Polymorphism subcell structures

Figure 3. Typical subcell structures of TAG polymorphs. The a, p, and p forms have hexagonal (H), orthorhombic perpendicular (0 ), and triclinic parallel(T//), respectively.

It has been reported extensively that fats solidify in more than one crystalline type (2-23). Triglycerides exhibit three main crystal types—ot, p, and p—with increasing degrees of stability and melting point. The molecular conformations and packings in the crystal of each polymorph have been reported. In the ot form, the fatty acid chain axes of the triglyceride are randomly oriented and the ot form reveals a freedom of molecular motion with the most loosely packed hexagonal subcell structure. 

Table 1 shows the polymorphic behavior of the three TAG in which the saturated fatty acid at the the in-1 and in-2,3 positions is stearic and the sn-2 acid varied from oleic (SOS), ricinoleic (SRS) to linoleic (SLS). As a reference, a typical feature of polymorphic transformation of SOS from a to 3j forms through y, P and P2 is illustrated in Table 1 (10). As briefly mentioned in die previous section, one of the unique polymorphic properties in SOS is that the chainlength structure converted from DCL (a) to TCL (y, P, P2, and Pj), and the subcell structures of stearic and unsaturated acid leaflets in the TCL polymorphs changed in different manners. This transformation behavior is caused by the steric hindrance of steric and unsaturated acid chains, as well as by the structural stabilization of the aliphatic chains and glycerol groups altogether, as briefly summarized in the following.

TAG molecules can aystaUize typically as one of three polymorphic aystal structures, a, P and P, each with different subcell structures (i.e., hexagonaL orthorhombic-perpendicular, and triclinic-paraUel, respectively) and with increasing melting points and enthalpies of fusion (Sato, 1999). If a liquid oil is cooled quickly, it will tend to nucleate... 

One may characterize the polymorphic forms of TAGs by thermal stability, subcell packing, and chain-length structure as described below. 

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

FIGURE 15.19 Crystal structure in various polymorphs, (a) Packing modes of paraffinic chains. The straight lines indicate the cross section of the subcell, (b) Packing modes of triglyceride molecules. 

By far the most popular method of quantifying the polymorphism of the crystallites formed during network formation is the determination of subcell and layered structures by X-ray diffraction. In addition to the standard methods of X-ray diffraction, a number of other techniques, such as vibrational spectroscopy, are employed in the identification of the different polymorphic forms [12-19]. Nuclear magnetic resonance (NMR) measurements have also been used to study the molecular mobility in polymorphs [20-27]. Atomic force microscopy has recently also been used to study the crystal structure of triglycerides [28]. 

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