Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 7 Schematic comparison of triacylglycerol bilayer and trilayler structures: (a) trilaurin
(LLL), (b) 2-caproyldipalmitin (PCP), (c) 2-oleyldipalmitin (POP), and (d) 2-palmitoyldiolein
(OPO). [Reproduced with permission from The Physical Chemistry of Lipids (D. M.
Small, ed.), Plenum Press, New York, 1986.]
A. Nomenclature
The complicated melting behavior and polymorphism of fats resulted in confusion
concerning terminology. Chapman [25] provided a good review of this controversy.
It is now accepted that fats and triacylglycerols primarily occur in any of three basic
polymorphic forms. The reference in Section II.B to the triclinic parallel (T�), orthorhombic
perpendicular (O�), and hexagonal (H) subcells addressed polymorphic stability.
The most stable and highest melting, T� is the � polymorph. Another polymorph,
with variable stability and a melting point lower than �, is��. Phases with
the hexagonal subcell have the lowest melting point and represent the � polymorph.
X-ray diffraction provides not only the characteristic short spacing measurements,
which define the lateral chain packing and subsequent polymorph assignment,
but also measurements of the lamella layer thickness (the ‘‘d-spacing’’). The dspacing
depends on the length of the molecule and the angle of tilt between the
chain axis and the basal lamellar plane. Although three basic polymorphs exist, subtle
variations in d-spacing lead to more than three polymorphic designations. Several of
these were illustrated [26] and are shown in Figure 8. Figure 9 illustrates the orientation
of the three primary polymorphs. The rarely isolated sub-� form is very
unstable and can be formed only at very low temperatures. The existence of more
than one �� form results from chain tilt relative to the basal plane. This can be
imagined by observation of the right-most projection of Figure 9 and the �� polymorph.
A second �� polymorph would have a different interlamellar angle of tilt.
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