Triacylglycerol polymorphism

Yano, J., F. Kaneko, M. Kobayashi, D.R. Kodali, D.M. Small, and K. Sato, Structural Analyses of Triacylglycerol Polymorphs with FT-IR Techniques II. P l-form of 1,2-Dipalmitoyl-3-myristoyl-sn-glycerol, y. Phys. Chem. B. 101 8120-8128 (1997). 

SPIN-LATTICE RELAXATION TIME MEASUREMENTS AS A PROBE FOR TRIACYLGLYCEROL POLYMORPHISM AND CRYSTAL SIZE... 

Ueno, S., Yano, J., Seto, H., Amemiya, Y., Sato, K. 2000. Synchrotron radiation X-ray diffraction study of polymorphic crystallization in triacylglycerols. In, Physical Properties of Fats, Oils and Emulsifiers (N. Widlak, ed.), pp. 64-78, AOCS Press, Champaign. 

In 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 crystaHine 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. 

Figure 10. Onset temperature of nucleation and polymorphic form of monoacid triacylglycerols with different chain lengths (nj at slow cooling rate (0.4°C/min). o.mp, P p and p p represent the melting temperatures of the different polymorphs (2).

TABLE 6. Occurrence of Polymorphic Forms in Representative Triacylglycerols. 

Figure 6. Polymorphic forms of fat crystals, including the possible polymorphic transitions, subcell packing structures, stability characteristic, and triacylglycerol stacking conformations.

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. 

Sato, K., M. Gato, J. Yano, K. Honda, D.R. Kodali, and D.M. Small, Atomic Resolution Structure Analysis of P Polymorph Crystal of a Triacylglycerol 1,2-Dipalmitoyl-3-myris-toyl-jn-glycerol, J. Lipid Res. 42 338-345 (2001). 

Fat is mainly composed of triacylglycerol (TAG) molecules. These TAG are polymorphic, i.e., they can crystallize under several crystalline forms (2). To get a stable food at the end requires the fat to be crystallized in the stable polymorphic form to avoid possible further transformation. Cocoa butter does not form in the stable Form V by simple cooling. It needs a well-controlled thermal path, performed under shearing conditions, called tempering. This specific temperature program is necessary to first nucleate enough crystals and then only keep the Form V crystals and melt the other metastable ones. Shear is also required during this process to get sufficiently rapid and intense nucleation of the stable forms. 

A great deal of information about polymorphic changes has been obtained from studying pure triacylglycerols (TAG). However, in commercial fats, especially if they contain several ingredients, it may be more difficult to predict the likelihood of p crystal formation to occur. Factors that influence the stability of the p poly-... 

Polymorphic Tendency of the Major Solid Triacylglycerols (TAG) in Vegetable Margarines and Shortenings of 16 and 18 Carbon Chain-Length Fatty Acids ... 

---