Food Lipids: Chemistry, Nutrition, and Biotechnology

9
Crystallization and Polymorphism
of Fats
PATRICK J. LAWLER
McCormick and Company Inc., Hunt Valley, Maryland
PAUL S. DIMICK
The Pennsylvania State University, University Park, Pennsylvania
I. CRYSTALLIZATION: GENERAL PRINCIPLES
Crystallization can be considered a subset of overall solidification. Solids are crystalline,
semicrystalline, or amorphous. The crystallization process from solution first
requires supersaturation; supercooling is a prerequisite for crystallizing from a melt.
These phenomena lead to nucleation and crystal growth. This chapter emphasizes fat
crystallization from the melt.
Once formed, crystals can have different shapes, called ‘‘habits’’ or morphologies.
Stable crystals modify their habit, whereas metastable ones undergo phase
transitions [1]. Both these processes result in polymorphic behavior, a behavior common
to fats and other lipids. Further, most crystals ‘‘ripen’’ and disappear, as a result
of changes in the degree of supersaturation [1]. Supersaturation ‘‘evolves’’ as crystal
growth proceeds, and the liquid phase becomes less supersaturated. This reduced
supersaturation results in a stability requirement for larger crystals, since crystals
below a critical size will return to solution or the melt.
A. Supercooling
Crystallization requires a solute concentration greater than the concentration of the
saturated solution. Observation of the schematic saturation–supersaturation curve in
Figure 1 aids in understanding this phenomenon. The solid line represents a satu-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.