, glucose and sorbitol are the most used raw materials in industry. The sugar-based emulsifiers most used in the food industry are sorbitan and sucrose esters.

7.2.1. Sorbitan esters

Sorbitol is a hexameric alcohol, obtained by the hydrogenation of glucose. Its free hydroxyl groups can react with fatty acids to form sorbitan esters (SE). In SE production, a reaction mixture containing a specific fatty acid, sorbitol and the catalyst (sodium or zinc stearate) is heated in an inert atmosphere to promote simultaneous esterification and cyclization reactions. The fatty acid/sorbitol mole ratio determines the formation of monoesters and triesters. The SE most well-known and used industrially include lauric, palmitic, stearic and oleic acids [17]. Figure 5 shows the chemical structure of a sorbitan tristearate.

Sorbitan tristearate (STS or 65) and sorbitan monostearate (SMS) are recognized for their ability to efficiently modify crystal morphology and consistency of fats, such as anti-bloom agents in confectionery products containing cocoa butter and in substitutes of cocoa butter, indicated as potential controllers of crystallization. It is assumed that these compounds can delay or inhibit the transition of fat crystals to a more stable form. Moreover, the SE showed to be particularly effective in stabilizing the polymorph β’ in margarines and modification of the solid fat content of fats in general, promoting fusion profiles adequate for the body temperature [18]. They can also be selective as dynamic controllers of polymorphic transitions in fat, due to their ability to create hydrogen bonds with neighboring TAGs, in a process known as The Button Syndrome, whereby the presence of a specific emulsifier does not form a preferred polymorph, but rather controls the degree of mobility of the molecules and their potential to undergo configurational changes. In this process, emulsifiers can modulate the polymorphic transformations in the solid state or via the liquid state, and the temperature regime used to control the physical state of crystals during the polymorphic transition and extension of the mobility of the molecules, thereby regulating the rate of polymorphic transformation [4].

According in [91], STS is the additive with greatest potential for modification of crystallization in cocoa butter, particularly in inhibiting the βV →β VI transition and fat bloom due to its high melting point (55°C) and chemical structure similar to the TAGs present in the oils and fats, permitting facilitated co-crystallization by this emulsifier and formation of solid solutions with these TAGs. In [93], the addition of 0.5% (w/w) of STS to base fats for margin had a stabilizing effect on the polymorph β’. According in [11] observed the formation of small crystal aggregates in mixtures of palm oil/palm kernel olein when adding 0.09% (w/w) of STS, in addition to increasing the rate of crystallization of these mixtures. In a review article, in [16] emphasized the use of STS and/or combinations thereof with other emulsifiers such as soy lecithin, the current alternative of greatest interest for the control of polymorphic transitions and structuring of the crystal lattice in fats, since the TAGs-STS interaction promotes the formation of regular crystals that melt at 40°C, the melting point characteristic of most base fats for industrial applications.

7.2.2. Sucrose esters

Sucrose fatty esters can be used in a wide range of food applications and are mainly utilized in the bakery, confectionery, desserts and special emulsion industries [94]. Sucrose esters, particularly mono-and di-esters, are extremely functional emulsifiers, since they provide a number of unique advantages for the food industry. They are non-toxic compounds, without taste or odor, easily digested sucrose and fatty acids, as well as biodegradable under aerobic and anaerobic conditions. They are produced by interesterification of sucrose and fatty acids by various reaction types and conditions. Their structure is typically composed of polar and nonpolar groups in the same molecule as other emulsifiers, but the eight possible positions for esterification with fatty acids allow for these molecules to obtain different lipophilic/hydrophilic properties. Partially esterified sucrose esters, especially the mono-, di-and tri-esters, are more versatile for use in food applications, where the degree of esterification is controlled by the fatty acids/sucrose ratio in the reaction mixture. Monoesters (~70% of monoesters) are hydrophilic, while the di-, tri-, and polyesters are increasingly hydrophobic [95]. The degree of saturation and size of fatty acid chains used also significantly influences the properties of these compounds [17, 86]. Figure 6 shows the chemical structure of a sucrose ester of stearic acid and that of behenicacid.

The fatty acids most commonly used in sucrose esters are the lauric (C12), myristic (C14), palmitic (C16), stearic (C18), oleic (C18) and behenic acids (C22). By changing the nature or number of fatty acid groups, a wide range of HLB values can be obtained. Commercial sucrose esters are mixtures with various degrees of esterification, due to their complexity, and exhibit diverse behaviors, like lipids. Consequently, they are used in studies on the crystallization of fats. The sucrose esters most studied to date are esters of stearic acid and palmitic acid, especially in the studies of [9, 96, 97]. However, according to [9], few studies explore the effect of these emulsifiers on the induction period, and the rate of crystallization and development of polymorphic forms in fatty systems.

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Acknowledgments

We thank the financial support of FAPESP (Brazil) Grant Proc. 2009/53006-0. M. A. F. Domingues was the recipient of a scholarship from the Brazilian Ministry of Education (CAPES).

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Maria Aliciane Fontenele Domingues, Ana Paula Badan Ribeiro, Theo Guenter Kieckbusch, Luiz Antonio Gioielli, Renato Grimaldi, Lisandro Pavie Cardoso and Lireny Aparecida Guaraldo Gonçalves (May 6th 2015). Advances in Lipids Crystallization Technology, Advanced Topics in Crystallization, Yitzhak Mastai, IntechOpen, DOI: 10.5772/59767. Available from:

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Crystallization of fats and oils

Control of crystallization of Fats and Oils is important in many food products, including margarine, chocolate, butter, and shortenings.



In these products, the aim is to produce the appropriate number and size distribution of fat crystals in the correct polymorphic form (i.e. type of crystal) because this solid fat crystalline phase plays a large role in the product properties as appearance, texture, spreadability, and functionality. It’s not just about the amount of fat crystals but it is also about the type of fat crystals.

Due to the complex composition of most natural fats, our understanding of fat crystallization and it’s application remains a challenge.

Composition of Fats and Oils

Fats are made up primarily of TriAcylGlycerols (TAGs), approximately 98%, with the remainder of the fat being more polar lipids like DiacylGlycerols (DAGs), MonoacylGlycerols (MAGs), Free Fatty Acids (FFAs), phospholipids, glycolipids, sterols, and other minor components. In refined fats, these minor lipids are much lower in concentration than in unrefined fats.

Although the TAGs form the main crystalline phase, the minor components, or impurities, can often play a large role in how crystallization occurs and crystallization may be substantially different in a refined oil than in the unrefined starting material.

TAGs

TAGs are composed of three fatty acids arranged on a glycerol molecule, and with variations in chain length and degree of saturation of the fatty acids, a wide range of components is possible. This range of composition leads to complexities in crystallization. Knowledge about the detailed structures of the triglycerides present in fats and oils is important because they define some of the physical characteristics of the oil. The melting points of triglycerides are dependent on the structures and position of the fatty acids on the glycerol backbone. They also affect the crystallization behavior of the oil.

Palm oil is composed of about 40 different types of TAGs. However, there are three important types of Triglycerides (e.g. POP, PPP and POO for the experts). The semisolid nature of palm oil for instance at room temperature has been attributed to the presence of the POP. Oil modifications process can lead to a product richer in POO (Palm Olein) and a product richer in PPP (Palm Stearin).

Complex Crystallization Process

The crystallization process is divided into nucleation and crystal growth phases. During cooling a crystalline nucleus forms and, it begins to grow by incorporating other molecules. Crystallization kinetics has a profound influence on fats’ final structures and is intrinsically related to their rheological and plasticity properties.

Fats’ tendency to crystallize is of fundamental concern to processing techniques. Triacylglycerols generally crystallize initially into the α polymorphic forms, which then transforms to the β′ and/or β forms which are more stable. This polymorphic transformation is an irreversible process from the less stable to the more stable form, and depends on the temperature and time involved.

Fats with crystals in the β′ form offer greater functionality, because they are softer, support aeration better, and offer creaming properties. The β′ form is thus generally the polymorph of greatest interest for producing high-fat foods, such as margarines and confectionary and bakery products.

The speed of the transformation of the α polymorphic form to the more stable form depends on which type of crystals are crystallizing. PPP type crystals will crystallize fast and transform rapidly to the stable form whereas POP type crystals will transform slowly to the more stable form.

The art of processing fat products in the case of paste products for instance is to control processing settings to allow the right consistency at packing (liquid or semi solid) and which in time will achieve a harder consistency which is solid and with a nice texture.

Sours: https://www.sonneveld.com/en/service/about_our_raw_materials/crystallization_of_fats_and_oils
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Crystallization modifiers in lipid systems

Ana Paula Badan Ribeiro

School of Food Engineering, University of Campinas, Campinas, Brazil

School of Food Engineering, Fats and Oils Laboratory, Cidade Universitária “Zeferino Vaz”, University of Campinas – UNICAMP, Bertrand Russel Street, 13083-970 Campinas, Brazil

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Monise Helen Masuchi

School of Chemical Engineering, University of Campinas, Campinas, Brazil

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Eriksen Koji Miyasaki

School of Chemical Engineering, University of Campinas, Campinas, Brazil

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Glazieli Marangoni de Oliveira

School of Chemical Engineering, University of Campinas, Campinas, Brazil

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Theo Guenter Kieckbusch

School of Chemical Engineering, University of Campinas, Campinas, Brazil

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Author informationArticle notesCopyright and License informationDisclaimer

School of Food Engineering, University of Campinas, Campinas, Brazil

School of Chemical Engineering, University of Campinas, Campinas, Brazil

School of Food Engineering, Fats and Oils Laboratory, Cidade Universitária “Zeferino Vaz”, University of Campinas – UNICAMP, Bertrand Russel Street, 13083-970 Campinas, Brazil

Ana Paula Badan Ribeiro, Email: [email protected]

corresponding authorCorresponding author.

Revised 2014 Aug 11; Accepted 2014 Sep 18.

Copyright © Association of Food Scientists & Technologists (India) 2014

This article has been cited by other articles in PMC.

Abstract

Crystallization of fats is a determinant physical event affecting the structure and properties of fat-based products. The stability of these processed foods is regulated by changes in the physical state of fats and alterations in their crystallization behavior. Problems like polymorphic transitions, oil migration, fat bloom development, slow crystallization and formation of crystalline aggregates stand out. The change of the crystallization behavior of lipid systems has been a strategic issue for the processing of foods, aiming at taylor made products, reducing costs, improving quality, and increasing the applicability and stability of different industrial fats. In this connection, advances in understanding the complex mechanisms that govern fat crystallization led to the development of strategies in order to modulate the conventional processes of fat structuration, based on the use of crystallization modifiers. Different components have been evaluated, such as specific triacyglycerols, partial glycerides (monoacylglycerols and diacylglycerols), free fatty acids, phospholipids and emulsifiers. The knowledge and expertise on the influence of these specific additives or minor lipids on the crystallization behavior of fat systems represents a focus of current interest for the industrial processing of oils and fats. This article presents a comprehensive review on the use of crystallization modifiers in lipid systems, especially for palm oil, cocoa butter and general purpose fats, highlighting: i) the removal, addition or fractionation of minor lipids in fat bases; ii) the use of nucleating agents to modify the crystallization process; iii) control of crystallization in lipid bases by using emulsifiers. The addition of these components into lipid systems is discussed in relation to the phenomena of nucleation, crystal growth, morphology, thermal behavior and polymorphism, with the intention of providing the reader with a complete panorama of the associated mechanisms with crystallization of fats and oils.

Keywords: Oils, Fats, Crystallization, Industrial processing, Minor lipids, Nucleating agents, Emulsifiers

Introduction

Oils and fats

Lipids are represented by fatty acids and their derivatives, or functionally and biosynthetically substances related with these compounds. Edible oils and fats are essential nutrients in the human diet, having a vital role for providing essential fatty acids and energy (Scrimgeour 2005).

Chemically, oils and fats are multi-component mixtures composed predominantly of triacylglycerols (TAGs), which are esters of glycerol and fatty acids. TAGs consist of a glycerol moiety with each hydroxyl group esterified to a fatty acid. In nature, they are synthesised by enzyme systems, which determine that a centre of asymmetry is created about carbon-2 of the glycerol backbone, so they exist in enantiomeric forms, with different fatty acids in each position (sn-1, sn-2 or sn-3), according to Fig. 1. Additional components include polar (or minor) lipids, such as diacylglycerols (DAGs), monoacylglycerols (MAGs), free fatty acids, phospholipids, glycolipids and sterols (Nichols and Sanderson 2003).

The physical behavior of lipids depends on the characteristics of the alkyl chain of the fatty acids: saturated or unsaturated fatty acids, cis or trans configuration and chain size. The melting point increases with chain length and decreases with increased unsaturation. Among saturated acids, odd chain acids are lower melting than adjacent even chain acids. The presence of cis-double bonds markedly lowers the melting point, the bent chains packing less well. Trans fatty acids have melting points much closer to those of the corresponding saturates (Scrimgeour 2005).

Particularly, the triacylglycerol composition determines the physical properties of the oils and fats, affecting the structure, stability, flavor and the sensory and visual characteristics of foods. The oils are liquid at room temperature and the fats are apparently solid, all formed of a complex mixture of TAGs (O’Brien 2008).

Crystallization of lipids

Plastic fats are a crystalline network on a continuous oil matrix (Sato 2001). The crystallization process is the system arrangement as a result of a driven force, characterized by total or partial restriction of movement caused from the physical or chemical bonds between the TAGs molecules. Differences in crystalline forms result from different molecular packing. Therefore, crystals consist of molecules arranged in fixed patterns known as reticulates. Their high degree of molecular complexity allows a same set of TAGs to be packed in several different structures that are relatively stable (Foubert 2007).

Crystallization of lipids has important implications in the industrial food processing, since these products have physical characteristics that largely depend on fat crystals. Such products include chocolates, margarines, spreads, fats for bakery and confectionery, dairy products and general-purpose shortenings (Sato 2001).

Crystallization of fats provides some important properties of processed foods: consistency and plasticity of rich fat products, such as butter, margarine and chocolate, during the stages of production and storage; sensorial properties, such as melting sensation in the mouth; physical stability related to the formation and growth of crystals, oil migration and coalescence of particles and emulsions; and appearance, such as the brightness in chocolates (Foubert et al. 2007). Crystallization of TAGs is generally considered the most important event in fat structuring, although the crystallization of minor lipids, such as DAGs, MAGs and phospholipids, represents a fundamental role in the quality of various products (Metin and Hartel 2005).

Mechanism of crystallization of lipids

The process of crystallization includes the nucleation and crystal growth. Nucleation involves the formation of aggregates of molecules that have exceeded a critical size and are therefore stable. Once a crystal nucleus has formed, it begins to grow by incorporating other molecules from the adjacent liquid layer, which is continuously filled by the supersaturated liquid that is around the crystal (Boistelle 1988).

A nucleus is the smallest crystal that can exist in a solution in a certain temperature. The formation of a nucleus from the liquid phase, or the nucleation process, requires the organization of molecules in a crystalline lattice of critical size, from overcoming an energy barrier. Nucleation mechanisms are generally classified as primary nucleation, which can be homogeneous or heterogeneous, and secondary nucleation. Homogeneous nucleation occurs from the junction of isolated molecular species, which form dimers, trimers and subsequently continue the accumulation process up to when a possible nucleus can be formed depending on the temperature and supersaturation conditions. This kind of nucleation, however, rarely occurs under the conditions of industrial processes. In practice, the nucleation of most systems is usually dominated by the heterogeneous mechanism, in which external catalytic sites or surfaces, such as molecules of differentiated composition, serve to reduce the energy barrier. Although the exact mechanism of heterogeneous nucleation is not yet fully elucidated, the phenomenon can be described as the result of interactions between the solid particle and the supersaturated fluid, causing the local ordering of molecules to form the nucleus. The secondary nucleation is the formation of a new nucleus in the presence of existing crystals, which may occur if microscopic crystalline elements are separated from a crystalline surface already formed, therefore resulting in the fracture of crystals in small stable nuclei (Metin and Hartel 2005; Lawler and Dimick 2002).

When the nuclei formed reach favorable dimensions, these elements become crystallites, whose growth depends not only on external factors (supersaturation, solvents, temperature, impurities), but also on internal factors (structure, links, defects). Therefore, the crystal growth rate can vary by several orders of magnitude. The growth occurs by the binding of molecules to a crystalline surface. At the same time in which molecules are bound to the surface of a crystal, some molecules are also deactivated. There is a continuous movement of molecules on the surface of the crystal, and the result of these processes determines the growth rate, which is directly proportional to the subcooling and varies inversely with the viscosity system (Foubert et al. 2007). Although nucleation and crystal growth are often considered as distinct events, they are not mutually exclusive. Nucleation also occurs while the crystals grow from the molecular clusters formed by the breaking of other existing crystals (Wright et al. 2000a, b).

Crystallization kinetics

The crystallization kinetics intensively influences the final structure of fats and is intrinsically linked to their rheological and plasticity properties. By monitoring the formation of crystalline solid material as a function of time, the nature of the process of crystallization can be determined (Foubert 2007).

The characterization of the crystallization kinetics can be done according to the induction period (τSFC) or nucleation period (relative to the beginning of crystal formation) and maximum solid content - SFCmax. Induction time reflects the time required for a stable nucleus of critical size to be formed in the liquid phase (Himawan et al. 2006). As a definition, τSFC is the time required to obtain a crystal nucleus per volume. The τSFC generally increases with increasing isothermal crystallization temperature and with decreasing melting point of the sample. Another useful parameter for the evaluation of isothermal crystallization is the crystallization stabilization time (tec), defined as the total time for the stabilization of the solid fat content at a given temperature. This parameter is the sum of the characteristic times for nucleation and crystal growth (Hachiya et al. 1989).

