Crystallization of Fats and Oils

Crystallization of Fats and Oils1. Serpil Metin1,2. Richard W. Hartel2AbstractControl of crystallization of lipids is important in many food products, including margarine, chocolate, butter, and shortening. In these products, the aim is to produce the appropriate number and size distribution of crystals in the correct polymorphic form because the crystalline phase plays a large role in such food properties as appearance, texture, spreadability, and flavor release. Thus, understanding the processes that control crystallization is critical to controlling quality in these products. Controlling crystallization requires an understanding of the driving force that leads to crystallization, the process of forming the crystalline phase (nucleation), and then subsequent crystal growth and polymorphic transformation to obtain the final crystalline phase volume in equilibrium with the remaining liquid fat. Because of the complex composition of most natural fats, our understanding of these processes remains somewhat uncertain.Keywords:fats; lpids; crystallization; nucleation; polymorphism; crystal growth

1.1Control of Lipid CrystallizationIn many food products and even some processing operations, it is important to be able to control lipid crystallization to obtain the desired number, size distribution, polymorph, and dispersion of the crystalline phase. In most foods, it is crystallization of triacylglycerols (TAG) that is most important, although, at times, crystallization of other lipids (i.e., monoacylglycerols, diacylglycerols, phospholipids, etc.) may also be important to product quality.Proper control of the crystalline microstructure leads to products with the desired textural properties and physical characteristics. For example, tempering of chocolate prior to molding or enrobing is designed to control crystallization of the cocoa butter into a large number of very small crystals that are all in the desired polymorphic form. When controlled properly, the cocoa butter crystals in chocolate contribute to the desired appearance (shine or gloss), snap, flavor release, melt-down rate upon consumption, and stability during shelf life (fat bloom). Similar arguments can be made for other products such as butter, margarine, whipped cream, ice cream, shortening, peanut butter, and a host of others.During processing of fats, crystallization is often used to modify the properties of the fat. For example, winterization of vegetable oils is needed to ensure that the oil remains a clear liquid even when stored at low temperatures for extended time periods. The process of fractionation of fats to produce components of natural fats with different melting properties also requires control of crystallization to optimize the separation process. Many fats, including palm oil, palm-kernel oil, milk fat, and tallow, are fractionated by crystallization to produce different functional fats.1.2Crystallization of Natural FatsThere are several aspects of lipid crystallization that make it unique from crystallization of other components in foods (like water, sugars, salts, etc.). These are related to the complex molecular composition of natural fats and the orientation of the triacylglycerol molecules.Fats are made up primarily of 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.Natural fats also contain a wide range of TAG species with fatty acids of different chain length and degree of unsaturation. Milkfat, for example, contains hundreds of different TAG species with no single species present at greater than about 5%. 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 interesting complexities in crystallization.The nature of the TAG molecule is such that it can often take multiple forms in a crystal lattice. That is, the same molecule can crystallize into different crystalline forms dependent on processing conditions. The phenomenon is called polymorphism. Although there are numerous molecules that exhibit polymorphism in nature (many in the pharmaceutical field), polymorphism is somewhat unique to lipids in the food industry (although some sugar alcohols also form polymorphs).In this chapter, the complex nature of lipid crystallization, primarily related to TAG, will be discussed.

2Lipid Phase Behavior2.1Nature of the Liquid PhaseIt is important to understand the nature of the liquid phase prior to crystallization to understand how crystals form. It is widely recognized that lipids retain some degree of ordering in the liquid phase, with temperatures well above the melting point needed to fully dissociate this ordering. When melting fats, this liquid ordering is termed a crystalline memory effect, where subsequent recooling leads to formation of a different (usually more stable) phase than would occur if the fat was heated to higher temperatures to destroy the liquid memory (1-3).In nucleation, or the formation of the crystalline phase from the liquid, some organization of molecules is expected. In lipids, the natural ordering of the liquid phase leads to crystal formation. In fact, rapid cooling of liquid lipids results in the formation of a diffuse crystalline phase (low-energy polymorph) because of the ordering structure in the liquid phase. Such rapid cooling of other systems, most notably sugars and starches, often results in the formation of a glassy state consisting of molecules that are randomly organized together with no long-term ordering.Upon slower cooling from the liquid, the lipid molecules have time to organize into lamellae (1) and eventually can form coherent, three-dimensional crystals (shown schematically in Figure1). The arrangement of the molecules into the crystalline state depends on such factors as the cooling rate, the temperature at which crystallization occurs, the agitation rate, and the composition of the lipid phase.

