Crystal structure, fats melting points, polymorphs

In many instances, TGs exist in polymorphic forms. Crystal structure is very important to the properties of margarines, shortenings, and specialty fats. The very unstable a form is readily transformed to the more stable ft form, which in some TGs has a higher melting point (more stable) than the /3 form. In single-acid TGs, the order is a — f3 — a, but some mixed TGs show a lower-melting (less stable) (3 form. This is further complicated by the existence of multiple / and /3 forms, depending upon the detailed TG structures at hand. 

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. 

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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