The most widely used model for the description of the isothermal phase transformation kinetics is the Avrami model, developed in 1940, which relates the kinetic experimentally determined with the growth form and final structure of the crystalline network (Narine et al. 2006). The Avrami equation gives an indication of the nature of the growth process of crystals:

Where: SFC (t) describes the solid fat content (%) as a function of time; SFC (∞) is the limit of the solid fat content when time tends to infinity; k is the Avrami constant (min−n), which takes into consideration both the nucleation and crystal growth rate; and n is the Avrami exponent, which indicates the crystal growth mechanism (Wright et al. 2000a, b). From the effects of the combination of k and n, we can calculate the crystallization half time (t½), which reflects the magnitude of the Avrami constant, being defined as the time required to achieve 50 % of the crystals (Saberi et al. 2011).

t½ =  (0.693/k)1/n

Currently, the most common analytical technique to investigate the crystallization kinetics of fats is the nuclear magnetic resonance (NMR). However, several analytical techniques such as differential scanning calorimetry (DSC), polarized light microscopy (PLM), as well as the rheological and turbidimetric techniques can be employed successfully. The understanding of the phenomena involved in the crystallization kinetics is better achieved with the combined use of several instrumental methods (Cerdeira et al. 2004).

Polymorphism

Long-chain compounds, such as fatty acids and their esters, can exist in different crystalline forms. Solids with the same composition that can exist in more than one crystalline form are called polymorphs. Polymorphism can be defined in terms of the ability to manifest different unit cell structures as a result of various molecular packings. The crystal habit is defined as the crystal form. From a crystallographic perspective, the habit reflects the direction of the growth within the crystal, while the morphology describes the set of faces determined through the symmetric elements of the crystal. This distinction allows crystals of the same morphology to have different crystalline habits (Lawler and Dimick 2002).

In a fat, crystals are solids with atoms arranged in a regular three-dimensional pattern. A cell is the repeating unit that makes up the integral structure of a given crystal. A sub-cell, in turn, is the smallest periodic structure that exists in the actual cell unit, being defined as the transverse mode of packing of the aliphatic chains in TAGs. The polymorphic forms of a fat are identified based on their sub-cell structure (Boistelle 1988). In lipids, three specific types of sub-cells are predominant, the polymorphs α, β′ and β, according to the current polymorphic nomenclature (Fig. 2). Form α is metastable, with hexagonal chain packing. Form β′ has intermediate stability and orthorhombic perpendicular packing, while form β has greater stability and triclinic parallel packing. The melting temperature increases with increased stability (α → β′ → β), as a result of differences in density of the molecular packing (Martini et al. 2006).

TAGs generally crystallize initially in forms α and β′, although form β is more stable. This phenomenon is related to the fact that form β has a higher free energy of activation for nucleation. The polymorphic transformation is an irreversible process going from the less stable to the more stable form (monotropic phase transformation), depending on the temperature and time involved. At constant temperature, the forms α and β′ can become, as a function of time, form β through the liquid–solid or solid-solid mechanisms (Herrera and Marquez Rocha 1996). The transition rate is dependent on the degree of homogeneity of the TAGs. Fats with low variability of TAGs quickly turn into the stable form β. Fats that are the random distribution of TAGs may have the form β′ indefinitely. In addition, factors such as formulation, cooling rate, crystallization heat and level of agitation affect the number and type of crystals formed. However, as fats are complex mixtures of TAGs, at a certain temperature, different polymorphic forms and liquid oil can coexist (Sato 2001).

Fats prone to crystallization in form β′ include the soybean, peanut, canola, corn and olive oils and lard (O’Brien 2008). In contrast, palm and cottonseed oil, milk fat and tallow tend to produce β′ crystals, which tend to persist for long periods (Foubert et al. 2007). In particular, for cocoa butter, there are six polymorphic forms, as a result of its TAG composition, where symmetrical monounsaturated TAGs are prevalent. The characteristic classification of polymorphs of cocoa butter is based on the Roman numbering system (I to VI), in which form I is the less stable one and form V is associated with the crystal habit desirable in chocolates, which can turn into form VI during storage, which offers greater stability. However, usually we can see combinations of this nomenclature with the Greek nomenclature, where the forms V and VI are recognized as βV and βVI (Loisel et al. 1998; Schenck and Peschar 2004). Table 1 shows the crystal tendencies of the more commonly used edible fats and oils.

Table 1

Classification of fats and oils according to crystal habit (Woerfel 1995)

Beta-prime-type (β′)Beta-type (β)
SoybeanCottonseed
SafflowerPalm
SunflowerTallow
SesameMilk fat
Peanut
Corn
Olive
Coconut
Palm kernel
Lard
Cocoa butter

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The crystal structure of fats is important in the formulation of shortenings, margarines and fat products in general, since each crystalline form presents unique properties regarding plasticity, texture, solubility and aeration (Bell et al. 2007). Fats with crystals in form β′ feature increased functionality, as they are softer; provide good aeration and creaminess properties. Thus, form β′ is the polymorph of interest for the production of foods high in fat such as margarine and confectionery and bakery products. For the production of chocolates with good physical and sensory characteristics, however, form βV is the desirable polymorph, because it is associated with properties such as brightness, uniformity, characteristic snap and improved shelf life (O’Brien 2008).

X-ray diffraction is used to identify the polymorphism of crystals by determining the dimensions of the crystal unit and sub-cells. Due to the different geometric configurations, the polymorphs diffract the x-rays at different angles. In fats, diffraction in high angles corresponds to short spacings (distances between parallel acyl groups in TAG) of sub-cells and allows the checking of different polymorphs (Campos 2005).

Microstructure

The lipid composition and crystallization conditions influence the crystal habit - different crystalline morphologies are possible. The crystals aggregate into larger structures forming a lattice, which characterizes the microstructural level of a fat. The concept of microstructure includes information about the condition, quantity, form, size, spatial relationship and interaction between all components of the crystal lattice and has enormous influence on the macroscopic properties of fats (Shi et al. 2005).

According to Narine and Marangoni (2005), the microstructural level or mesoscale of the crystal lattice of a fat can be defined as the set of structures with dimensions between 0.5μm and 200μm. Its quantification is obtained primarily through the visualization of its geometry. The levels of structure in a typical crystalline lattice are defined when the fat crystallizes from its complete melting. As nanostructural elements (0.4–250 nm), the TAGs crystallize in particular polymorphic states. Most TAGs crystallizes as spherulites, which implies that the crystalline growth occurs radially. The formed crystals grow up to dimensions of 1 to 4 μm and then join together forming clusters (above 100 μm), through a process governed by heat and mass transfer. The aggregation process continues until a continuous three-dimensional lattice is formed from the amalgamation of these microstructures, trapping the fat liquid phase (Marangoni and Narine 2002). This structural hierarchy was recognized by several researchers (Acevedo and Marangoni 2010; Campos et al. 2010; Mazzanti et al. 2011). However, the arrangement of molecules in the crystalline state also depends on factors such as cooling rate, crystallization temperature and shear rate if required (Tang and Marangoni 2007).

Crystal growth can occur in one, two or three dimensions, characterizing the formation of needle, disc or spherulite-shaped crystals, respectively (McGauley and Marangoni 2002). According to Herrera et al. (1998), the application of fats in food products requires the average diameter of the crystals to be less than 30 μm to avoid the sandiness feeling in the mouth.

Currently, a considerable number of new techniques of microscopy has been used to visualize the surface of foods. In particular, studies on the formation of fat bloom in chocolate are including, in addition to the PLM, techniques such as fluorescence microscopy, scanning electron microscopy, magnetic resonance, atomic force microscopy, laser scanning microscopy and confocal microscopy.

Control of crystallization

The crystallization behavior, polymorphic form and microstructure of fats are due to the combination of the individual physical properties of TAGs and phase behavior of the mixture of TAGs. In general, the specific composition of a fat is one of the most important factors for the final development of the crystalline structure (Vereecken et al. 2009).

The crystallization of fats is a critical factor associated with the structure and properties of most foods. The industrial crystallization process consists of a sequence of steps at different temperatures (undercooling) and application of mechanical force. Three parameters in the crystallization process should be controlled simultaneously in order to obtain good solidification: temperature, crystallization time and agitation rate (O’Brien 2008).

The stability of many processed products is influenced by changes in the physical state of fats and changes in the processes of crystallization, as the crystal growth and nucleation events occur simultaneously at different rates, since they are affected by conditions such as degree and rates of supercooling, viscosity and shear (Toro-Vazquez et al. 2005).

The relative rates of nucleation and crystal growth determine the distribution, form and size of the crystals, parameters that are directly associated with the consistency and texture characteristics. However, during the storage phase, several post-crystallization phenomena may occur, which can considerably affect the properties and stability of foods. These include polymorphic transitions to thermodynamically more stable phases, formation of new crystals and crystal growth, migration of oil or small crystals. We highlight that, however, such events are not chronological; polymorphic transitions can occur even in the early stages of processing (Himawan et al. 2006).

Additionally, in the processes of post-crystallization, recognized phenomena such as agglutination of adjacent surfaces or sintering, and the spontaneous dissolution, also known as Ostwald ripening, can be seen. The term sintering is described as the formation of solid bonds between the crystals of fat, with the formation of a cohesive network associated with the undesirable increase in hardness of the fat phase. The Ostwald ripening is, in turn, associated with the dissolution of small crystals previously existing in the fat phase and the development of crystals with undesirable dimensions and weaker crystal lattices, which entails the loss of consistency of products (Johansson and Bergenstahl 1995).

In addition, in some specific products, the controlling of crystallization means, above all, avoiding this process, even if it is thermodynamically favored or by processing or storage conditions (Cerdeira et al. 2005). Thus, the control of crystallization and polymorphic transitions in fats consists of a factor of fundamental importance for the food industry.

Crystallization of fats of industrial relevance

Cocoa butter

Cocoa butter is an important component used in the manufacture of chocolates. It represents the solid phase of the product, serving as dispersant matrix for sugar, milk and cocoa solids. Cocoa butter is responsible for various characteristics of the final product quality, such as hardness and snap at room temperature, complete melting in the mouth, brightness, contraction during demoulding and fast release of aroma in tasting. Moreover, cocoa butter is one of the most expensive ingredients of chocolate, representing 25 to 36 % (w/w) of the cost of the end product (Hindle et al. 2002).

Cocoa butter is composed of approximately 97 % of TAGs, mainly the saturated-unsaturated-saturated type. Among these, the TAG species 1-palmitoyl-2-oleoyl-palmitin (POP), 1-palmitoyl-2-oleoyl-stearin (POS) and 1-stearyl-2-oleoyl-stearin (SOS) represent between 80 and 90 % of all TAGs. Small amounts of MAGs and DAGs (approximately 2 %), free fatty acids and phospholipids are part of the composition of crude cocoa butter (Lipp and Anklan 1998).

The differentiated thermal and structural behavior of cocoa butter reflects its TAG composition, as all majority TAGs in its composition have complex polymorphism. The polymorphism of cocoa butter is directly linked to the quality of the product and the processing performance. The crystal modifications in cocoa butter, except for form βVI, can be obtained directly from the liquid state, in proper cooling conditions. This fact suggests that the βV → βVI transition is mediated only by solid-solid transformation (Garti et al. 1986).

In the production of chocolate, cocoa butter must be pre-crystallized or tempered before the molding or coating steps. Tempering, characterized by certain protocols time x temperature, is employed to promote the crystallization of the more stable polymorph. In chocolate, tempering should induce the formation of nuclei of type βV crystals, which give the desirable characteristics to the product (Loisel et al. 1998). The tempering process begins with the complete melting of the fat phase of the chocolate, at 40 °C. Then, there is the controlled cooling, stirred, to induce fat crystallization. The cooling rate should be close to 2 °C/min up to a temperature of approximately 28 °C. In this step, besides the formation of the desired type βV crystals, there is also the formation of smaller amounts of unwanted crystals, e.g. type β′, which is unstable and has a lower melting point, which are eliminated with a subsequent heating of the mass at 30–32 °C (Quast et al. 2007). This procedure is the usual technique to control the polymorphic crystallization of cocoa butter (Sato 2001).

The tempering is the critical step of processing, not only regarding the quality of the chocolate produced, but also regarding economic terms, as problems in its control (temperature and uniformity) has caused the need for reprocessing, burdening energy costs and area use in industrial plants. Chocolates with different ingredients often require different tempering protocols. Moreover, improperly tempered or crystallized chocolates are associated with the fat bloom phenomenon, considered the main problem in quality in chocolate and confectionery industry, which leads to rejection by the consumer and considerable damage in the marketing of these products (Depypere et al. 2009).

The formation of fat bloom in chocolate adversely affects the appearance and texture attributes, since it generally promotes the formation of whitish and non-uniform surfaces. The generally accepted perception is that the visual fat bloom is a function of the migration of the cocoa butter to the surface, which is deposited in the form of crystals with dimensions between 4 and 5 μm. However, a variety of fat bloom types may occur, depending on the type of chocolate and the conditions of storage, and such variation further hinders the understanding of this phenomenon (Kinta and Hatta 2007).

Although many studies have been devoted to fat bloom, its causes and forms of remediation are not yet fully addressed in the technical literature. It is considered that the fat bloom in chocolate can be resulted from various situations. Inefficient tempering, for example, is responsible for its quick development. Fat bloom is also verified when fats incompatible with cocoa butter are added to chocolate. Other common causes are incorrect cooling of tempered chocolates and temperature fluctuations during storage. The specific mechanism through which it occurs is still unknown, although several theories have been proposed. The most accepted theory is that fat bloom is formed because of the polymorphic transition of cocoa butter. The transformation of unstable crystals into stable crystalline elements would result in fat bloom, primarily associated with the βV→ βVI transition. Recently, another visual form of fat bloom characterized by lightly colored surfaces was reported. In this differentiated type of bloom, the formation of crystalline nuclei in the form βV was not observed (Lohman and Hartel 1994; Lonchampt and Hartel 2004).

During the production of chocolate, in addition to the desirable polymorphism, parameters such as the proportion of solid fat present, number and size of crystals, crystal morphology and microstructure are fundamental in determining the finished product, modulating the rheological and mechanical properties of chocolate (Afoakwa et al. 2008). To conclude, the production of good quality chocolate is modulated by the proper manipulation of cocoa butter, associated with favorable storage conditions. However, the costs associated with the tempering process, as well as the difficult stabilization of the cocoa butter crystallization during and after processing are still problematic factors for the industrial sector.

Palm oil

Palm oil is obtained from the mesocarp of the fruit Elaesis guineensis. It is semi-solid at room temperature, consisting mainly of TAGs of palmitic and oleic acids. Palm oil is the world’s largest vegetable oil used in the food industry (Oil World 2013).

Compared to other vegetable oils, palm oil has a unique and differentiated fatty acid composition, containing similar percentages of saturated and unsaturated fatty acids. Also, it has significant saturated fatty acid content (10 to 16 %) in the position sn-2 of TAGs, in addition to significant levels of palmitic acid (44 %). Besides these features, palm oil has small percentages of MAGs and DAGs as minority components, which are produced during the maturation of the palm fruits and oil processing. The DAGs, specifically, correspond to 4–8 % of the composition of the palm oil, with variations according to geographical origin and processing conditions. The removal of these compounds, however, is difficult even under optimum conditions of refining (O’Brien 2008; Okawachi and Sagi 1985).

Interesterified palm oil and palm stearin, obtained through the respective interesterification and fractionation processes, represent fat bases with extensive use by the food industry. The hard fraction of palm oil, known as hard palm midfraction, is usually employed as an ingredient in cocoa butter equivalents (CBEs), being characterized by the high content of the TAG species 1-palmitoyl-2-oleoyl-palmitin (POP) and fast melting between 30 and 35 °C, as cocoa butter (Hashimoto et al. 2001).

The crystallization behavior of palm oil is extremely important from the commercial point of view, due to crystal habit β′, which, associated to its characteristics of plasticity, ensures its application in margarines, spreads, bakery and confectionery fats and general-purpose shortenings. The functional properties of palm oil and its fractions are strongly related to their composition and to the amount and type of crystals formed on the application temperature range. However, the crystals of palm oil need a long time for the α → β′ transition, a factor considered as inappropriate from the perspective of industrial processes. The resistance to change into β′ is mainly attributed to DAGs. Recent studies on the interactions between TAGs and DAGs of palm oil during crystallization show that the latter have deleterious effect on the characteristics of crystallization, with intensity proportional to the concentration of these minor lipids in palm oil or its fractions (Che Man et al. 2003; Chong et al. 2007). According to Watanabe et al. (1992), the negative effect of DAGs on the crystallization of palm oil would be related to the low nucleation rate of TAGs in the presence of these compounds.

In addition to the slow crystallization of palm oil, another factor of great concern in the industry is its post-processing stability. Palm oil is often associated with hardening problems during storage. In some products based on this raw material, there is the undesirable growth of crystals, which results in sandiness texture and low spreadability (Omar et al. 2005). These crystalline forms can reach dimensions greater than 50 μm in a few weeks of storage, causing the non-uniformity of the end products (Watanabe et al. 1992). In margarines, specifically, there is the formation of crystal clusters with an average diameter between 0.1 to 3 mm, which can easily be observed with the naked eye (Garbolino et al. 2005).