Figure1.Proposed mechanism (highly schematic) for nucleation of triacylglycerols (TAGs). Straight chains indicate crystallized TAGs, whereas bent chains indicate fluid TAGs (4).2.2PolymorphismPolymorphism is the ability of a molecule to take more than one crystalline form depending on its arrangement within the crystal lattice. In lipids, differences in hydrocarbon chain packing and variations in the angle of tilt of the hydrocarbon chain packing differentiate polymorphic forms. The crystallization behavior of TAG, including crystallization rate, crystal size, morphology, and total crystallinity, are affected by polymorphism. The molecular structure of the TAG and several external factors like temperature, pressure, rate of crystallization, impurities, and shear rate influence polymorphism (5).TAGs are oriented in a chair or tuning fork configuration in the crystalline lattice. The TAG can take either a double or triple chain-length structure as seen in Figure2. The fatty acids of TAG pairs overlap in a double chain-length structure whereas in triple chain packing, the fatty acids do not overlap. The height of these chair structures and the distance between the molecules in the chair structures are found by using the X-ray spectra as the long and short spacings, respectively.

Figure2.Packing arrangements of triacylglycerol molecules in the crystal lattice (4).The polymorphic forms of fats are often simply classified into three categories, , , and , in increasing order of stability. The form is the least stable polymorph with the lowest melting point and latent heat of fusion. The form is the most stable, with the highest melting point and latent heat. Each polymorphic form has distinct short spacings (the distances between parallel acyl groups on the TAG) that are used to distinguish the polymorphic forms based on their X-ray diffraction patterns, as summarized in Table1. Based on the unique configuration of the molecules within the crystal lattice, each polymorph has a different crystallographic unit cell, also shown in Table1.Polymorphic FormUnit CellLines and Short Spacings (A)

HexagonalA single strong and very broad @ 4.15

OrthorhombicTwo strong lines @ 4.2 and 3.8

TriclinicA strong line @ 4.6

Table1.Identification of Polymorphic Forms of Fats Based on X-ray Analysis of Short Spacings6

In general, TAGs with three saturated fatty acids crystallize in double chain-length packing, whereas triple chain-length packing is obtained if the TAG contains fatty acids with different structures (chain length and unsaturation). Lutton (7) stated that if the fatty acids of a TAG differ in length by more than four carbons, it forms a triple chain-length structure. Triple chain-length packing is also observed in TAG containing acis-unsaturated fatty acid because this causes a kink in the structure, as seen in Figure2.Cis-unsaturated fatty acids do not mix in one layer with saturated fatty acids, and triple chain-length crystals are formed (8). It should be noted thattrans-unsaturated fatty acids incorporate into a crystal structure in the same way as the saturated fatty acids (8). The chain-length structure influences the mixing-phase behavior of different types of TAGs in solid phases (5). The triple chain-length structure has greater long spacings than does the double chain-length structure.Lipids exhibit monotropic polymorphism, where unstable forms are the first to crystallize in a subcooled fat because of their lower energy state, according to the Gibbs free energy (5). Subsequent transformation of unstable polymorphs into more stable forms occurs over time until, eventually, the most stable polymorph for a given lipid is reached. Transformation of unstable to stable polymorphs can be achieved by a slight increase in temperature above the melting point of the less-stable forms. This increase in temperature first causes the melting of the unstable forms and then solidification in a more stable form. Transformation to a more stable form can also take place without melting as seen in Figure3. The difference in Gibbs, free energy between polymorphs is the driving force for this transformation, as the molecules become more tightly arranged in the crystal lattice. It is assumed that the chair structure is maintained during polymorphic transformations (9). The layer arrangement of the polymorph does not change when it is transformed to the polymorph, although its lateral chain packing and angle of tilt changes during polymorphic transformation.

Figure3.Monotropic polymorphism of lipids where,, andare the melting temperatures of the , , and polymorphs, respectively.The hydrocarbon chain packing of the polymorph is denser than that of the polymorph. The denser chain packing in the polymorph gives increased stability compared with the polymorph. In addition, stable polymorphs have higher melting point and higher heat of fusion than the less-stable forms. The different polymorphic forms typically crystallize at rates in order of their stability (