Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 1 Saturation–supersaturation diagram.
ration or solubility curve. Below this, crystallization is impossible. Above this continuous
line, the system is supersaturated: for example, at point 2, and crystallization
is possible. Although crystallization is possible at point 2, it will not occur with
agitation or seeding. This contrasts with point 3, where crystallization is spontaneous.
The saturation curve results from thermodynamic factors; whereas the position of
the unstable boundary (dashed line in Fig. 1) depends on kinetic factors, particularly
the rate of cooling [2].
Review of the following equations, relative to melt systems, provides a conceptual
understanding of how solubility (i.e., saturation) depends on the size of the
solute. From Eq. (1),
2�V
ln X = (1)
rRT
where X is the solubility increase versus crystals of infinite size, � the surface tension,
V the molar volume, and r the spherical crystal radius, it is clear that larger crystals
would be less soluble. Equation (2),
2�VT
�T = (2)
r �H
where �T is ‘‘supercooling’’ and r is the smallest size of nuclei possible at a specific
crystallization temperature T � �T, indicates that the more a melt is supercooled,
the smaller the critical radius necessary for stability.
B. Nucleation
Only when sufficient supercooling has been achieved can crystallization centers or
nuclei form. A crystal nucleus is the smallest crystal that can exist in a solution with
defined temperature and concentration [1]. Crystal nuclei were further distinguished
from crystal ‘‘embryos’’ as molecular aggregates that continue to grow rather than
redissolve [3].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.