It is important to understand the nature of the liquid phase prior to crystallization to understand how crystals form. It is widely recognized that lipids retain some degree of ordering in the liquid phase, with temperatures well above the melting point needed to fully dissociate this ordering. When melting fats, this liquid ordering is termed a crystalline memory effect, where subsequent recooling leads to formation of a different (usually more stable) phase than would occur if the fat was heated to higher temperatures to destroy the liquid memory (1-3).
In nucleation, or the formation of the crystalline phase from the liquid, some organization of molecules is expected. In lipids, the natural ordering of the liquid phase leads to crystal formation. In fact, rapid cooling of liquid lipids results in the formation of a diffuse crystalline phase (low-energy polymorph) because of the ordering structure in the liquid phase. Such rapid cooling of other systems, most notably sugars and starches, often results in the formation of a glassy state consisting of molecules that are randomly organized together with no long-term ordering.
Upon slower cooling from the liquid, the lipid molecules have time to organize into lamellae (1) and eventually can form coherent, three-dimensional crystals (shown schematically in Figure 1). The arrangement of the molecules into the crystalline state depends on such factors as the cooling rate, the temperature at which crystallization occurs, the agitation rate, and the composition of the lipid phase