Tanaka et al. (2007) verified that the main TAGs of palm oil, 1-palmitoyl-2-oleoyl-palmitin (POP) and 1-palmitoyl-diolein (POO), have limited miscibility among themselves, which results in the formation of large POP crystals enveloped by POO. When these clusters are formed, there is the junction of other saturated TAGs, in a process that promotes the β′ → β transition. Therefore, to ensure the stability of the polymorph β′ in products based on palm oil is a matter of great industrial interest, given the vast economic importance associated with the use of this raw material.

General purpose fats

Most natural oils and fats have limited application, imposed by their particular composition in fatty acids and TAGs. This way, oils and fats for various industrial applications are chemically modified, through hydrogenation or interesterification, or physically modified, through fractionation or mixture (Erickson 1995). Although used for a long period, the partial hydrogenation results in the expressive formation of trans fatty acids, associated with negative health effects (Hunter 2005).

Worldwide, the controversial issues about the role of trans fatty acids in food caused progressive modifications in legislation, aiming at the inclusion of more information for consumers, requiring the declaration of the levels of these components on the nutrition labeling of foodstuffs. In response, the industries have opted for the progressive replacement of trans fat in various products, through the development of fat bases with economic viability and functionality equivalent to partially hydrogenated fats, without increasing the content of saturated fatty acids in foods (Ribeiro et al. 2007).

In this sense, interesterification proved to be the main alternative for obtaining plastic fats with low levels of trans isomers or even absence of these compounds. Particularly, the chemical interesterification of liquid oils with fully hydrogenated oils (hardfats) is currently the alternative of greater versatility to obtain zero trans fats, producing fat bases with favorable characteristics to prepare general-purpose shortenings (Ribeiro et al. 2009). The use of blends, i.e. mixtures of fats with different physical properties, and fractionation are also additional alternatives for obtaining fat bases with proper plasticity and physical characteristics to be used in several products, although with potential limited by the chemical composition of the raw materials (Foubert et al. 2007).

Although the processes of interesterification, fractionation and blending are very functional under the technological point of view, the replacement of partially hydrogenated fats in food products, mainly in shortenings and confectionery, is currently a challenge, as suitable crystallization and texture properties are difficult to obtain in the absence of trans fatty acids (Reyes-Hernandez et al. 2007).

Especially, the appropriateness of the crystallization kinetics of these fat bases is of is of paramount importance for their use can be adjusted to the limitations of industrial processes and to improve the control of the processing steps involving the recrystallization of the fat fraction, ensuring the quality of the end product (Foubert et al. 2006). Otherwise, processing times and equipment already standardized need to be changed according to the characteristics of the fat used. This fact has become particularly important as new fat bases began to replace partially hydrogenated fats in most industrial applications, mainly in the production of biscuits and bakery products, in which it was noted that fats with the same apparent solid profile presented very different crystallization properties (Bell et al. 2007). In the specific case of interesterified fats, the formation of partial acylglycerols, such as MAGs and DAGs, as a result of chemical interesterification, can influence the crystallization kinetics through changes in the process of nucleation of crystals (Herrera et al. 1999). According to Minal (2003), 0.1 % of sodium methoxide catalyst, employed for randomization, can produce between 1.2 and 2.4 % of MAGs + DAGs. Since the typical content of the catalyst used industrially varies between 0.1 and 0.4 %, the resulting levels of these minor lipids can be greater than 9 %. Although minor lipids have influence on the properties of crystallization of these fats, their complete removal is still difficult and costly, especially on a large scale (Metin and Hartel 2005).

Considering that this replacement process is relatively recent, the crystallization behavior problems because of the non-suitability of new fat bases are numerous and exacerbated, mainly because of regional climatic differences and conditions of transportation and storage. In this context, we can highlight problems like undesirable polymorphic transitions, exudation of oil, development of fat bloom, formation of crystalline clusters, as well as fat bases with maximum content of solid fat or induction periods incompatible with certain industrial applications. Studies on the modification, stabilization and control of the crystallization of these materials thus have crucial importance for the development of the industry of edible oils.

Crystallization modifiers in oils and fats

From the considerations above, the modification, control and/or stabilization of the crystallization and polymorphic transitions of fat raw materials can be done mainly through three alternatives, used individually or combined, according to the requirements of each fat basis: (i) removal, addition or fractionation of minor lipids in fat bases; (ii) use of nucleating agents to modulate the process of crystallization (seeding); (iii) dynamic control of the crystallization in lipid systems through the use of emulsifiers.

Historically, it was understood that the crystalline modification in oils and fats would be linked primarily to physicochemical changes provided by processes such as hydrogenation, interesterification, fractionation and blending, as well as the use of specific time x temperature protocols (Omar et al. 2005).

The alternatives for modification, control and/or stabilization of crystallization here mentioned are based on recent research studies on the technology of oils and fats, which in recent decades showed great advances by employing highly sensitive analytical techniques, which enabled evaluations under crystallographic, micro-structural, and kinetic perspectives which were unknown before in the science of lipids (Sato and Ueno 2005).

Removal, addition or fractionation of minor lipids

Minor lipids (ML) include lipids of greater polarity and with an amphiphilic structure, such as DAGs, MAGs, free fatty acids, phospholipids and sterols. These constituents have been considered molecular agents that affect crystallization. In some cases, the presence of ML can promote crystallization, while, in some systems, there is the effect of inhibition (Metin and Hartel 2005). According to Toro-Vazquez et al. (2005), these compounds modulate all the crystallization process, from the nucleation to the post-crystallization events.

The action of ML on the crystallization of fats is the subject of several recent studies. Some of them evaluate the ML of a given set source; others are directed to the evaluation of a specific component, such as DAGs or phospholipids. Most research studies, however, have focused DAGs, class of ML that is predominant in oils and fats (Mazzanti et al. 2004).

There are some potential mechanisms described in the literature that support the hypothesis that ML can affect crystallization. It is considered the occurrence of interactions of ML with crystals of TAGs in growth, which generates a structural competitive effect or permanent incorporation in the crystalline structure, in order to prevent or enhance growth. In this case, ML could also limit the transference rate of TAGs to the sites of incorporation in the crystalline structure. Through these processes, ML would affect the properties of crystallization rate, polymorphic forms and microstructure of crystals, through the preferential inhibition or promotion of the development of certain crystal faces. Some authors also associate the effect of ML to the heterogeneous nucleation induction, according to the proposition that these compounds are organized separately in micelar structures, acting as bases for the nucleation process. However, the distinction of these effects and their selectivity between crystalline growth or nucleation phases is not yet fully established in the literature, being a subject of great interest in the science of lipids (Foubert et al. 2004; Wright and Marangoni 2002; Metin and Hartel 2005).

In general, the effects of the various ML on the crystallization of fats show that this process can be controlled by adding, removing or even selectively fractionation components between the glycerol classes or even within the same class of compounds (Oh et al. 2005).

Minor lipids

The effect of the set of typical ML of a specific fat has been recently evaluated in literature. Toro-Vazquez et al. (2005) studied the effect of removing ML from cocoa butter. Purified cocoa butter exhibited a shorter time for crystallization and higher rate in the polymorphic transitions, especially in relation to the α → β′ transition. However, the removal of these compounds did not modify the mechanism of crystalline growth. Bunjes et al. (2003) reported that the presence of ML slowed the α → β′ transition into tripalmitin (PPP). This behavior was attributed to the formation of less ordered polymorph α, because of the ML-PPP interactions. According to Mazzanti et al. (2004), the removal of ML prevents the accommodation of a wide variety of molecules during the crystal formation. Thus, TAGs in purified fats would be free to be accommodated in the more stable polymorphic phase, this way accelerating the α → β′ → β transitions.

In studies with milk fat, Wright et al. (2000a, b) and Herrera et al. (1999) verified that the ML slowed the nucleation period at temperatures above 25 °C. However, their observations on the crystal growth rate are contradictory and were attributed to differences in composition. Mazzanti et al. (2004) found that the ML present in milk fat slowed the beginning of crystallization and reduced the rate of growth of the crystals, promoting, in addition, the instability of form β′.

Studies of the crystallization of cocoa butter with added milk fat (10 %) were performed by Tietz and Hartel (2000). The evaluations were made for the complete removal and natural (2.5 % w/w) and duplicate (5.0 % w/w) levels of ML in milk fat. Removing the ML resulted in an increased nucleation period, formation of primary and secondary irregular crystals, with inclusion of liquid fat and rapid formation of bloom in chocolates produced with this fat basis. ML in natural levels have been associated with the formation of spherical and uniform crystals, while the increase of concentration of these compounds produced accelerated crystallization. The authors suggest that ML act as catalytic sites of nucleation when present in low levels and may interfere with crystallization in high concentrations.

In general, the latest research studies show some degree of consensus that ML in natural or slightly higher concentrations are associated with the uniformity of crystals and reduction of the phenomenon of fat bloom; the precise effect on the crystallization and polymorphic habit is still controversial, according to the different results verified for the same type of raw material and the paucity of studies on the joint action of ML (Metin and Hartel 2005).

Phospholipids

Phospholipids crystallize at higher temperatures compared to TAGs, and they can act as crystallization nuclei. In studies of nucleation in cocoa butter, Arruda and Dimick (1991) showed that isolated crystalline nuclei presented in their composition levels 12 times higher of phospholipids compared to the concentration of phospholipids originally present on this fat. This study suggested that phospholipids, particularly phosphatidylcholine and phosphatidylethanolamine, develop crystallization nuclei for the subsequent crystallization of TAGs. Chaiseri and Dimick (1995) associated the nucleation rate of cocoa butter with the polarity of phospholipids. High concentrations of polar phospholipids, such as lysophosphatidylcholine and phosphatidylinositol, were related to slow nucleation rates, whereas for rapid nucleation low concentrations of these compounds and significant levels of phosphatidylcholine were observed. Additional results indicated that the removal of phospholipids by refining had the effect of decreasing the crystallization rate of cocoa butter, with increase induction period in comparison to the original raw material, therefore suggesting that phospholipids are necessary elements to supply the crystallization nuclei of cocoa butter.

The effect of the addition of phospholipids on the crystallization of refined cocoa butter was evaluated by Lawler and Dimick (2002). The incorporation of 0.1 % of phosphatidylcholine promoted increase in the crystallization rate of the samples. In addition to this effect, the authors observed an increase of solid fat content of cocoa butter after tempering, suggesting that the addition of certain phospholipids, in appropriate concentrations, can handle the crystallization and assist in the development of polymorphic forms that are more stable in cocoa butter. Vanhoutte et al. (2002) reported that the increase in concentration of phospholipids in milk fat slowed the onset of crystallization. A similar effect was observed by Miura et al. (2006), in a study on the crystallization of pure butter oil containing added phospholipids.

The incorporation of different lecithin, at concentrations of 0.1, 0.5 and 1.0 % (w/w), on the crystallization behavior of palm oil and an interesterified fat, indicated that the added lecithins showed tendency to slow the formation of fat crystals. This may prove to be one of the solutions in industrial processes where this effect is desired, when replacing trans fats by zero trans fats in food processing and stability (Correa et al. 2011).

Smith (2000) also verified that phospholipids, added at concentrations of 0.1, 0.2, 0.5 and 1 % (w/w), showed interactions with the nucleation and crystal growth in palm oil, trilaurin (LLL) and tristearin (SSS). The study showed that the types of interaction phospholipids-TAGs would be dependent on the chemical structure of phospholipids and the chain size similarity between these compounds. Some phospholipids showed to be nucleation inhibitors, while others had pronounced effect on the crystallization rate, such as the studies mentioned above. Additionally, changes were observed in the shape, size and polymorphism of the crystals, in all concentrations of phospholipids evaluated.

Although the isolated function of phospholipids on the crystallization behavior of fats is not yet completely understood, some events are related to the presence and/or addition of these compounds, including: the formation of nuclei or crystallization seeds, microstructural changes and modifications of the typical processes of nucleation and crystal growth of certain lipid materials. The findings, still contradictory, reported in the literature, can be explained by the natural chemical diversity of the evaluated fats, and as a consequence the use of different instrumental methodologies and supercooling conditions (Toro-Vazquez et al. 2005).

Diacylglycerols (DAGs)

DAGs represent the class of ML of greatest interest to the studies of crystallization of lipids, as they occur in higher concentrations in virtually all vegetable or animal fats (O’Brien 2008).

Studies on the action of DAGs have primarily focused milk fat. Wright and Marangoni (2002) evaluated the effect of the complete removal of ML from milk fat and subsequent addition of DAGs of these components, at a concentration of 0.1 % (w/w). The DAGs slowed the crystallization process, with increased in the induction time, but they did not modify the microstructure of the milk fat. Foubert et al. (2004) reported that the addition of 0.5 and 1.0 % (w/w) of diestearin and diolein to milk fat showed dependent effects on temperature and concentration. The type of fatty acid determined the effect of DAGs on crystal growth and nucleation: diestearin induced nucleation at low temperatures and modified crystal growth at high temperatures; diolein had an effect only on nucleation, in a wide temperature range.

Tietz and Hartel (2000) verified that the 1,2-diacyl-glycerols in a blend of cocoa butter/milk fat slowed the crystallization; in contrast, the 1,3-diacyl-glycerols promoted a significant change in the crystallization rate. According to the authors, this fact shows that DAGs at specific levels may have greater importance in comparison to the total concentration of DAGs in a lipid system. Similarly, Chaiseri and Dimick (1995) reported that the presence of DAGs high in stearic acid promoted fast nucleation of cocoa butter.

Siew and Ng (1999) observed that palm oil DAGs inhibited the process of nucleation and hampered the crystal growth of TAGs. In palm olein, dipalmitin and 1-palmitoyl-2-oleoyl-acylglycerol had positive and negative effects on the crystallization rate, respectively, while diolein showed a neutral effect. Long et al. (2005) suggest that the precise effect of DAGs on the crystallization of palm oil is dependent on their concentration and chemical structure. This proposition shows agreement with other studies in literature. Smith and Povey (1997) reported that the most significant delays on the crystallization of trilaurin were observed when there was a similarity between the chain size of the added DAGs and lauric acid. In a study of crystallization of coconut oil, Gordon and Rahman (1991) verified that the addition of dilaurin increased the crystallization induction period; the addition of diolein showed no effect on this parameter. Studies of Martini and Herrera (2008) evaluated the influence of incorporation of saturated and unsaturated DAGs to different blends of palm oil/palm kernel oil/soybean oil/sunflower oil. DAGs modified the shape and number of crystals, which affected the solid fat content. DAGs with similar chemical composition to the chemical composition of the blends slowed or inhibited crystallization more efficiently.

Saberi et al. (2011) studied the effect low and high concentrations of palm DAGs on the crystallization kinetics of palm oil. The addition of 2 and 5 % of DAGs decreased (negligibly and significantly, respectively) the rate nucleation, the rate of crystallization and the mechanism of crystal growth in palm oil. According to the authors, low concentration of DAGs (10 %) can be used as a stabilizer agent of β′ polymorphous in palm-based products. However, the addition of 30 and 50 % of DAGs significantly increased the rate of nucleation and crystallization, and it also significantly changed the crystal microstructure of palm oil. From the standpoint of industrial applications, the use of high concentrations of DAGs (40 %) in palm-based margarines might be interesting to inhibit post-crystallization hardening in such products.

Therefore, regarding the action of DAGs on the nucleation and crystal growth rates, literature shows that this glycerol class can have a promoting or inhibiting effect on crystallization, primarily as a function of compatibility between its composition and the composition of the lipid raw materials. According to Wright and Marangoni (2002), the ability of the DAGs as modifiers of the crystalline behavior of TAGs is primarily related to the similarity in the chemical composition between these glycerol classes. The studies presented so far suggest that the parameters that define this degree of similarity are the isomerism or stereospecificity of DAGs and their composition in fatty acids, in relation to the chain size and saturation. Also, the effect of the addition of DAGs is dependent on their concentration and more pronounced at low degrees of supercooling.

In general, the polymorphic stability of fats increases significantly with the addition of DAGs, even at low concentrations. Again, their molecular structures, in particular the chain size of their fatty acids and position in the glycerol molecule, show importance for this purpose (Oh et al. 2005). According to Wright et al. (2000a, b), the structural complementarity between the DAGs and TAGs molecules would allow the cocrystallization of these compounds, stabilizing polymorphs and avoiding undesirable transitions. So far, studies indicate that the 1,2-diacyl-glycerols slow polymorphic transitions more effectively than their 1,3-diacyl-glycerols isomers, a difference attributed to the fact that the first preferably exhibit orthorhombic packing, while the later exhibit triclinic arrangement (Oh et al. 2005).

In studies with palm oil, Berger and Wright (1986) showed that α polymorph showed lifetime from 16 to 55 min with the addition of 6.4 and 20 % of DAGs, respectively. Under the same crystallization conditions, Chong et al. (2007) reported that the lifetime of the polymorph α was reduced by half when removing these compounds.

The addition of 5 % of DAGs was effective in slowing the β′ → β transition in margarines, according to a study of Hernqvist and Anjou (1983). In cocoa butter, the presence of DAGs inhibited the βV → βVI transition, according to Tietz and Hartel (2000). In rapeseed oil with high levels of stearic acid and a small amount of erucic acid, the addition of diestearin showed the greatest stabilizing effect on β′ polymorph, when compared to dipalmitin and diecosanoin. The use of 1,2-diacyl-glycerols had greater effect on the stability of the forms α and β′ than 1,3-diacyl-glycerols (Hernqvist et al. 1981).

