Oleic acid polymorphic forms

Di- and tri-acid triglycerides with one or two unsaturated chains have more complex phase behaviour. Cocoa butter which is dominated by triglycerides with oleic acid in the 2-position and palmitic and stearic acid in the 1- and 3-positions, exhibits six crystal forms. Transitions into the most stable form causes so-called blooming of chocolate. An interesting study of the polymorphism and the... 

Alkenoic acids also have polymorphic crystalline forms. For example, both oleic and elaidic acids are dimorphic with melting points of 13.6 and 16.3°C for oleic, and 43.7 and 44.8°C for elaidic acid (14). 

The complexity of chocolate manufacture arises from the polymorphic nature of its constituent fats, which can come in at least five crystal forms, each with an individual melting point. Cocoa butter is chemically a multicomponent mixture of triglycerides and trace compounds (Davis and Dimick 1986). Approximately 85% of the composition consists of just three triglycerides POP ( 20%), POS ( 40%) and SOS ( 25%), where palmitic (P), oleic (O) and stearic (S) are the fatty acids attached to the glycerol base. The precise composition depends on factors such as growing conditions and therefore can vary between batches, especially from different geographic regions (Chaiseri and Dimick 1989). 

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.

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