When the temperature of a fat melt is decreased below its maximum melting temperature, it becomes supersaturated in the higher-melting triacylglycerol (TAG) species present in the mixture. This so-called undercooling or supercooling (below the melting temperature) represents the thermodynamic driving force for the change in state from liquid to solid. Fats usually have to be undercooled by at least 5-10C before they begin to crystallize. For a few degrees below the melting point, the melt exists in a so-called metastable region. In this region, molecules begin to aggregate into tiny clusters called embryos.
At these low degrees of undercooling, embryos continuously form and breakdown, but do not persist to form stable nuclei. The energy of interaction between triacylglycerol molecules has to be greater than the kinetic energy of the molecules in the melt so as to overcome Brownian effects. For these flexible molecules, it is not sufficient to simply interact; molecules have to adopt a specific conformation in order to form a stable nucleus. The adoption of this more stable conformation is relatively slow, thus explaining the existence of a metastable region. As the undercooling is increased (i.e., at lower temperatures), stable nuclei of a specific critical size are formed. The Gibbs free energy change associated with the formation of a crystal embryo (Gn) includes contributions from both surface (positive) and volume changes (negative) and is defined by