Some studies also investigate the effect of adding DAGs to pure TAGs. Smith et al. (1994) reported that the addition of 1,2-dilaurin to trilaurin showed a greater effect on the β′ → β stabilization than the addition of 1,3-dilaurin. Oh et al. (2005) evaluated the influence of incorporating dipalmitin, diestearin, diolein and dilinolein (5 % w/w) to tristearin. DAGs showed a stabilizing effect in the α → β′ transition during storage at 53 °C, with maintenance of the α form, and stabilization of the β′ →β transition in storage at 59 °C.

A recent study suggested that the addition of DAGs changed the crystallization process of tristearin (SSS), with changes in the crystallization and melting profiles. The melting curve of pure tristearin presented two endothermic peaks that correspond to the polymorphic forms α and β, and a exothermic peak between the two endothermic ones, corresponding to the events of the polymorphic transition α-β′-β. The addition of diestearin in the system increased the peak area of polymorphic form α compared to pure SSS, while form β had its peak area reduced. The addition of dipalmitin increased both peak area α and β. The presence of diolein caused the peak area concerning form α to be reduced, while peak area β was increased. The exothermic peak was also affected by the presence of DAGs. The results indicate that the addition of OO promotes the polymorphic transition, whereas PP and SS slow the polymorphic transition to the more stable crystalline form (Silva et al. 2014).

Other properties described in the literature on the influence of DAGs on the crystallization of fats include changes in the shape and number of crystals, neutral effect on melting point and solid fat content, decreased tempering temperature and consistency of cocoa butter (Metin and Hartel 2005).

Monoacylglycerols (MAGs)

The MAGs are present in smaller amounts in fats, and few studies are available on the effect of these compounds on the crystallization behavior of lipid systems. The studies available so far show acceleration of fat crystallization, changes in the shape and number of crystals, as well as decreased consistency and solid fat content in an extensive temperature range (Foubert et al. 2004).

Sambuc et al. (1980) investigated the effects of MAGs in the crystallization of various vegetable fats. The addition of 4 % of the monopalmitin/monostearin blend decreased the induction period for all samples evaluated. Smith et al. (1994) showed that the incorporation of monolaurin accelerated the crystallization of trilaurin, with additional decrease in the size of the crystals. Miura et al. (2002) reported the effect of MAGs from myristic, palmitic, stearic, lauric and behenic acids (0.4 % w/w) on the crystallization behavior of palm oil. The content of palm oil solid fat decreased with the addition of MAGs from myristic, palmitic and stearic acids; MAGs from lauric and behenic acids showed no effects on crystallization.

Addition of nucleating agents - seeding

The knowledge on the mechanisms to control the crystallization of fats promoted the development of a new proposal with great potential to modulate and substitute the conventional processes of crystallization and tempering of fats for various industrial purposes, based on the addition of nucleation agents, a technique known as seeding. This alternative is based on the fact that the crystallization of fats can be promoted by adding solid material with properties of nucleation agents – or crystallization seeds. The incorporation of crystallization seeds into liquid fats may promote two effects associated with the control of crystallization: availability of numerous additional nuclei (known as ready-made nuclei) and/or surfaces for crystal growth. Moreover, another technical advantage associated with the use of nucleating agents is related to their great potential as promoter of specific polymorphic forms. Active nucleation agents with specific crystal habit may induce crystallization of fats in the desirable polymorphic forms, as the information for the crystalline packing is provided by the seeds that control this process (Padar et al. 2008; Metin and Hartel 2005).

From a thermodynamic perspective, the use of crystallization seeds is ideal for directing the crystallization in fat bases. The binding of TAG molecules to a pre-existing crystal face is favored, without the energy requirement to create a crystal nucleus (Lonchampt and Hartel 2004). However, in the process of seeding, the temperature control is critical to the permanence of stable crystalline seeds. Additionally, the extension of its effect depends on parameters such as ratio of the mass of the solid material incorporated and the mass of the melted fat to be crystallized, inoculation temperature and cooling rate (Debaste et al. 2008).

Recent studies indicate the use of crystallization seeds as the alternative with greater technological potential for replacement or improvement of conventional tempering, used in the production of chocolates. This technics would solve the problems of insufficient tempering, incomplete crystallization, heterogeneous nucleation during cooling, crystalline instability in temperature fluctuations during storage and, consequently, prevention or inhibition of fat bloom. Additionally, the process of seeding exhibits favorable effects as a crystallization accelerator of palm oil and as a modulator of the crystallization kinetics of fat bases for specific purposes (Smith et al. 2008). Generally, the following advantages are attributed to the use of the seeds of crystallization: reduced sensitivity to variations in temperature, fast solidification and superior fat products (Lonchampt and Hartel 2004).

The crystallization agents used in the seeding technique consist of saturated or unsaturated TAGs. For the crystallization of cocoa butter, particularly, the use of symmetrical disaturated TAGs, with a higher tendency to the formation and permanence of the polymorph βV, is recommended. The use of TAGs as active nucleation agents is also associated with the anti-bloom effect, by avoiding the βV→βVI transition (Smith et al. 2008). Pure TAGs are potential nucleation agents in fats for bakery and general-purpose shortenings as enhancers of the process of crystallization and controllers of the polymorphic form of these raw materials. On the fractionation of palm oil, process improvements have been based on the addition of tripalmitin (PPP) to liquid fat, facilitating the crystallization of 1-palmitoyl-2-oleoyl-palmitin (POP) (Vereecken et al. 2009). Basso et al. (2010) evaluated the addition of tripalmitin on the crystallization features of palm oil. Campos et.al (2010) studied the effects of the addition of tristearin and trilinolein to cocoa butter. The modifying potential of tripalmitin and tristearin in safflower and soybean oils was reported by Dibildox-Alvarado et al. (2010).

According to Sato (2001), specific molecular interactions between materials of the crystallization agents and the fat to be crystallized are prerequisites for the seeding effect, in terms of polymorphic correspondence, similarity of aliphatic chain and thermal stability. Regarding the polymorphic matching, the seed material must have the same polymorphic form of the desired polymorphic form for the nucleation of the mother phase, usually β′ or β. The similarity of the aliphatic chain involves two meanings: chain length and chemical structure of fatty acids. Takiguchi et al. (1998) suggest that the chain length of the fatty acids crystallization seeds must not differ by more than four carbon atoms in relation to the predominant fatty acid in the mother phase. The chemical structure of the fatty acids refers to the degree of saturation. When the mother phase is made up of a blend with a large amount of unsaturated TAGs, crystallization seeds without unsaturated fatty acids will be less effective. Finally, the thermal stability is simply due to the fact that the seeding material should not melted in the liquid phase of the fat at the inoculation temperature. Therefore, the melting point of crystallization seeds must be higher than the melting point of the fat being crystallized. Consequently, the selection of materials with proper polymorphism and compatible melting point is essential for the seeding process (Himavan et al. 2006).

Despite the huge potential of this technics to modify crystallization of fats, few studies are found in literature. Hachiya et al. (1989) proved that the crystallization nuclei formed spontaneously in cocoa butter developed by grouping the TAG species 1-stearyl-2-oleoyl-stearin (SOS) constitute surfaces for aggregation of other TAGs. Sato (2001) evaluated the performance of the TAGs 1-stearyl-2-oleoyl-stearin (SOS), 1-behenyl-2-oleoyl-behenyl (BOB) and tristearin (SSS), at variable concentrations between 0.1 and 5.0 % (w/w) as crystallization seeds of cocoa butter. The ideal outcome was obtained for BOB, having the author concluded the following advantages of the seeding process for the production of chocolates: (i) accelerated crystallization of cocoa butter directly in form βV; (ii) no need for the tempering process; (iii) significant improvement of chocolate stability to the fat bloom. In practice, after inoculation of the TAGs in the liquid chocolate mass, only cooling to approximately 15 °C was required, without need for any additional step. The effect of BOB (at concentrations of 0.5, 1.25 and 2.5 %) on the inhibition of fat bloom in dark chocolate was confirmed by Walter and Cornillon (2001).

Van Malssen et al. (2001), cited by Schenk and Peschar (2004), patented a technique for the production of chocolates that replaces the traditional tempering by using crystallization seeds in form βV, obtained by spray freezing. The chocolates presented excellent brightness and good shelf-life stability. Pore et al. (2009) and Gwie et al. (2006) reported the production of crystallization seeds with tripalmitin and cocoa butter, also by the spray freezing process. Vereecken et al. (2009) evaluated the potential of combining different TAG as crystallization seeds. The authors concluded that the best nucleation agents were formed by blending crystals tripalmitin (PPP) and 1-palmitoyl-2-oleoyl-palmitin (POP) and tristearin (SSS) and 1-stearyl-2-oleoyl-stearin (SOS). The studies on the effect of the seeding on the crystallization of fats are still scarce, in relation to the type of fat being crystallized and to the nature of the materials with favorable characteristics for effective action as crystallization seeds. Few TAG species were tested, as well as their combined action or synergism as crystallization regulators.

Current research studies indicate the use of fully hydrogenated oils, or hardfats, as potential crystallization modifiers of oils and fats, in order to obtain better quality products and industrial processing with significant cost reduction. Oliveira et al. (2011) found that the addition of hardfats promoted drastic changes in the of crystallization profile of palm oil and, increasing the consistency of this raw material. The incorporation of 1 % of hardfats from palm, cottonseed and soybean oils into palm oil promoted a significant reduction in induction time of crystallization, which decreased from 34 min to 29, 27 and 21 min, respectively. Recent study by Ribeiro et al. (2013), showed that use of hardfats from palm, soybean, cottonseed and crambe oils presents effective potential as modifiers of the physical properties of cocoa butter, promoting increased hardness and changes in microstructure of this raw material. The results guide for use of these hardfats, added at low concentrations, as active agents in lipid modification processes, a highly viable option for situations in which the adequacy of the physical properties of cocoa butter is required.

Emulsifiers

Emulsifiers are functional additives of utmost importance in the food industry. They are amphiphilic molecules, usually with long hydrocarbon chains, characterized by simultaneous hydrophilic and lipophilic properties. Beyond their emulsifying and stabilization functions, emulsifiers can modify the solid phase behavior of a food product, giving it specific benefits. In foods high in fat, emulsifiers can be used to control or modify the crystallization properties of the fat phase. The study of the effects of emulsifiers in lipid systems is of great interest for the improvement of industrial fat bases, particularly regarding fats for use in chocolates, confectionery and bakery. However, the role of these compounds as crystallization modifiers in natural and commercials fats has not been completely elucidated in literature (Hasenhuettl 2008). So far, the vast majority of studies on the use of emulsifiers as modifiers of the fat crystallization process were conducted with fully hydrogenated oils, model-systems or pure TAGs, and do not reflect, therefore, the need to control the crystallization of fats of industrial use (Rousseau et al. 2005; Cerdeira et al. 2006).

Emulsifiers with different hydrophobic properties can affect the dynamics of crystallization of fats and oils, accelerating or slowing down this process, as well as the polymorphic transitions. Also, emulsifiers can act as inhibitors of fat bloom. These compounds promote changes in the surface properties of lipids, resulting in changes related to the size and morphology of crystals and crystalline density (Garti 2002).

In general, the effect of emulsifiers is related to different crystal organizations and the creation of imperfections. Some of them may slow the transformations through steric hindrance, while others promote these transformations by favoring molecular displacements (Aronhime et al. 1987). Two different mechanisms have been described in literature in order to interpret the effects of emulsifiers on the crystallization of fats. The first one refers to the performance of these additives as heteronuclei, accelerating the crystallization through the direct catalytic action as impurities. During crystal growth, the emulsifiers would be adsorbed on the surface of the crystals and therefore would change the rate of incorporation of TAGs and the crystal morphology. The second mechanism considers that TAGs and emulsifiers would be likely to suffer cocrystallization because of the similarity between their chemical structures. Thus, the structural dissimilarity would also entail delay on the nucleation and possible crystal growth inhibition (Cerdeira et al. 2003; Garti 2002). According to this mechanism, the emulsifiers are associated with the TAG molecules by their hydrophobic groups, especially through acyl-acyl interactions. The acyl group of emulsifiers determines its functionality in relation to TAGs. The main effects of these additives on the crystallization of fats would occur during the stages of nucleation, polymorphic transition and crystal growth, changing physical properties such as crystal size, solid fat content and microstructure. The question of promoting or inhibiting crystallization, however, is still controversial. In general, studies indicate that emulsifiers with acyl groups similar to the fat being crystallized accelerate this process (Miskandar et al. 2007).

According to Garti (2002) and Miskandar et al. (2006), the behavior of the emulsifiers during fat crystallization can be divided in three events: (1) limited miscibility between the emulsifiers molecules and TAGs: in this situation the emulsifier acts as an impurity and the interaction results in imperfect crystals, which can promote or slow the crystal growth and polymorphic transitions, according to the compatibility of the hydrophobic chains in their structures; (2) high degree of miscibility between emulsifiers and TAGs, which promotes the formation of molecular compounds; (3) total immiscibility between emulsifiers and TAGs, where emulsifiers can act as crystallization seeds and microstructural modifiers.

With regard to the polymorphic transitions, the extension of the protection provided by the emulsifiers is not yet fully known and need further studies, although the literature shows very favorable results (Hasenhuettl 2008).

The selectivity of these additives such as dynamic controllers of polymorphic transitions in fats have been explained by their ability to create hydrogen bonds with neighboring TAGs, by a process known as Button Syndrome, in which the presence of a specific emulsifier does not dictate the formation of a specific polymorph, but controls the degree of mobility of the molecules and their ease to undergo configuration changes. In this process, the emulsifiers can modulate the polymorphic transformations in the solid state or through the liquid state, and the temperature program controls the physical state of the crystals during the polymorphic transition and the extent of the mobility of the molecules, thus regulating the rate of polymorphic transformation (Aronhime et al. 1987).

The literature shows that, in the solid-solid transformation, the promoting or inhibiting effect of emulsifiers on the β′ → β transitions are mainly dependent on the chemical structure of these compounds. This fact is particularly important when considering the βV → βVI transition in cocoa butter, which requires emulsifiers with high melting point, whose carbon chains must be packed with great proximity to produce a rigid structure that hinders the molecular mobility of TAGs. However, for transformations through the liquid phase, the β′ → β transition is avoided by the majority of solid emulsifiers (Aronhime et al. 1988).

In particular, the more efficient emulsifiers in terms of inhibition of fat bloom have three main effects: (i) increase in the crystallization rate and reduction of crystal size; (ii) increase in the melting point of the fat base, providing greater heat resistance to the product; and (iii) prevention of polymorphic transitions (Lonchampt and Hartel 2004).

The emulsifiers with the greatest potential for controlling the crystallization of fat bases include the sorbitan esters of fatty acids, fatty esters and polyesters from saccharose, natural lecithin and chemically modified lecithin, and polyglycerol polyricinoleate (Lonchampt and Hartel 2004). In general, literature shows that these emulsifiers affect the crystallization induction times, the composition of the nucleation seeds, crystal growth rates and polymorphic transitions. However, the results are still very incomplete, because the different parameters regarding the process of crystallization have not been studied to the same extent for each one of these compounds. Besides this factor, few studies address the use of these emulsifiers in real systems and the potential synergy between them as dynamic controllers of fat crystallization under conditions of industrial processing and storage, as well as the systematic understanding of their mechanisms to optimize the use in fat bases (Weyland and Hartel 2008).

Sorbitan esters (SE)

Sorbitol is an alcohol in the hexahedral alcohols group, obtained through hydrogenation of glucose. Its free hydroxyl groups can react with fatty acids to form sorbitan esters (SE). In the production of SE, a reaction mixture containing a specific fatty acid, sorbitol and catalyst (sodium hydroxide or zinc stearate) is heated in inert atmosphere to promote simultaneous reactions of esterification and cyclization. The molar ratio sorbitol/fatty acid determines the formation of monoesters or triesters. The SE that are most well-known and used industrially include SE from lauric, palmitic, stearic and oleic acids (Hasenhuettl 2008). Figure 3 shows the chemical structure of a sorbitan monoester.

The SE are recognized for their ability to modify the crystalline morphology and consistency of fats, with effectiveness as anti-bloom agents in confectionery products containing cocoa butter and cocoa butter substitutes, highlighting the sorbitan monostearate (SMS) and the sorbitan tristearate (STS) as potential crystallization controllers. These compounds may slow or inhibit the polymorphic transition of fat crystals more stable forms. Moreover, SE are especially effective in stabilizing the polymorph β′ in margarine and modifying the solid fat content of fats in general, to promote appropriate melting profiles at body temperature (O’Brien 2008).

The initial assessments on the effect of SE on the crystallization of lipids involved the study of pure TAGs. Aronhime et al. (1988) reported the inhibition of β-crystallization in tristearin crystallized directly from the melted state, when using SMS. According to the authors, the long hydrocarbon chain in this additive allows its solidification at temperatures close to the solidification of trisatured TAGs and its cocrystallization with them, interfering in the polymorphic transitions through steric hindrance. Sato and Kuroda (1987) investigated the incorporation of SMS and STS, at concentrations of 5 % w/w, on the crystallization properties of tripalmitin. The additives slowed the crystallization and the polymorphic transitions α → β′ → β.

Subsequent studies addressed the effect of emulsifiers on the crystallization of raw materials such as cocoa butter and some fat bases, such as blends for different industrial uses. Garti et al. (1986) studied the effects of SE (1–10 % w/w) on the polymorphism of cocoa butter. The authors found an increase in the βIV→βV transition rate, but a significant delay in the βV → βVI transition. Lonchampt and Hartel (2004) highlighted that the SE from palmitic and stearic acids show stabilizing effect on the intermediate form βV of cocoa butter. Garbolino et al. (2005) reported that the addition of several SE (monolaurate, monopalmitate, monostearate and STS; 2 % w/w) to palm oil/interesterified palm oil and palm kernel oil/sunflower oil blends resulted in a significant modification of the crystalline morphology and consistency properties of this fat basis.

