Food Lipids: Chemistry, Nutrition, and Biotechnology

where A = B = C and all are saturated or trans-unsaturated, the form is �-2;
where A = C and are saturated and B is cis-unsaturated or has 4 or more carbons
less than A and B, the form is �-3;
where, in mixed saturated/unsaturated systems, A = B or B = C (asymmetrical),
the form is ��-3;
where, in mixed saturated/unsaturated systems, A = C (symmetrical), the form
is �-3.
The packing requirements for �� are less stringent than for �; therefore, mixed
fatty acid triacylglycerols, such as those in lauric fats, tend to be ��-stable [28].
B. Phase Behavior
Timms [24,28] provided a complete review of the phase behavior of fats. A phase
is a physical state of matter that is homogeneous and separated from other phases
by a definite boundary. Therefore, if only the solid and liquid states of matter are
considered, solid–solid, liquid–solid, and liquid–liquid phases can exist. Triacylglycerol
mixtures are ideally miscible when liquid and therefore show no heat or
volume changes when mixed. As a consequence, distinct liquid–liquid phases are
not apparent. Many natural fats clearly exhibit several distinct solid phases with
partial miscibility, which leads to compound crystals and solid solutions. Identification
of these solid solutions with X-ray powder diffraction is effective, particularly
with fats in equilibrium or relatively stable metastable forms. Thermal analysis using
differential scanning calorimetry (DSC) provides for observation of the effect of
changing temperature [28].
An idealized set of DSC thermograms is depicted in Figure 10. Melting profiles
can be correlated to definitive X-ray determination, which allows subsequent estimation
of polymorphic form using DSC alone. However, polymorph analysis using
Figure 10 Schematic representation of polymorph identification by means of differential
scanning calorimetry. [Reproduced with permission from The Physical Chemistry of Lipids
(D. M. Small, ed.), Plenum Press, New York, 1986.]
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.