Triacylglycerols polymorphic forms

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.

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). 

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 systematic infrared spectroscopic study of the mono acid triacylglycerols of the homologous fatty acids C2-C22 in solid state at —180 °C (and as liquids) for the different polymorphic forms was performed (De Ruig, 1971). The wagging, rocking-twisting and C-C stretching chain absorptions were co-ordinated with the well-established distributions of vibrational modes in hydrocarbon spectra. 

Figure 3.6 Arrangement of polymorphic forms of triacylglycerols In crystals.

Modified triacylglycerols can occur in different polymorphic forms, and these influence the melting properties of the fat, the softening point, the solids content, etc. Analysis of such properties is commonly performed using differential scanning calorimetry (DSC). DSC has been used to determine the physical properties of sTAG formed from interesterification products of triolein and tristearin (Seriburi and Akoh, 1998) and from acidolysis reactions between stearic acid and coconut oil (Rao et al., 2001). 

On cooling, fats may assume various polymorphic forms. On slow cooling, or reheating of the vitreous form, they change to crystalline forms. The form showing the least amount of crystalline order for a triacylglycerol, as determined by X-ray diffraction and infrared spectroscopy, is called the alpha (a) form. The most compact crystalline form, with the lowest free energy and the... 

This experiment thus suggested that trilaurin and tripalmitin tend to form co-crystals, which behave similarly to the triacylglycerol in excess, and that polymorphic modification of trilaurin is slowed down by the presence of tripalmitin. 

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