According to Weyland and Hartel (2008), STS is an additive with greater potential for the modification of cocoa butter crystallization, particularly for inhibition of the βV → βVI transition and fat bloom, due to its high melting point (55 °C) and chemical structure similar to TAGs from cocoa butter, allowing this emulsifier an easy cocrystallization and the formation of solid solutions with these TAGs. Berger (1990), cited by Weyland and Hartel (2008), also verified that STS showed inhibition of fat bloom and increased brightness in cake toppings based on palm kernel oil. Young and Wassel (2008) reported that the addition of 0.5 % (w/w) of STS in fat bases for margarines had a stabilizing effect on the polymorph β′. Miskandar et al. (2006) observed that aggregates of small crystals were formed in palm oil/palm kernel olein blends when adding 0.09 % (w/w) of STS, in addition to increasing the crystallization rate of these mixtures. Pernetti et al. (2007) emphasized that the use of STS and/or its combination with other emulsifiers is the most important alternative for the control of polymorphic transitions and structuring crystal fat networks, as the TAGs-STS interaction promotes the formation of regular crystals with melting point around 40 °C, characteristic of many fat bases used in industrial applications.

In order to evaluate the effect of different sorbitan monoesters as crystallization modifiers, the components sorbitan monolaurate, monopalmitate, monostearate and monooleate were added to cocoa butter at concentrations of 0.5, 1.0 and 1.5 % (w/w) (Masuchi et al. 2012). Table 2 presents the solid content according to AOCS official method (2009) for pure cocoa butter, pure emulsifiers and their blends, evaluated through Nuclear Magnetic Resonance (NMR). An increase in the solid fat content was noted, mainly at the temperatures of 10 and 15 °C, for samples containing cocoa butter with sorbitan monostearate and monopalmitate at the different concentrations, but with greater increase in solid fat content for samples containing 1.5 % of these emulsifiers.

Table 2

Solid fat content (SFC), in %, of pure cocoa butter (CB), pure sorbitan monoesters: monolaurate (SMLa), monopalmitate (SMP), monostearate (SMSt) and monooleate (SMO), and cocoa butter added with 0.5, 1.0 and 1.5 % (w/w) of each emulsifier

Temperature (°C)
Sample1015202530354565
CB76.471.965.554.827.90.6
SMLa43.021.54.52.31.60.9
CB SMLa 0.5 %78.672.465.154.125.80.2
CB SMLa 1.0 %78.272.365.953.225.80.4
CB SMLa 1.5 %76.372.165.053.525.80.6
SMP92.390.889.187.785.982.449.10.4
CB SMP 0.5 %79.672.966.654.626.50.3
CB SMP 1.0 %79.973.466.854.525.60.7
CB SMP 1.5 %79.374.867.355.125.90.8
SMSt96.495.394.193.192.190.472.00.5
CB SMSt 0.5 %79.473.267.054.927.10.4
CB SMSt 1.0 %80.373.366.254.027.70.6
CB SMSt 1.5 %81.474.768.055.929.10.6
SMO2.91.91.30.80.60.0
CB SMO 0.5 %77.570.965.453.327.20.7
CB SMO 1.0 %76.770.965.253.226.60.6
CB SMO 1.5 %76.370.964.153.225.60.5

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Figure 4 presents the crystallization events by differential scanning calorimetry (DSC) for pure cocoa butter, cocoa butter samples with 1.5 % of sorbitan monostearate (CB SMSt 1.5 %) and 1.5 % of sorbitan monopalmitate. The samples CB SMSt 1.5 and CB SMP 1.5 % showed variations in the onset crystallization temperature of 4.7 and 3.6 °C, respectively, compared to pure cocoa butter. Thus, the addition of these emulsifiers induces crystallization of cocoa butter, anticipating the solidification process.

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Fig. 4

Crystallization curves obtained by Differential Scanning Calorimetry for pure cocoa butter (CB) and samples of cocoa butter added with 1.5 % (w/w) of sorbitan monostearate (CB SMSt 1.5 %) and 1.5 % (w/w) of sorbitan monopalmitate (CB SMP 1.5 %)

Sucrose esters of fatty acids (SEFAs)

Sucrose esters of fatty acids (SEFAs) are mono-, di- and tri-esters of sucrose with food fatty acids, prepared from sucrose and methyl and ethyl esters of food fatty acids or by extraction from sucroglycerides (Hasenhuettl 2008). SEFAs are extremely functional emulsifiers, as they are characterized by unique advantages to the food industry. They are nontoxic compounds, tasteless and flavorless, easily digested, as well as biodegradable under aerobic and anaerobic conditions. The SEFAs are produced by interesterification between sucrose and fatty acids through different reaction mechanisms. They have typical structure of polar and nonpolar groups in the same molecule like other emulsifiers, but the eight possible positions for esterification with fatty acids allow different lipophilic/hydrophilic properties for these molecules. The partially esterified SEFAs, mainly the mono-, di- and triesters, are the most versatile components for use in food, and their esterification degree is controlled by the fatty acids/sucrose ratio in the reaction system. The degree of saturation and the size of the fatty acid chain also significantly influence the properties of these compounds. Sucrose fatty polyesters (SFPs) are synthesized with a high degree of substitution (six to eight groups per molecule), usually from a interesterification reaction in two stages (Garti 2002; Hasenhuettl 2008). Figure 5 shows the chemical structure of a sucrose fatty diester.

In spite of its great versatility of use in food products, including applications such as synthetic fat replacers, studies on the use of SEFAs as crystallization modifiers in fats are recent, but have great potential,, especially when considering issues of adequacy of fats for industrial use. Few works have explored the effect of these emulsifiers on the induction period and crystallization rate and the development of polymorphic forms in fat systems. Regarding the use of SFPs, studies are even more scarce (Cerdeira et al. 2006). According to Lonchampt and Hartel (2004), these additives should also be evaluated as inhibitors or retardants of fat bloom in chocolate and similar products. Oh et al. (2005) consider that these emulsifiers can improve the quality and stability of foods with the desirable polymorph β′.

In studies with pure TAGs, Elisabettini et al. (1996) reported that the addition of 5 % of sucrose monostearate to tristearin (SSS) efficiently slowed the α →β and β′→β transitions. Moreover, Oh et al. (2005) found that the polymorphic transitions of tristearin in the presence of SFP were dependent on the molecular structure of these compounds, especially with respect to the chain size and the degree of substitution of fatty acids.

Some studies show the effect of the incorporation of SEFAs in fat blends. Yuki et al. (1990), cited by Martini et al. (2004), studied the crystallization behavior of blends of 60 % partially hydrogenated soybean oil/30 palm oil/10 % canola oil, with incorporation of 0.5 % (w/w) of different SEFAs. The SEFAs from palmitic and stearic acids accelerated the crystallization, while the SEFA from lauric acid slowed this process. Nasir (2003) verified that the addition of sucrose tetrastearate (1.0 % w/w) to the blend consisting of 90 % partially hydrogenated soybean oil /10 % cottonseed oil produced an increase in the crystallization rate and solid fat content of this basis. According to the author, the use of SEFAs in fats can reduce the time of conventional tempering in several applications in the food industry. Cerdeira et al. (2003) evaluated changes in the properties of crystallization of sunflower oil/fat blend, from the use of different SEFAs, at the concentrations of 0.1 and 0.5 % (w/w). According to this study, these components slowed the nucleation processes of samples. Particularly, the SEFAs from palmitic and stearic acids showed a significant effect in decreasing the crystal sizes and in the modification of crystalline distribution, positively associated with changes in fat bases with large crystals, such as blends of hardfats and liquid oils. In a later study with the same raw materials and additives, Cerdeira et al. (2006) observed that, under static crystallization, the additives favored the formation of the β′ polymorph and slowed the formation of the β polymorph; in dynamic crystallization conditions, action of the additives resulted in a decrease of the crystal sizes, but showed no effect on the crystalline morphology.

In the only study found in literature using cocoa butter added of SEFAs from lauric, myristic, palmitic, stearic and oleic acids, at the concentration of 5 % w/w, Oh and Swanson (2006) observed that the addition of emulsifiers to cocoa butter changed the rate of the βV→ βVI transitions, and this effect depends on the fatty acid composition of the emulsifier. The SEFAs containing fatty acids of similar size to the fatty acids predominant in cocoa butter fully inhibited this transition.

Lecithin

In general usage, lecithin refers to a complex, naturally occurring mixture of phospholipids, lipids containing a phosphoric acid residue; they are nature’s principal surface-active agents. They are found in all living cells, whether of animal or plant origin. Lecithin is obtained by water-degumming crude vegetable oils and separating and drying the hydrated gums. It is, however, the phospholipid portion of lecithin that is mainly responsible for giving form and function to commercial lecithin (Szuhaj 2005).

Lecithin is the emulsifier with higher functionality for use in the food industry. Commercially, soybean oil is the main source of this additive. Food-grade standard lecithin is a complex mixture of phosphatides, which contains mainly phosphatidylcholine (12–18 %), phosphatidylethanolamine (10–15 %), phosphatidylinositol (8–11 %) and phosphatidic acid (3–8 %), combined with other substances, such as TAGs, free fatty acids and carbohydrates (Fig. 6). According to the oil content, lecithin can be classified as liquid, plastic or semi-solid, in which the percentages of phospholipids vary between 60 and 65 %; in the oil free form, lecithin presents phospholipids levels higher than 90 % (O’Brien 2008). Further, standard lecithin can be modified, giving rise to compounded lecithin (combined with other surfactants or additives), fractionated lecithins and chemically modified lecithins (hydrogenated, hydroxylated, halogenated, sulfonated and acetylated). In this last group, the hydroxylated and acetylated lecithins stand out as the species with greater functionality (Garti 2002).

Lecithin is the most widely used additive in confectionery fats and chocolates. As its sensory properties are very similar to those of fats, the use of lecithin enables the reduction of fat levels in many formulations. In chocolates, the use of lecithin reduced viscosity, improved snap and resistance to fat bloom and temperature variations. For example, the addition of 0.5 % of lecithin in a chocolate coating provides reduction in viscosity similar to the addition of 5 % of cocoa butter or vegetable oil (Timms 2003).

As a result of their pronounced amphiphilic characteristics, natural or modified lecithins have drawn attention as modifiers agents of fat crystallization processes, especially regarding the nucleation process and microstructure. Although lecithin is present in many fat matrices, such as margarine and chocolate, its functional use as an active agent in the microstructural and kinetic modifications of fats was evaluated only in a few studies, and just recently the studies were directed to the cocrystallization events and structural modification associated with the use of this emulsifier (Pernetti et al. 2007). Different kinds and concentrations of lecithin and their combinations with other additives in a fat system can display very different effects with regard to the changes in the crystallization rate and crystal morphology, in addition to reducing viscosity and consistency. Thus, the understanding of the mechanisms related to these changes is essential so that the use of this emulsifier can be optimized as an effective controller of crystallization in fat bases for different foodstuffs (Weyland and Hartel 2008).

The influence of incorporating lecithin in chocolate was recently the subject of study of Afoakwa et al. (2008). The authors evaluated the addition of two different levels (0.3 and 0.5 % w/w) of standard lecithin to dark chocolate. The incorporation of lecithin influenced the degree of crystallinity and the melting events of the chocolate mass; the increased lecithin content reduced the size of the crystals and decreased the values of final temperature, peak temperature and melting enthalpy. According to this study, the amphiphilic nature of lecithin would be responsible for crystal deagglomeration, with effects on the physical properties. The authors also highlight that the knowledge on the influence of the type and content of lecithin would have important applications in defining the quality of chocolates, with respect to the characteristics of morphology and dimensions of crystals and polymorphic stability.

In studies on the processes of fats post-crystallization, Johanson and Bergenstahl (1995) reported that lecithin avoided the sintering phenomenon in soybean oil/palm stearin/fully hydrogenated palm kernel oil/partially hydrogenated canola oil blends. This effect was later proven by Harada and Yokomizo (2000), who verified that lecithin adsorb in crystalline interfaces, slowing or even inhibiting the process of sintering in fats during storage.

Regarding to the influence of lecithin on the crystallization kinetics and crystalline microstructure, studies are scarce in literature. Evaluations made by Dhonsi and Stapley (2006) suggested that the addition of 0.2 % of standard lecithin to cocoa butter slightly increased the induction period of crystallization. Miskandar et al. (2006, 2007) observed that 0.03 % lecithin content promoted the formation of small and homogeneous crystals in palm olein/palm oil blends and accelerated the crystallization of the samples. In contrast, the incorporation of lecithin at the levels of 0.06 and 0.09 % inhibited the crystallization of these mixtures.

In spite of its large availability and industrial use, there is a wide gap of knowledge on the accurate effect of the use of lecithin as a crystallization modifier in fats. The available results indicate the formation of more homogeneous crystalline networks, decreased size of crystals and increased induction period, as well as post-processing stability. However, no precise information is available on the percentages of this emulsifier favorable to the processes of crystallization in relation to specific fats. Farther, studies on the functions of chemically modified lecithins on the crystallization of fat bases are still unknown, but have great potential according to the diversification of the functional groups in the molecules of these additives (Pernetti et al. 2007; Weyland and Hartel 2008).

Miyasaki et al. (2012) studied the influence of modified soybean lecithin (acetylated, hydroxylated, enzymatically hydrolyzed and deoiled) on the process of crystallization of cocoa butter in different concentrations (0.2, 0.5 and 0.8 % w/w). The results were compared with the data obtained using standard lecithin. The TAG composition of the cocoa butter was 19.89 % for POP, 39.74 % for SOS and 21.60 % for POS and the amounts of trisaturated, mono-, di- and tri-unsaturated TAGs were 1.55, 87.70, 9.99 and 0.76 %, respectively. In all samples tested, the average values of solid fat content did not show differences and were approximately 76 % (10 °C), 70 % (15 °C), 63 % (20 °C), 53 % (25 °C), 27 % (30 °C), 0.5 % (35 °C) and 0.3 % (40 °C). The curves showed a typical sigmoidal shape of crystallization isotherms of cocoa butter. The addition of modified lecithins in different concentrations changed the induction time and the Avrami parameters. In relation to the samples with concentrations of 0.2 % of emulsifier, they showed an effect that was more pronounced or similar to those with a concentration of 0.5 % in relation to the crystallization rate. In Fig. 7, we present the isotherms of the samples of CB+ lecithins at the concentration of 0.2 %, obtained by Nuclear Magnetic Resonance (NRM). The equilibrium in the solid fat content (SFC (∞)) was reached for all samples in approximately 110 min. The behavior of the samples containing 0.5 % of emulsifier was similar to those with 0.2 %, whereas samples with 0.8 % presented curves that approached those of cocoa butter without emulsifier. Additionally, we can observe two regions where there is a visible distinction of the effect of emulsifiers. One refers to the nucleation onset and crystal growth, in the period between 10 and 12 min, and the other refers to the period of intense crystalline growth, between 35 and 80 min. In the latter region, a discrepancy can be seen in terms of CGS of samples with added emulsifier and pure cocoa butter. The Avrami parameter n was obtained from the data of isothermal crystallization at 15 °C and was approximately 2, suggesting uniformity of the crystal types formed and growth mechanisms. Samples with 0.2 % of emulsifier enabled a better differentiation between effects. Among the emulsifiers tested, the enzymatically hydrolyzed lecithin was the most effective in accelerating the crystallization, followed by standard, hydroxylated, deoiled and acetylated lecithin, in that order. The values of the Avrami parameters, n and k, for samples with emulsifier concentration of 0.2 %, were: 2.10 and 2.80 E-04; 2.16 and 2.12 E-04; 2.16 and 2.12 E-04; 2.15 and 2.13 E-04; 2.28 and 1.29 E-04, respectively. Pure cocoa butter presented values of n and k equal to 2.34 and 8.42 E-05, respectively. Based on these results, enzymatically hydrolyzed lecithin demonstrates great potential to be used in the production of chocolate and other confectionery applications.

Polyglycerol polyricinoleate (PGPR)

Polyglycerol polyricinoleate (PGPR) consists of polyglycerol esters of interesterified fatty acids present in castor oil. It is insoluble in water and ethanol and soluble in ether (Hasenhuettl 2008). Polyglycerol esters are formed by the reaction of fatty acids with glycerol, containing polymers with 2 to 10 molecules. The production of these emulsifiers includes polymerization and esterification processes, which must be carefully controlled in order to obtain specific properties. They are multifunctional additives, property that allows their use as emulsifiers or fat substitutes (O’Brien 2008).

PGPR is one of the more hydrophobic emulsifiers used in foods. This additive has gained attention recently because of its approval for use in confectionery fats, notably in chocolate-based products. The obtaining of PGPR consists of three phases: polymerization of glycerol at elevated temperatures, forming of polycondensated ricinoleic acid and esterification between polyricinoleic acid and polyglycerol at mild temperatures for the formation of oligomers (Garti 2002). Its chemical structure is shown in Fig. 8.

The functionality of PGPR in chocolate has been linked mainly to the reduction of consistency (or yield value), with limited effect on viscosity. Its synergistic effect with lecithin has recently been documented, but the specific mechanisms of action have not yet been fully clarified. Weyland and Hartel (2008) state that the use of PGPR associated with lecithin is the best alternative for viscosity modification in coatings and chocolates, allowing significant adjustments in the levels of cocoa butter or vegetable fat in a particular formulation, with significant reduction of costs. Schantz and Rohm (2005) point out that the optimization of the use of these additives combined in chocolates and similar products would allow the development of innovative products such as thin coatings with high stability.

