Food Lipids: Chemistry, Nutrition, and Biotechnology

Table 1 The Seven Crystal Systems
System Angles and axial lengths
Triclinic All axes unequal and none at right angles
a ≠ b ≠ c and � ≠ � ≠ � and ≠90�
Orthorhombic All axes unequal and all at right angles
a ≠ b ≠ c and � = � = � and =90�
Hexagonal Two axes = 120� and the third at 90� relative to them
a = b ≠ c and � = � =90� and � = 120�
Cubic All axes equal and all at right angles
a = b = c and � = � = � and =90�
Tetragonal Two of three axes equal and all at right angles
a = b ≠ c and � = � = � and =90�
Rhombohedral All axes equal and none at right angles
a = b = c and � = � = � and ≠90�
Monoclinic Three unequal axes having one pair not =90�
a ≠ b ≠ c and � = � =90� ≠ �
Source: Ref. 6.
within a crystal, whereas morphology describes the set of faces determined by means
of the symmetry elements of the crystal [1]. This subtle distinction allows crystals
of the same morphology to nevertheless occupy different habits.
Polymorphism is defined in terms of an ability to reveal different unit cell
structures resulting from varied molecular packing. Polytypism refers to altered
stacking direction of the crystal lamellae [8]. Each lamella is configured as identical
polymorphs, but the direction of tilt of the hydrocarbon axis from the methyl endgroup
plane varies (Fig. 4).
II. FAT CRYSTALLIZATION
A. Lipid Classification
All fats are lipids, but the converse is not true. Therefore, a distinction must be
drawn to provide an accurate perspective when considering fat crystallization as
discussed in this chapter. Fats are defined as follows: ‘‘A glyceryl ester of higher
fatty acids.... Such esters are solids at room temperature and exhibit crystalline
Figure 4 Polymorphism versus polytypism. (Reproduced with permission from Ref. 8.)
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.