Polymorphism of Natural Fats

It is difficult to simply define the polymorphism of natural fats composed of multiple TAGs because of the following two reasons ... 

The polymorphic nature of the multicomponent TAG systems is related to phase behavior that is affected by molecular interactions among the component TAGs. The fat crystals in a miscible phase may exhibit simple polymorphic properties. By contrast, the immiscile eutectic phase may show complicated polymorphic properties as a superposition of the polymorphic forms of the component TAGs. Furthermore, if the molecular compound is formed by specific TAG components, the polymorphic behavior becomes complicated, as shown for the case of POP-OPO (see Section 5.2). Therefore, knowing the phase behavior of the principal TAG components is a prerequisite for precise understanding of the polymorphism of natural fats. 

Crystallization from the emulsified state may lead to different nucleation processes than observed for the same fat in bulk liquid form. It has been suggested that nucleation often occurs at the interface of the droplet where surface-active agents are located. The general similarity of the lipophilic components of surfactants oriented at the surface may provide some ordering and structure for the lipid molecules within the droplet and enhance nucleation, as found for example by Kaneko et al. (40) for a hydrocarbon emulsion. Walstra (11) also suggests that formation of compound crystals from emulsions of natural fats may be different than the same fat crystallized from bulk liquid. The initial polymorph formed may also be different, with more stable polymorphs more likely to form in the emulsion (38). 

In this section, the polymorphic properties of natural fats are briefly discussed by highlighting miMat, cocoa butter, and palm oil fraction based on recent research into the effects of external factors on the polymorphic crystallization such as shear stress, ultrasound stimulation, and addition of food emulsifiers. 

This chapter described polymorphic properties of principal TAGs and natural fats based on recent research work to clarify fundamental aspects of polymorphsim of fats and oils. The authors hope that the basic understanding of the polymorphism of the principal TAGs would be useful to elucidate rather complicated polymorphic properties of natural fats and oils that contain TAGs with very heterogeneous fatty acid compositions. 

The relevance of the molecular structural diversity of the TAG to practical application may be understood by taking some examples of natural fats cocoa butter with major TAG of POP (l,3-dipalmitoyl-2-oleoyl-jn-glycerol), POS (1,3-palmitoyl-stearoyl,2-oleoyl-rac-glycerol) and SOS (l,3-distearoyl-2-oleoyl-jn-glycerol), milk fats whose major TAG are saturated-unsaturated mixed-acid TAG, and mixed-acid TAG with saturated fatty acids having different chainlengths (2). In these natural fats, few mono-acid TAG are present as major TAG components, and the major fats are composed of the mixed-acid TAG. Therefore, it is required to elucidate for the polymorphic structures of the mixed-acid TAG. 

The distribution of fatty acids in the TAG of natural fats also determines dieir polymorphic behavior and, therefore, the texture of these fats. This is exemplified by the difference between lard and tallow (Table 17). Lard has none of the symmetric SUS glycerides, whereas in tallow there is 21%. The result (Table 18) indicates that lard crystallizes in the P form and tallow in die P form. The levels of TAG 54 and 16 0 in the high-melting glycerides of these fats are roughly similar. 

Emulsifiers are key players in polymorphic transformation studies. They alter the fat surface properties, resulting in changes in crystal size and nature. Early reviews by van den Tempel [52] and Garti [53] showed that many types of emulsifiers tend to reduce the crystal growth rate of natural fat blends. Since then, further work has been performed on the effects of different emulsifiers on fats not only in bulk but also in emulsion systems. Garti and Yano [54] discuss in great detail the progress made in this field in recent years. 

D Souza, V, JM deMan, L deMan. (1990). Short spacings and polymorphic forms of natural fats and commercial solid fats A review. J Am Oil Chem Soc 67 835-843. 

If the fat is cooled to some point below the melting point of the highest melting component and allowed to fully equilibrate (crystalhze to the maximum extent in the most stable polymorph), there will be some ratio of sohd to liquid fat dependent on the nature of the TAG mixture in the natural fat. This solid fat content (SFC) is often measured by a pulsed nuclear magnetic resonance (NMR) technique. A plot of the maximum amount of fat crystallized (SFC) at sequentially higher temperatures... 

The phase behavior of the mixed TAG system is influenced by polymorphism. For example, a miscible phase is formed in a and p polymorphs, but it transforms into a eutectic phase in p, as revealed in the SSS-PPP mixture. Then, the polymorphic occurrence is largely affected by cooling rate and temperature fluctuation, and it is therefore necessary to observe the polymorphic properties of the natural fats by varying the rate of coohng or by fluctuating the temperature (so-called tempering). 

Sato, K., Polymorphism of Pure Triacylglycerols and Natural Fats, in Advances in Applied Lipid Research, Vol. 2, edited by F.B. Padley, JAI Press, London, 1996, pp. 213-268. 

The separation and purification of naturally occurring fatty acids, based on distillation, salt solubility and low temperature crystallisation, are described by K.S.Markley (Ed.), Fatty Acids, 2nd Edn, part 3, Chap. 20, Interscience, New York, 1964, see also N.Reavley Essential Fatty Acids Book Media Publ, 2002, ISBN 9780958157643 G.Grati and K.Sato (Eds) Crystallisation and Polymorphism of Fats and Fatty Acids Marcel Dekker, 1988, ISBN 9780824778750. 

In products containing lipids, control of the crystal polymorphic form is also necessary. Lipids form different crystalline structures, or polymorphs, depending on the nature of the fat and the processing conditions. Transitions from less stable to more stable polymorphs are also dependent on composition and processing conditions. For example, tempering (or precrystallization) of chocolate is a process through which the chocolate is sequentially cooled and warmed to promote crystallization of cocoa butter into the desired polymorphic form. Controlling crystallization to produce the proper size distribution of this polymorph provides ... 

D Souza, V., de Man, J.M., and de Man, L. Short spacings and polymorphic forms of natural and commercial solid fats a review. Journal of the American Oil Chemists Society, 67, 835-843. 1990. Hagemann, J.W. Thermal behavior of acylglycerides (eds. N. Garti and K. Sato). Crystallization and Polymorphism of Fats and Fatty Acids. Marcel Dekker, New York, pp. 9-95. 1988. 

Solid substances with the same chemical composition and different crystal structure are called polymorphic forms or modifications. Each polymorphic form has characteristic properties (such as specific volume and melting point). The formation of a given polymorphic form depends on many factors (such as temperature, cooling rate of the liquid phase and type of solvent). In the solid crystal state, one polymorphic form is transformed into the other polymorphic forms without previous melting of the crystals. If one of the forms and another form is stable, these two polymorphic forms are called monotropic. The transformation occurs only in one direction, the stable form arises from the metastabile form. All natural fats are monotropic. 

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