Rousseau et al. (2005) showed that the polymorphic form and crystalline morphology of the canola oil/4 % fully hydrogenated cottonseed oil blend can be manipulated through the use of PGPR (0.125 % w/w) under different tempering conditions. The presence of PGPR in the concentration evaluated altered the crystal habit and the morphology of the blends, in stirred (240 rpm) crystallization at 5 °C, with a significant increase in the proportion of form β′. The authors suggest that PGPR could produce changes in short spacings that are characteristic of a fat, with the combined use of stirring (factor responsible for the incorporation of PGPR in the inclusions of crystalline forms), thus significantly slowing the α → β′ →β transitions.

Stroppa et al. (2011) evaluated the impact of the use of lecithin and PGPR on the rheological characteristics (Casson model), temper index, snap of chocolate bars and crystallization kinetics through a simulation of the conventional process of chocolate production: tempering with optimized conditions of time and temperature, but statically, and a subsequent isothermal cooling to complement the crystallization. Expected effects of reduction of plastic viscosity and yield limit were found. The authors highlight that the use of PGPR could introduce beneficial effects on the crystallization of fats, through easier and faster tempering processes, microstructural modifications and possible prevention of fat bloom. However, the contribution of PGPR as controller or modifier of crystallization and polymorphism in fat systems is almost unknown and has become the subject of interest to food technology. Fundamental studies are still needed to explain the reactivity of this emulsifier in the processes of crystallization, primarily in relation to cocoa butter (Garti 2002; Lonchampt and Hartel 2004).

Conclusions

This study conducted a comprehensive review of the effects of several compounds as crystallization modifiers of lipid phases, to provide input to the knowledge on the crystallographic, microstructural and kinetic phenomena involved in the processes for modifying the crystallization of fats and oils. Minor lipids, specific TAGs, in addition to a series of emulsifiers, employed at low concentrations, proven effective agents in the processes of lipid modification, representing a highly viable option, in economic terms, for modulating the crystallization properties of industrial oils and fats.

Acknowledgments

To the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP – Proc. 2009/53006-0) for financial support. The authors thank Espaço da Escrita – Coordenadoria Geral da Universidade - UNICAMP - for the language services provided.

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Sours: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4486597/
Effects of Tripalmitin and Tristearin on Crystallization and Melting Behaviour of Coconut Oil

Modification of palm oil crystallization by phytosterol addition as a tool for structuring a low saturated lipid blend

Monise Helen Masuchi Buscato
    School of Chemical Engineering, University of Campinas, Albert Einstein Avenue, 500 13083-852, Cidade Universitaria Zeferino Vaz, Campinas, Sao Paulo, Brazil
Barbara Gallani Zaia
    School of Food Engineering, University of Campinas, Brazil
Kamila Ramponi Rodrigues de Godoi
    School of Food Engineering, University of Campinas, Brazil
Ana Paula Badan Ribeiro
    School of Food Engineering, University of Campinas, Brazil
Theo Guenter Kieckbusch
    School of Chemical Engineering, University of Campinas, Albert Einstein Avenue, 500 13083-852, Cidade Universitaria Zeferino Vaz, Campinas, Sao Paulo, Brazil
About the authors

Fat structural modifications promoted by phytosterol addition ( a hypocholesterolemic component ( to palm oil and a mixture of palm oil and canola oilwere evaluated in order to develop fats with reduced saturated fatty acids. Palm oil added with free or esterified phytosterols was investigated in terms of triacylglycerol composition, microstructure, solid fat content, and crystallization behavior before and after chemical interesterification. The addition of 10% of free phytosterols to samples before interesterification built up a denser crystal fat network structure. After interesterification, the free phytosterols lost their structuring ability and behaved as the esterified form. Free phytosterols were subsequently added to blends of palm oil and canola oil (50:50 w/w%) at different concentrations. Consistency measurements and microscopic observation confirmed that, at concentrations of 6, 8, and 10%, the free phytosterols upgraded the fat structure forming a strongly cohesivefat crystal network.

Keywords:
phytosterols; palm oil; canola oil; saturated fatty acids; fat crystal network.

Oils and fats are raw materials present in the formulation of most processed food and are considered important nutritional constituents. According to current international health organizations, the lipid matrices in food formulations are recommended to contain low levels of saturated fatty acids (low sat) and absence of fatty acids with trans isomers (zero trans), which are associated with increasing cardiovascular diseases (Keys et al., 1965Keys, A., Anderson, J. T., Grande, F., Serum cholesterol response to changes in the diet, IV. Particular saturated fatty acids in the diet. Metabolism, 7, 776-787 (1965).). Along with these nutritional issues, oils and fats should display appropriate structural and sensorial characteristics for the manufacture and for the consumer acceptability of processed food.

According to Haighton (1959)Haighton, A. J., The measurement of the hardness of margarine and fats with cone penetrometers. Journal of the American Oil Chemists Society, 36(8), 345-348 (1959)., fat blends can be classified by ranges of yield value (YV) as soft, plastic or too hard, a practical parameter used mainly for applications in margarines and shortening. Lipid blends exhibiting yield values between 200 and 800 gF/cm2 for example, are characterized as satisfactory plastic and spreadable. For values lower than 100 gF/cm2, the material is classified as soft or very soft, not spreadable, and pourable. Thus, in compliance with health organizations recommendations and also according to specific industrial, commercial and product identification needs, new or alternative zero trans and low sat fats should be developed (Pernetti et al., 2007Pernetti, M., van Malssen, K. F., Flöter, E., Bot, A., Structuring of edible oils by alternatives to crystalline fat. Current Opinion in Colloid and Interface Science , 12, 221-231 (2007).; Rogers, 2009Rogers, M. A., Novel structuring strategies for unsaturated fats - Meeting the zero-trans, zero-saturated fat challenge: A review. Food Research International , 42(7), 747-753 (2009).).

Several reviews present innovative materials and advanced techniques for the development of fat structuration aiming the reduction of saturated fatty acids level and the zero trans approach (Bot et al., 2009Bot, A., Veldhuizen, Y. S. J., den Adel, R., Roijers, E. C., Non-TAG structuring of edible oils and emulsions. Food Hydrocolloids, 23(4), 1184-1189 (2009).; Co and Marangoni, 2012Co, E. D., Marangoni, A. G., Organogels: An alternative edible oil-structuring method. Journal of the American Oil Chemists’ Society, 89(5), 749-780 (2012).; Dassanayake et al., 2011Dassanayake, L. S. K., Kodali, D. R., Ueno, S., Formation of oleogels based on edible lipid materials. Current Opinion in Colloid and Interface Science, 16(5), 432-439 (2011).; Pernetti et al., 2007Pernetti, M., van Malssen, K. F., Flöter, E., Bot, A., Structuring of edible oils by alternatives to crystalline fat. Current Opinion in Colloid and Interface Science , 12, 221-231 (2007).; Rogers, 2009Rogers, M. A., Novel structuring strategies for unsaturated fats - Meeting the zero-trans, zero-saturated fat challenge: A review. Food Research International , 42(7), 747-753 (2009).; Siraj et al., 2015Siraj, N., Shabbir, M. A., Ahmad, T., Sajjad, A., Khan, M. R., Khan, M. I., Butt, M. S., Organogelators as a Saturated Fat Replacer for Structuring Edible Oils. International Journal of Food Properties, 18(9), 1973-1989 (2015).). Numerous polar and nonpolar additives are being considered as promising structuring agents ( also termed oleogelators ( of lipid systems such as high-melting triacylglycerols, monoacylglycerols, diacylglycerols, fatty acids, fatty alcohols, waxes, wax esters, ceramides, sorbitan monostearate, sorbitan tristearate, lecithin, phytosterols/oryzanol, and 12-hydroxystearic acid.

In addition to the application of these different additives to lipid raw materials, interesterification reactions and thermal fractionation are also examples of process implementations, separately or in association, in order to obtain zero trans and low sat fats with industrial applicability (Wassell and Young, 2007Wassell, P., Young, N. W. G., Food applications of trans fatty acid substitutes. International Journal of Food Science & Technology, 42(5), 503-517 (2007).).

Most of the scientific developments regarding the applications of phytosterols for crystal network structuration in lipid materials are currently being performed with mixtures of phytosterols (mainly β-sitosterol) and γ-oryzanol - the esterified form of β-sitosterol with ferulic acid (Bot and Agterof, 2006Bot, A., Agterof, W. G. M., Structuring of edible oils by mixtures of γ-oryzan with beta-sitosterol or related phytosterols. Journal of the American Oil Chemists'Society, 83(6), 513-521 (2006).; Bot et al., 2009Bot, A., Veldhuizen, Y. S. J., den Adel, R., Roijers, E. C., Non-TAG structuring of edible oils and emulsions. Food Hydrocolloids, 23(4), 1184-1189 (2009).). Figure 1 presents a structural representation of an esterified and non-esterified β-sitosterol. According to Bot et al. (2009)Bot, A., Veldhuizen, Y. S. J., den Adel, R., Roijers, E. C., Non-TAG structuring of edible oils and emulsions. Food Hydrocolloids, 23(4), 1184-1189 (2009)., molecular aggregation of β-sitosterol and γ-oryzanol mixtures forms thin fibrillar building blocks, exhibiting a high specific surface area, enabling them to participate in network connections as an outstanding structuring agent. Ract and Gioielli (2008)Ract, J. N. R., Gioielli, L. A., Modified lipids obtained from milk fat, sunflower oil, and phytosterols esters for application in spreads (In Portuguese). Quimica Nova , 31, 1960-1965 (2008). evaluated the addition of esterified phytosterols to sunflower oil aiming to customize the consistency of milk fat by chemical interesterification of a mixture of these three components. They observed a decrease in consistency of the interesterified blend at low temperatures and increased hardness at room temperatures. Acevedo and Franchetti (2016Acevedo, N. C., Franchetti, D., Analysis of co-crystallized free phytosterols with triacylglycerols as a functional food ingredient. Food Research International, 85, 104-112 (2016).) assessed mainly structural properties of blends containing 0, 20 and 25% (w/w%) of free phytosterols -β-sitosterol or stigmasterol - in soybean oil and fully hydrogenated soybean oil, observing the formation of a co-crystallized system and arise in the melting temperature.

Figure 1
Structural representation of an esterified and non-esterified β-sitosterol.

Considering the interesterification reaction of a fat blend containing phytosterols, Ferrari et al. (1997)Ferrari, R. A., Esteves, W., Mukherjee, K. D., Alteration of steryl ester content and positional distribution of fatty acids in triacylglycerols by chemical and enzymic interesterification of plant oils. Journal of the American Oil Chemists’ Society , 74, 93-96 (1997). reported that the chemical structure of the added non-esterified phytosterols was changed after the interesterification process, since the free phytosterol form was transformed into an esterified one.

In addition to the possible uses of phytosterols as fat and oil structuring agents, they also contribute to very important health benefits bylowering the levels of blood cholesterols. Katan et al. (2003)Katan, M. B., Grundy, S. M., Jones, P., Law, M., Miettinen, T., Paoletti, R., Efficacy and safety of plant stanols and sterols in the management of blood cholesterol levels. Mayo Clinic Proceedings, 78(8),965-978 (2003)., for instance, reported that phytosterols are responsible for the decrease inabout 10% of the LDL cholesterol in the human blood with an average daily intake of about 2.0 g.

In the food industry, phytosterols have been added in healthier margarine formulations, generally in the esterified form, easier to solubilize in lipid materials due to the fatty acid moiety (Belitz et al., 2009Belitz, H. D., Grosch, W., Schieberle, P., Lipids. In: Food Chemistry, 4th ed., Springer, Berlin, p.158-247 (2009).).

The crystallization modification effects promoted by the addition of phytosterols (mainly the non-esterified form) to low saturated lipid matrices are still insufficiently investigated.

In this context, this research aimed to evaluate phytosterol additions in the esterified and non-esterified forms as potential modifier agents of the crystal structure of palm oil. The blend was then submitted to interesterification as a possible way to facilitate the incorporation of phytosterols, mainly the non-esterified form, into the lipid structures. After this initial evaluation of the phytosterols incorporation and crystallization ability in the palm oil structure, a low saturated lipid blend (50% palm oil and 50% canola oil) added with non-esterified phytosterols was considered in order to combine the structuring ability of phytosterols and the development of healthier fat blends. The structured samples were evaluated in terms of fatty acid and triacylglycerol compositions, solid fat content, crystallization behavior, consistency, and morphological structure.

Materials

Refined palm oil (PO) was provided by Agropalma (Brazil). Canola oil was purchased at a local market. Free and esterified phytosterols obtained mainly from soya beans were purchased at Cognis (Germany). According to the supplier, the free phytosterol composition was: brasicasterol - 3.6%, campesterol - 26.2%, stigmasterol - 14.0%, β-sitosterol - 47.4%, others - 8.8%; the esterified phytosterols (mainly derived from soybean) composition was: brasicasterol - 2.4%, campesterol - 26.7%, campestanol - 23.0%, β-sitosterol - 43.7%, others - 4.2%. Sodium methoxide (95%) used in the chemical interesterification was purchased from Sigma Aldrich (United States).

Sample preparation

The two forms of phytosterols, free (FPh) and esterified (EPh), were added separately to palm oil, obtaining, respectively, the samples named POF and POE with 10% (w/w) of the respective phytosterol form in each sample. A blank sample (named PO) with no addition of phytosterols was considered for comparison. The blends were melted at 100°C and maintained at this temperature during 10 minutes under constant stirring for complete homogenization and effective incorporation of phytosterols. The three samples - PO, POF and POE - were chemically interesterified, originating, respectively, the samples POi, POFi and POEi. In a subsequent study (Section "Crystallization of a 50:50 blend of canola and palm oil, added with free phytosterols"), a blend of canola oil and palm oil (50:50) received 2, 4, 6, 8, and 10% of free phytosterols prepared as described above, but without chemical interesterification.

Chemical interesterification reaction

The reaction was carried out in a closed vessel using 100 g of each sample and 0.4% (w/w) of sodium methoxide at 90°C under constant stirring (500 rpm) during 20 min, according to optimized conditions described by Grimaldi et al. (2005)Grimaldi, R., Gonçalves, L. A. G., Ando, M. Y., Optimization of chemical interesterification of palm oil (In Portuguese). Quimica Nova, 28, 633-636 (2005)..

Fatty acid composition

An Agilent 6850 Series Gas Chromatograph (GC) system (Santa Clara, CA, USA) equipped with a capillary column and flame ionisation detector (FID) was used to obtain fatty acid composition of palm oil and canola oil, according to Hartman and Lago (1973). The fatty acid methyl esters were separated as described by the AOCS Method Ce 2-66 (AOCS, 2009AOCS. Official Methods and Recommended Practices of the American Oil Chemists’ Society (5th ed.). Champaign (2009).), using a 60 m length DB-23 Agilent capillary column (Santa Clara, CA, USA) (50% cyanopropyl-methylpolysiloxane) with internal diameter of 0.25 mm and coated with a 0.25 μm film. The operating conditions for the oven temperature program were 110°C for 5 min, followed by heating to 215°C at a rate of 5°C/min and holding at 215°C for 24 min. Other running conditions: detector temperature: 280°C, injector temperature: 250°C; helium used as stripping gas; split ratio of 1:50; and 1.0 µL injection volume. Qualitative composition was determined by comparing peak retention times with the respective standards for fatty acids. Samples were analysed in duplicate and the values reported are means of two injections.

Triacylglycerol composition

Triacylglycerol composition determination was performed in triplicate using the same capillary GC CGC Agilent 6850 Series GC System (Santa Clara, CA, USA), with a DB-17 HT Agilent capillary column (Santa Clara, CA, USA) (50%-phenyl-methylpolysiloxane) 15 m in length and 0.25 mm of internal diameter, coated with a 0.15 μm film. The operating conditions were: 1:100 split injection ratio; initial column temperature of 250ºC, heated up to 350ºC at 5ºC/min; helium as stripping gas at 1.0 mL/min; injector temperature of 360ºC; detector temperature of 375ºC; 1.0 µL injection volume and sample concentration of 10 mg/mL in tetrahydrofuran. The triacylglycerols were identified by comparing retention time with standard samples and were quantified based on relative peak area (Antoniosi Filho et al., 1995Antoniosi Filho, N. R., Mendes, O. L., Lanças, F. M., Computer Prediction of Triacylglycerol Composition of Vegetable Oils by HRGC. Chromatographia, 40(9-10), 557-562 (1995).). The results are expressed as the mean of three replicates followed by standard deviations.

Solid fat content

Solid fat content (SFC) was measured using a nuclear magnetic resonance spectrometer Bruker pc 120 Minispec (Silberstreifen, Rheinstetten, Germany) according to AOCS direct method Cd 16b-93 (AOCS, 2009AOCS. Official Methods and Recommended Practices of the American Oil Chemists’ Society (5th ed.). Champaign (2009).). Sample preparation for the measurement was the following: melting at 100ºC for 15 min, conditioned at 60°C during 5 min, chilled to 0°C for 90 min, and then kept at each measurement temperature (10, 15, 20, 25, 30, 35 and 40ºC) during 30 min in a high precision dry bath (TCON 2000, Duratech, Carmel, IN, USA), prior to SFC measurements. The results are expressed as the mean of three determinations.

