of crystals, and secondary nucleation, when some crystals are
already present to induce crystallization. Primary nucleation can
occur through a heterogeneous mechanism in the presence of
impurities or interfaces that elicit the process, or through a homogeneous
pathway, based on the spontaneous formation of
crystalline germs in bulk. Nucleation occurs when the temperature
of a molten fat decreases under the temperature of fusion of at
least one of its components. At a high cooling rate, the liquid phase
may transiently remain in a metastable undercooled state, and the
involved components become supersaturated (Marangoni &
Wesdorp, 2013). This phenomenon reflects the existence of an
energy barrier due to surface tension between the crystals and
the liquid phase. Molecules start to aggregate in clusters that need
to reach a critical size to further grow. After formation of large
enough nuclei, the crystalline growth proceeds by accretion of
molecules from the melt at the interfacial level. The growth can
happen in several dimensions and depends on the structure of
the crystal facets. It can be kinetically limited by the diffusion of
molecules in the liquid phase and/or by their adsorption at the
solid/liquid interface.
Although TAGs are the major compounds of fats, minor lipids
such as free fatty acids (FFAs), monoacylglycerols (MAGs),
diacylglycerols (DAGs) and phospholipids can influence the nucleation
stage, the crystal growth and/or the polymorphic behavior