Crystallization behavior

Thermal behavior was determined with a differential scanning calorimetry (DSC), model Q2000 (TA Instruments, USA) according to the AOCS Method Cj 1-94 (AOCS, 2009AOCS. Official Methods and Recommended Practices of the American Oil Chemists’ Society (5th ed.). Champaign (2009).). Melted samples were weighted (approximately 10 mg) in aluminum pans and hermetically sealed with covers. An empty sealed aluminum pan was used as reference and the equipment was previously calibrated using indium. The operation conditions were: initial temperature of 140ºC maintained for 5 min, cooling to -80ºC at a rate of 10ºC/min, and maintained at -80ºC for 5 min. The crystallization onset temperature (Tonset) of each sample was calculated by Advantage Software (TA Instruments, USA) through linear peak integration. The results are expressed as the mean of three determinations followed by the respective standard deviation.

Consistency

Consistencies were determined by the texture analyzer TA-XT Plus (Stable Micro Systems, Surrey, UK). Initially the samples were heated to 100°C for complete melting of the fat crystals, then conditioned in 50 mL-incubators at 5°C for 24h, for fat crystallization and, subsequently conditioned for 24h at each of the pre-determined temperatures (15, 20 and 25°C). The probe was a Plexiglas® cone with non-truncated tip angle of 45°. The penetration depth applied was 10 mm with probe velocity of 2 mm/s. The compression force obtained is given in gram force (gF) (Campos, 2007Campos, R., Experimental methodology. In: Fat Crystal Networks. Ed. Marangoni, A.J., Marcel, Dekker, p. 267-349 (2007).). According to Haighton (1959)Haighton, A. J., The measurement of the hardness of margarine and fats with cone penetrometers. Journal of the American Oil Chemists Society, 36(8), 345-348 (1959)., the compression force is converted to yield value (YV) by the following equation (Eq. 1):

where YV is yield value, in gF/cm2; K is the constant that is dependent on the cone angle. For a cone with 45° the value is 4700; W is compression force, in gF; p is penetration depth, in units of 0.1 mm. The samples were analysed in quadruplicate and the results are expressed as the means of the replicates.

Crystal morphology

Images were acquired by a polarized Light Microscope (Olympus, model BX 51, USA), coupled to a digital video camera (Media Cybernetics, USA). Samples were first melted at 100°C and then approximately 1 drop of each sample was placed on a pre-heated glass slide and covered with a coverslip. Slides were incubated for 3h at 25°C and then placed on the heated stage (Mettler Toledo, FP82 Microscope Hot Stage, USA) at the same temperature (25°C) for image acquisition. The images were captured by the software Image Pro-Plus version 7.0 (Media Cybernetics, USA), under polarized light and amplification of 40 times. Randomly, three visual areas of each slide were chosen and focused for a qualitative evaluation of the fat crystal network.

Visual observations

Aliquots of 35 mL of the samples 50:50 PO:CO added with 2, 4, 6, 8, and 10% of non-esterified phytosterols were placed in a 70mL-glass-flasks with caps. First, the samples were allowed to melt at 100°C, and then maintained at 5°C during 24h to crystallize, and then stabilized at 25°C for another 24h. After this stabilization period, the flasks were inverted and exposed at oscillating room temperature between 25 and 30°C during 30 days.

Images were taken with a digital camera after 1 minute, 24 hours, and 30 days of the inversion of the flasks.

Statistical Analysis

Yield value (gF/cm2) and crystallization onset temperature (°C) were statistically analyzed by one-way analysis of variance (ANOVA) using STATISTICA V.8 software (StatSoft Inc., USA). Tukey's post hoc test was applied for statistical comparison among the means with a significance level of 5% (p<0.05).

The knowledge of the fatty acid composition is useful for anticipating the lipid materials characteristics and their physico-chemical behavior. For the development of fats with reduced saturated fatty acids content, this insight is of fundamental importance. Table 1 presents the fatty acid composition and the total amount of saturated and unsaturated fatty acids of palm oil, canola oil and a blend made of 50% (w/w%) of PO and CO, named PO:CO 50:50.

Table 1
Fatty acids composition (%) of palm oil (PO), canola oil (CO) and the blend of 50:50 (w/w%) of PO:CO.

The fatty acid composition of palm oil presented approximately 50% of saturated fatty acids and 50% of unsaturated fatty acids, as usually reported in the literature (Basiron, 2005Basiron, Y., Palm oil - Edible oil and fats product: Chemistry, Properties, and health effects. Bailey’s Industrial Oil and Fat Products, p. 333-425 (2005).). This distinguishing composition of palm oil is the main factor that accounts for the wide usage of palm oil in different food applications and also as a raw material for obtaining other lipid materials such as palm oil stearin and olein by fractionation processes.

Canola oil composition presents 9.4% of saturated fatty acids and, as also observed for the palm oil sample, contains no trans fatty acids. Due to the high amount of unsaturated fatty acids, canola oil can be added to lipid formulations to effectively decrease the relative content of saturated fatty acids. In this context, the 50:50 sample of canola oil and palm oil induced a reduction of 40% of the saturated fatty acids originally in palm oil.

Crystallization of palm oil added with phytosterols: effect of chemical interesterification

In order to evaluate the potential modification action on the palm oil crystal structure promoted by phytosterols, samples of pure palm oil received 10% of free phytosterols (FPh) and 10% of the esterified form (EPh). The control sample PO (palm oil with no phytosterol addition) and the other two samples POE (palm oil added with EP) and POF (palm oil added with FPh) were chemically interesterified and the respective interesterified samples POi, POEi and POFi were obtained.

Triacylglycerol composition

To characterize the chemical changes in the lipid systems induced by the interesterification reaction as well as triggered by the phytosterol addition, the samples before (PO, POE and POF) and after (POi, POEi and POFi) the interesterification were evaluated in terms of triacylglycerol composition, solid fat content, crystallization behavior, and crystal morphology.

The triacylglycerol compositions, determined by gas chromatography of the samples before and after the chemical interesterification reaction, are listed in Table 2.

Table 2
Triacylglycerol composition (TAG, in %w/w) of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification reaction.

The triacylglycerol compositions shown in Table 2 indicate slight alterations between the PO and POi samples, especially in the amounts of PLP, POO and PLO triacylglycerols. This was expected since the reaction was performed using only a single natural lipid material - palm oil. Larger differences between the triacylglycerol compositions of the initial and interesterified samples can be found when the lipid materials are physically different, such as a liquid oil and a fully hydrogenated fat.

In samples added with esterified and non-esterified phytosterols before (POE and POF) and after (POEi and POFi) the chemical interesterification reaction, the triacylglycerol compositions indicate apparent changes in the POO triacylglycerol content. However, proper identification of the different triacylglycerols in the four samples was impaired by the presence of peaks related to the phytosterols. The 8 to 11% unidentified content of triacylglycerols observed in the POFi and POEi is probably related to the presence of free and/or esterified phytosterols in these samples.

Solid fat content

In Figure 2, the solid fat content (SFC, in %) is shown at temperatures of 10, 15, 20, 25, 30, 35, and 40°C. All samples showed some difference in solid fat content at most measurement temperatures before and after the reaction. At lower temperatures - 10, 15 and 20°C - larger differences can be observed. At 10°C, for instance, the samples can be divided in two distinct groups: the first group presenting an average of 48% of SFC, and the second, approximately 40% of SFC. The first group - showing 10% more solid fat content than the other - includes PO, POi and POF samples. The group with lower solid fat content is composed by POE, POEi, and POFi. At higher measured temperatures such as 25, 30, and 35°C,the curves indicate that the sample POF presents the highest content of solid fat: approximately 2% higher than the other samples.

Figure 2
Solid fat content (in %) of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification, measured at different temperatures (°C)

The solid fat content curves suggest that the addition of esterified phytosterols decreased the solid fat content of palm oil, hampering the formation of the crystalline organization, probably due to their chemical structure which contain esterified fatty acids. It is known that the presence of fatty acids in plant sterol molecules favors solubility when added in lipid materials, due to the similarity between the molecular structures (lipids and esterified phytosterols). These components can organize themselves in a simpler way compared to the triacylglycerols in the lipid mixture, not favoring the creation of a different crystal network (Belitz et al., 2009Belitz, H. D., Grosch, W., Schieberle, P., Lipids. In: Food Chemistry, 4th ed., Springer, Berlin, p.158-247 (2009).; Rodrigues et al., 2007Rodrigues, J. N., Torres, R. P., Mancini-Filho, J., Gioielli, L. A. Physical and chemical properties of milkfat and phytosterol esters blends. Food Research International , 40, 748-755 (2007).). One of the main objectives of the present research of promoting the increment of solid fat content was not achieved by the addition of esterified phytosterols in palm oil. Furthermore, the simple addition of free phytosterols, i.e. sterols not esterified with fatty acids, to palm oil (POF) resulted in an increase in the solids content of the sample. After the interesterification (POFi), however, the palm oil sample with added free phytosterols showed reduced solids content compared to the sample before the reaction (POF). A similar trend was found by Ferrari et al. (1997)Ferrari, R. A., Esteves, W., Mukherjee, K. D., Alteration of steryl ester content and positional distribution of fatty acids in triacylglycerols by chemical and enzymic interesterification of plant oils. Journal of the American Oil Chemists’ Society , 74, 93-96 (1997)., who confirmed that free phytosterols have their chemical structure changed after the interesterification process, since the non-esterified forms are transformed in esterified phytosterols during this reaction. On the other hand, POFi exhibited higher SFC values at all measured temperatures compared to POEi, suggesting that not all free phytosterols were esterified during the chemical interesterification reaction.

Crystal morphology by microscopic observation

The crystal network morphology of the pure palm oil sample, with free phytosterols and esterified phytosterols before (PO, POE and POF) and after (POi, POEi and POFi) the chemical interesterification was observed under polarized light microscopy, after submission to isothermal crystallization during 3h at 25°C. The images, obtained undera 40-fold magnification, are presented in Figure 3.

Figure 3
Polarized light microscopy images of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification reaction. The images were taken after 3 h isothermal crystallization at 25°C and the scale bars represent 50 μm

Palm oil crystalline elements showed an average diameter of approximately 50 μm, distributed as a non-cohesive lipid network. The same morphology can be observed in POi, POE and POEi samples. However the micrographs of these two last samples added with esterified phytosterols show higher amount of liquid oil, represented by the black background in the images, and also larger features, in a more dispersed way and less densely compacted.

The morphology of the crystals of palm oil added with free phytosterol and not yet submitted to the interesterification reaction (POF) presented a completely different crystal network compared to the other samples. The POF image showed a network of smaller tightly packed crystals, and larger fibrillar crystals trapped within this frame can also be observed, which characterizes the typical morphology described in the literature for non-esterified phytosterols (Bot et al., 2009Bot, A., Veldhuizen, Y. S. J., den Adel, R., Roijers, E. C., Non-TAG structuring of edible oils and emulsions. Food Hydrocolloids, 23(4), 1184-1189 (2009).; Rogers, 2009Rogers, M. A., Novel structuring strategies for unsaturated fats - Meeting the zero-trans, zero-saturated fat challenge: A review. Food Research International , 42(7), 747-753 (2009).). The microstructural differences observed among the samples added with free and esterified phytosterols can be attributed to a self-assembly mechanism of the free phytosterol forms added to the palm oil sample, as described by Dassanayake et al.(2011)Dassanayake, L. S. K., Kodali, D. R., Ueno, S., Formation of oleogels based on edible lipid materials. Current Opinion in Colloid and Interface Science, 16(5), 432-439 (2011)..

The POFi image displayed crystal morphology analogous to the POE and POEi samples (Figure 3), repeating the trend found in the solid fat content results and supporting the assumption that the free phytosterol chemical structure was modified by esterification of their molecules during the interesterification reaction.

Crystallization behavior by DSC

The crystallization behaviors of free phytosterols (FPh) and esterified phytosterols (EPh) were determined by DSC technique and the profiles obtained are presented in Figure 4. The output indicated that both have distinct behavior in terms of crystallization onset temperatures. The EPh's onset of crystallization occurs at approximately 23°C, in the region of the lipid phase change temperature, while in FPh, this parameter is around 125°C. Therefore, the incorporation of esterified phytosterols into lipid materials should proceed smoothly, due to similarities in crystallization temperatures. The opposite behavior is expected for the incorporation of free phytosterols and it is known that they are poorly soluble in the fat phase (Belitz et al., 2009Belitz, H. D., Grosch, W., Schieberle, P., Lipids. In: Food Chemistry, 4th ed., Springer, Berlin, p.158-247 (2009).).The dissimilar structure and the polar characteristics of FPh, however, can favor the formation of a new fat crystal network. When a uniform blend of non-esterified phytosterols with lipid material at high temperatures is cooled, FPh solidifies first due to its polar and thermal characteristics and starts to self-assemble in fibrillar crystals (Bot et al., 2009Bot, A., Veldhuizen, Y. S. J., den Adel, R., Roijers, E. C., Non-TAG structuring of edible oils and emulsions. Food Hydrocolloids, 23(4), 1184-1189 (2009).; Dassanayake et al., 2011Dassanayake, L. S. K., Kodali, D. R., Ueno, S., Formation of oleogels based on edible lipid materials. Current Opinion in Colloid and Interface Science, 16(5), 432-439 (2011).; Rogers, 2009Rogers, M. A., Novel structuring strategies for unsaturated fats - Meeting the zero-trans, zero-saturated fat challenge: A review. Food Research International , 42(7), 747-753 (2009).), forming a specific frame. At lower temperatures, the triacylglycerols will crystallize, and the crystals, in turn, could be tightly trapped in the FPh-triacylglycerol network.

Figure 4
Crystallization profile of free phytosterols (FPh) and esterified phytosterols (EPh). Positive values for heat flow represent exothermic reactions

The DSC crystallization behavior of pure palm oil and samples added with 10% of each phytosterol are presented in Figure 5.An effective incorporation of an additive into the lipid matrix assumes that the crystallization events profiles obtained by DSC do not present isolated peaks, credited to the pure, isolated component. For instance, an isolated peak around 125°C in samples added with FPh would appear if this component was not effectively incorporated into the lipid material.

Figure 5
Crystallization profile of palm oil, palm oil added with 10% of esterified phytosterols, and non-esterified phytosterols before (PO, POE, POF) and after (POi, POEi, POFi) chemical interesterification reaction. Positive values for heat flow represent exothermic reactions

By comparison between Figure 4 and 5 , no peak in the same crystallization temperature range of pure FPh was detected, suggesting that the free phytosterol was effectively incorporated into the palm oil. However, the crystallization peak of the pure material could be observed at a smaller cooling rate. On the other hand, it is unreasonable to discuss the effect on the peak related to the pure Eph due to the presence of other peaks around 20°C related to palm oil. However, the incorporation of esterified phytosterols to lipid materials is facilitated by their chemical structure similarities and is reported to be easily dispersed in fat systems, like spreads (Belitz et al., 2009Belitz, H. D., Grosch, W., Schieberle, P., Lipids. In: Food Chemistry, 4th ed., Springer, Berlin, p.158-247 (2009).).

The crystallization onset temperatures (Tonset, ºC) of each sample, before and after the chemical interesterification, were determined through inspection of the crystallization profiles and their mean values and standard deviations are shown in Table 3.

Table 3
Crystallization onset temperature (Tonset, °C) of the peaks obtained from the DSC crystallization profile for palm oil, palm oil added with esterified phytosterols and with non-esterified phytosterols before (PO, POE, POF) and after (POi, POEi, POFi) chemical interesterification reaction.

Two main peaks - termed Peak 2 and Peak 3 in Table 3 - can be observed in all samples, as expected for palm oil crystallization profile by DSC (Basiron, 2005Basiron, Y., Palm oil - Edible oil and fats product: Chemistry, Properties, and health effects. Bailey’s Industrial Oil and Fat Products, p. 333-425 (2005).). These two peaks stand for the two main triacylglycerol groups in palm oil: disaturated (represented mainly by POP) and diunsaturated (mainly POO). Palm oil before the interesterification and with no phytosterol additions shows a crystallization onset temperature approximately at 19.4°C for the disaturated triacylglycerols, whilst for the diunsaturated ones, it is near 4.8°C.

Compared to the value of pure palm oil, the crystallization onset temperatures (Peak 2) after the addition of esterified phytosterols and without being interesterified decreased about two degrees. However, after the chemical interesterification, all the samples showed similar temperatures at the beginning of the crystallization.

In general, palm oil added with non-esterified phytosterols presented the most altered crystallization profile among the samples before the interesterification reaction. A third peak with the highest crystallization onset temperature, close to 47ºC, became apparent in the POF sample. This peak suggests that a different crystalline structure with modified crystallization parameters emerged as the result of the free phytosterols addition, before the interesterification reaction. Although the incorporation of non-esterified phytosterols was facilitated after the chemical interesterification reaction, the FPh probably turns to the esterified forms and its innovative crystallization ability vanished. This behavior, again, resembles the trends found in solid fat content and the microstructure observation for the sample added with non-esterified phytosterols and not interesterified. Consequently, a possible direct application of free phytosterols for the development of structured fat products with reduced saturated fatty acids can be achieved due to the increase in the solid fat content associated with a denser packed crystal morphology, as presented in the following section.

Crystallization of a 50:50 blend of canola and palm oil, added with free phytosterols

As seen in the former experiments, adding non-esterified phytosterols to palm oil generated a denser and more homogeneous crystal network. This effect can be attributed to the polar characteristics of non-esterified phytosterols and their difficulties in solubilising in lipid media. Tests indicate that, after adding free phytosterols to palm oil at 100°C, under constant stirring, a turbid solution is formed due to the polar characteristics of FPh molecules. If the solution temperature is gradually decreased below 20°C, under continuing stirring, the FPh molecules become less compatible with the lipid phase and they will self-assemble in a fibrillar structure forming a continuous network (Bot and Agterof, 2006Bot, A., Agterof, W. G. M., Structuring of edible oils by mixtures of γ-oryzan with beta-sitosterol or related phytosterols. Journal of the American Oil Chemists'Society, 83(6), 513-521 (2006).; Rogers, 2009Rogers, M. A., Novel structuring strategies for unsaturated fats - Meeting the zero-trans, zero-saturated fat challenge: A review. Food Research International , 42(7), 747-753 (2009).), generating together with the higher melting triacylglycerols of the sample the framework for the crystallization of the palm oil structure. The remaining liquid lipid phase as well as the lipid crystals are trapped inside this structure. On the other hand, when added to low saturated lipid media, like pure canola oil, a two-phase system is immediately formed and the phytosterol phase settles at the bottom of the mixture, blocking the FPh ability to modify the crystal structuration. Thus, this mechanism suggests that non-esterified phytosterols can only form a denser network when added to samples containing certainamounts of saturated fatty acids, and not those containing only unsaturated ones.

In order to seek a compromise between the structuring action of free phytosterols in palm oil and the shortcomings of low saturated fat systems, a blend denominated PO:CO 50:50 was prepared and its fatty acid profile was presented in Table 1. The data indicate 40% less saturated fatty acids content compared to pure palm oil. Five different concentrations - 2, 4, 6, 8, and 10% (in w/w%) - of non-esterified phytosterols (FPh) were added to this lipid system. For comparative purposes, a sample with no phytosterol addition was evaluated together with the structured blends. Samples were evaluated by visual observation, consistency, and crystal morphology.

Visual observation

In order to acknowledge a qualitative perception of the crystal network formation, Figure 6 shows the images taken of the inverted sample flasks added with free phytosterols (0, 2, 4, 6, 8, and 10%) at 1min, 24h and 30 days after flask inversion. Immediately after the inversion, only the sample with no addition of free phytosterols decanted, indicating that a fluid lipid blend shaped by a soft crystal network had been formed during the temperature protocol period. After 24h at room temperature, the samples added with 2 and 4% of free phytosterols lost their cohesive crystal structure and also decanted, probably due to the oscillating temperature effects. The additions of 6, 8, and 10% of non-esterified phytosterols to the palm and canola oil blend were able to assemble a tightly interconnected crystal network, establishing a stable structure, even after 30 days under oscillating room temperature. However, only samples added with 8 and 10% of free phytosterols showed no liquid oil exudation during the total period of observation.

Figure 6
Images of palm oil (PO) and canola oil (CO) blends (PO:CO 50:50), added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), stabilized at 5°C for 24h and 25°C for 24h, then inverted and exposed to oscillating room temperature (between 25 and 30°C) for 1min, 24h, and 30 days

Consistency

The consistencies of the samples were determined with a texture analyzer. Figure 7 shows the consistency expressed in yield value (gF/cm2) for PO:CO 50:50 samples added with 0, 2, 4, 6, 8, and 10% of free phytosterols (FPh), measured at 15, 20, and 25°C. A glance at Figure 7 confirms the substantial action of free-phytosterols as a structure forming component, at all temperatures, but mainly at 15°C. At this temperature, the increase in consistency is linear with the phytosterol concentration.

Figure 7
Yield value (in gF/cm2) of palm oil and canola oil (PO:CO 50:50, in w/w%) added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), measured at 15, 20, and 25°C. Same letters for the same temperature indicate that there are no significant differences between the means evaluated by Tukey's test (p<0.05)

The yield values presented in Figure 7 indicated that all the samples showed higher consistency at 15°C. However, only the samples added with 6, 8, and 10% of FPh showed YV greater than 200 gF/cm2 and, therefore, are classified as plastic materials at this temperature (Haighton, 1959Haighton, A. J., The measurement of the hardness of margarine and fats with cone penetrometers. Journal of the American Oil Chemists Society, 36(8), 345-348 (1959).). These three samples belong tothe group that presented self-stable structure in the visual observationtest at 25°C, mentioned above. However, considering the YV measured at 25°C, all samples can be classified as very soft and pourable, according to Haighton (1959)Haighton, A. J., The measurement of the hardness of margarine and fats with cone penetrometers. Journal of the American Oil Chemists Society, 36(8), 345-348 (1959)..

Crystal morphology by microscopic observation

The fat crystals formed after isothermal crystallization at 25°C, for 3h in the lipid blend added with free phytosterols were observed under polarized light microscope. The micrographs are presented in Figure 8.

Figure 8
Polarized light microscopy images of pure palm oil (PO), and blends of palm oil and canola oil (PO:CO 50:50, in w/w%) added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), after isothermal crystallization at 25°C, for 3h (the scale bars represent 50 μm)

The addition of canola oil to palm oil (PO:CO 50:50) increased the liquid oil fraction, recognizable by the dominant black background in the micrographs. By gradual addition of free phytosterols, mainly at concentrations higher than 4%, a crystal network with higher number of small crystals was formed. The presence of tightly structured small crystals favors the formation of a denser packed network, exhibiting higher consistency, as confirmed by the yield value results. Particularly in the micrograph of PO:CO 50:50 added with 10% of free phytosterols, besides the formation of a more cohesive crystal structure, crystals featuring larger diameter can also be observed, attributable to free phytosterol crystalline structure.

Esterified and non-esterified phytosterols were evaluated as possible crystallization modifiers in palm oil. Only the non-esterified or free phytosterols form was able to effectively modifythe palm oil microstructure. After chemical interesterification of the mixture containing free phytosterols in palm oil, the structuration ability of the free phytosterols disappeared probably because of their own esterification with fatty acids from palm oil. Furthermore, the structuration action of free phytosterols in palm oil and canola oil blends was evaluated and a compact fat crystal network was obtained, mainly with the addition of 6, 8, and 10% of free phytosterols. The results ratify non-esterified phytosterols as a powerful structuring agent when applied to low saturated fatty acids lipid blends.

The authors are grateful for the Brazilian financial support received from São Paulo Research Foundation (FAPESP - Proc. 2014/05365-0).

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  • Publication in this collection
    Jan 2018
  • Received
    31 May 2016
  • Reviewed
    21 Nov 2016
  • Accepted
    29 Nov 2016
Monise Helen Masuchi Buscato *
School of Chemical Engineering, University of Campinas, Albert Einstein Avenue, 500 13083-852, Cidade Universitaria Zeferino Vaz, Campinas, Sao Paulo, Brazil
Barbara Gallani Zaia
School of Food Engineering, University of Campinas, Brazil
Kamila Ramponi Rodrigues de Godoi
School of Food Engineering, University of Campinas, Brazil
Ana Paula Badan Ribeiro
School of Food Engineering, University of Campinas, Brazil
Theo Guenter Kieckbusch
School of Chemical Engineering, University of Campinas, Albert Einstein Avenue, 500 13083-852, Cidade Universitaria Zeferino Vaz, Campinas, Sao Paulo, Brazil

Figures | Tables | Formulas

Figure 1
Structural representation of an esterified and non-esterified β-sitosterol.
Figure 2
Solid fat content (in %) of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification, measured at different temperatures (°C)
Figure 3
Polarized light microscopy images of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification reaction. The images were taken after 3 h isothermal crystallization at 25°C and the scale bars represent 50 μm
Figure 4
Crystallization profile of free phytosterols (FPh) and esterified phytosterols (EPh). Positive values for heat flow represent exothermic reactions
Figure 5
Crystallization profile of palm oil, palm oil added with 10% of esterified phytosterols, and non-esterified phytosterols before (PO, POE, POF) and after (POi, POEi, POFi) chemical interesterification reaction. Positive values for heat flow represent exothermic reactions
Figure 6
Images of palm oil (PO) and canola oil (CO) blends (PO:CO 50:50), added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), stabilized at 5°C for 24h and 25°C for 24h, then inverted and exposed to oscillating room temperature (between 25 and 30°C) for 1min, 24h, and 30 days
Figure 7
Yield value (in gF/cm2) of palm oil and canola oil (PO:CO 50:50, in w/w%) added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), measured at 15, 20, and 25°C. Same letters for the same temperature indicate that there are no significant differences between the means evaluated by Tukey's test (p<0.05)
Figure 8
Polarized light microscopy images of pure palm oil (PO), and blends of palm oil and canola oil (PO:CO 50:50, in w/w%) added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), after isothermal crystallization at 25°C, for 3h (the scale bars represent 50 μm)
Table 1
Fatty acids composition (%) of palm oil (PO), canola oil (CO) and the blend of 50:50 (w/w%) of PO:CO.
Table 2
Triacylglycerol composition (TAG, in %w/w) of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification reaction.
Table 3
Crystallization onset temperature (Tonset, °C) of the peaks obtained from the DSC crystallization profile for palm oil, palm oil added with esterified phytosterols and with non-esterified phytosterols before (PO, POE, POF) and after (POi, POEi, POFi) chemical interesterification reaction.

Figure 1   Structural representation of an esterified and non-esterified β-sitosterol.

Figure 2   Solid fat content (in %) of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification, measured at different temperatures (°C)

Figure 3   Polarized light microscopy images of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification reaction. The images were taken after 3 h isothermal crystallization at 25°C and the scale bars represent 50 μm

Figure 4   Crystallization profile of free phytosterols (FPh) and esterified phytosterols (EPh). Positive values for heat flow represent exothermic reactions

Figure 5   Crystallization profile of palm oil, palm oil added with 10% of esterified phytosterols, and non-esterified phytosterols before (PO, POE, POF) and after (POi, POEi, POFi) chemical interesterification reaction. Positive values for heat flow represent exothermic reactions

Figure 6   Images of palm oil (PO) and canola oil (CO) blends (PO:CO 50:50), added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), stabilized at 5°C for 24h and 25°C for 24h, then inverted and exposed to oscillating room temperature (between 25 and 30°C) for 1min, 24h, and 30 days

Figure 7   Yield value (in gF/cm2) of palm oil and canola oil (PO:CO 50:50, in w/w%) added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), measured at 15, 20, and 25°C. Same letters for the same temperature indicate that there are no significant differences between the means evaluated by Tukey's test (p<0.05)

Figure 8   Polarized light microscopy images of pure palm oil (PO), and blends of palm oil and canola oil (PO:CO 50:50, in w/w%) added with 0, 2, 4, 6, 8, and 10% of non-esterified phytosterols (FPh), after isothermal crystallization at 25°C, for 3h (the scale bars represent 50 μm)

Table 1   Fatty acids composition (%) of palm oil (PO), canola oil (CO) and the blend of 50:50 (w/w%) of PO:CO.

Fatty Acids (%) Palm oil (PO)Canola Oil (CO)PO:CO 50:50 (w/w%)
Lauric acidC12:00.60.00.3
Myristic acidC14:00.90.10.5
Palmitic acidC16:040.55.022.8
Palmitoleic acidC16:10.10.30.2
Margaric acidC17:00.10.10.1
Stearic acidC18:04.82.73.8
Oleic acidC18:143.361.552.4
Linoleic acidC18:28.719.013.8
Linolenic acidC18:30.28.64.4
Araquidic acidC20:00.40.80.6
Gadoleic acidC20:10.21.10.6
Behenic acidC22:00.10.50.3
Erucic acidC22:10.00.10.1
Lignoceric acidC24:00.10.20.2
Σ Saturated fatty acids47.59.428.5
Σ Unsaturated fatty acids52.590.671.5

Table 2   Triacylglycerol composition (TAG, in %w/w) of palm oil, palm oil added with 10% of esterified phytosterols and 10% of non-esterified phytosterols, before (PO, POE, POF) and after (POi, POEi, POFi) the chemical interesterification reaction.

CNTAG (%)POPOiPOEPOEiPOFPOFi
C46PPM0.4 ± 0.10.5 ± 0.00.4 ± 0.10.5 ± 0.10.4 ± 0.11.0 ± 0.2
C48PPP7.9 ± 0.38.0 ± 0.17.9 ± 0.310.3 ± 0.27.9 ± 0.37.2 ± 0.1
C50PPS/POP*32.8 ± 0.333.2 ± 0.132.8 ± 0.330.7 ± 0.232.8 ± 0.330.3 ± 0.3
PLP7.3 ± 0.36.6 ± 0.37.3 ± 0.36.2 ± 0.37.3 ± 0.36.2 ± 0.2
C52POS/POO*31.8 ± 0.430.5 ± 0.431.8 ± 0.428.6± 0.631.8 ± 0.428.8 ± 0.4
PLO7.5 ± 0.26.4 ± 0.17.5 ± 0.26.6 ± 0.17.5 ± 0.26.0 ± 0.0
PLL1.8 ± 0.32.0 ± 0.21.8 ± 0.31.4 ± 0.51.8 ± 0.31.4 ± 0.1
C54SOO2.7 ± 0.02.5 ± 0.12.7 ± 0.02.3 ± 0.12.7 ± 0.02.5 ± 0.1
OOO4.7 ± 0.14.4 ± 0.24.7 ± 0.14.2 ± 0.14.7 ± 0.14.3 ± 0.1
OLO1.1 ± 0.11.1 ± 0.01.1 ± 0.10.9 ± 0.11.1 ± 0.10.9 ± 0.1
Others 2.0 ± 0.64.9 ± 0.52.0 ± 0.68.2 ± 0.32.0 ± 0.611.4 ± 0.5

Table 3   Crystallization onset temperature (Tonset, °C) of the peaks obtained from the DSC crystallization profile for palm oil, palm oil added with esterified phytosterols and with non-esterified phytosterols before (PO, POE, POF) and after (POi, POEi, POFi) chemical interesterification reaction.

Tonset (°C)
SamplePeak 1Peak 2Peak 3
PO-19.44 ± 0.20a4.83 ± 1.12a
POi-19.75 ± 0.62a5.69 ± 0.84a
POF47.39 ± 0.51a20.78 ± 0.55a4.24 ± 0.80a
POFi-20.07 ± 0.29a4.83 ± 0.80a
POE-17.39 ± 0.45b3.23 ± 0.67a
POEi-19.76 ± 0.55a3.96 ± 1.51a
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Crystallization oil

Introduction to Crude Oil Wax Crystallization Kinetics:  Process Modeling

We developed a method for measuring and monitoring the crystallization kinetics of waxy crude oils under quiescent milieu. Using a setup based on a cross-polar microscope (CPM), coupled with a high-resolution, autoranging picoammeter, power meter connected to a light detector and a high-definition−high-resolution image processing, we have been able to capture wax particle growth from 1 micron and higher sizes. Both isothermal and non-isothermal kinetics processes were studied. Analysis of the continuous wax particles nucleation, as well as their growth process, of various oil samples from different geographical locations is presented. Results indicate some degree of similarities to trends observed in crystallization kinetics described by both Ozawa (Polymer1971, 12, 150) for non-isothermal kinetics and Avrami (J. Chem. Phys.1939, 7, 1103) for isothermal kinetics. Empirical models for both isothermal and non-isothermal crude oils crystallization cases are introduced.

Sours: https://pubs.acs.org/doi/10.1021/ie061002g
Crystallization

Crystallization of Vegetable Oils

Sample

Rape seed oil, pressed Soybean oil, Homa brand Olive oil, Dante brand, Italian first pressing, Palmoil

Conditions

Measuring cell: DSC30

Pan: Aluminum standard 40 µl, hermetically sealed

Sample preparation: No special sample preparation

DSC measurement: Cooling from 50 °C to -100 °C at 10 K/min


Interpretation

Olive oil crystallizes below -10 °C. The triglyceride fraction, with in part saturated fatty acids, crystallizes between -10 °C and -35 °C. The main fraction of olive oil, the triglyceride with 3 oleic acid units (70%), crystallizes at lower temperatures and is recognizable as a crystallization peak at α-form 

Commercial palm oil crystallizes below +15 °C. It has a high percentage of saturated fatty acids (50% C12, 18% C14) and hence a high melting point; 8% unsaturated oleic acid lowers the melting point to values below room temperature.

Soybean oil contains a significant fraction of saturated fatty acids (10% C16) and also a high percentage of monobasic and dibasic unsaturated fatty acids (nutritionally valuable). Triglyceride specimens analyzed by HPLC show fractions with predominantly saturated fatty acids, with some unsaturated fatty acids and also with highly unsaturated fatty acids. These 3 triglyceride fractions are visible in the DSC curve at different crystallization temperatures. The high fraction of linolenic acid in soybean oil (50%) leads to a high fraction of glycerides with three linolenic acid units (melting point -45 °C), which is shown as a peak at -42 °C.

Rape seed oil contains virtually only unsaturated fatty acids (60% C18 monobasic, 25% C18 dibasic, 10% C18 tribasic) and crystallizes at a very low temperature. Only 5% of the fatty acids are saturated and this is indicated by ‘small peaks’ between -20 °C and -40 °C. Even though DSC cannot clearly identify fatty acids or triglyceride fractions, rapid characterization of the oils/fats is possible.

Conclusion

DSC is a rapid means of investigating the crystallization behavior of edible oils. For comparison purposes approximately identical sample sizes should be taken, e.g. 20 ± 5 mg. The sample size may influence the supercooling: the smaller the sample size the greater the degree of supercooling.

Crystallization of Vegetable Oils | Thermal Analysis Application No. HB 1001 | Application published in METTLER TOLEDO TA Application Handbook Food 

Sours: https://www.mt.com/us/en/home/supportive_content/matchar_apps/MatChar_HB1001.html

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