Food Lipids: Chemistry, Nutrition, and Biotechnology

structure.... The term ‘fat’ usually refers to triacylglycerols specifically, whereas
‘lipid’ is all-inclusive’’ [9]. Included in the more general term ‘‘lipid’’ are hydrocarbons,
steroids, soaps, detergents, all acylglycerols, phospholipids, gangliosides, and
lipopolysaccharides [9]. Since all these lipids crystallize with many degrees of complexity,
this chapter primarily addresses the crystallization of triacylglycerols, making
only limited reference to other lipids.
Lipids have been classified based on their interaction with water [10]. Triacylglycerols
belong to class I polar lipids. These are insoluble, nonswelling amphiphiles.
Triacylglycerols will spread at the aqueous interface and form a stable monolayer.
They have a low affinity for water compared to other class I polar lipids such
as diacylglycerol and cholesterol. Class II polar lipids include many of the phospholipids,
as well as glycolipids and monoacylglycerol. These, too, are insoluble; however,
they swell because water is soluble in their polar moieties. A result of this
interaction with water is the ability of class II polar lipids to undergo lyotropic
mesomorphism and develop into liquid crystals. It has been suggested that mesomorphism
of phospholipids influences the nucleation and solidification behavior of
cocoa butter [11].
B. Triacylglycerol Crystal Packing Structure
Both the technical and biological functions of lipids are better understood with
knowledge of their structural composition. X-ray analysis provides much of the structural
information known regarding all lipids. Shipley [7] presented a brief history of
the X-ray analysis of triacylglycerol single crystals. One of the first direct studies
performed [12], the examination of the triclinic, � form of trilaurin, set the groundwork
for determining the conformational nature of other, �-form triacylglycerols.
Of the seven crystal systems referred to in Sec. I.D, three predominate in the
crystalline triacylglycerols [13]. Usually, the most stable form of triacylglycerols has
a triclinic subcell with parallel hydrocarbon–chain planes (T�). A second common
subcell is orthorhombic with perpendicular chain phases (O�). The third common
subcell type is hexagonal (H) with no specific chain plane conformation [14]. Therefore
the hexagonal form exhibits the lowest stability and has the highest Gibbs free
energy, closest to the original melt. Figure 5 contains subcell representations of these
three common triacylglycerol conformations.
Interpretation of X-ray crystallography data from trilaurin and tricaprin [15–
17] resulted in representation of triacylglycerols in a tuning fork conformation when
crystalline. The fatty acid esterified at the sn-1 and sn-2 positions of glycerol are
extended and almost straight. The sn-3 ester projects 90� from sn-1 and sn-2, folds
over at the carboxyl carbon, and aligns parallel to the sn-1 acyl ester. Molecules are
packed in pairs, in a single layer arrangement, with the methyl groups and glycerol
backbones in separate regions (Fig. 6).
The schematic view presented in Figure 6 represents simple, monoacid triacylglycerols.
The polymorphic structures described for these simple triacylglycerols
are valid for natural fats that contain complex triacylglycerols, given their common
X-ray short spacings (axes normal to the chain direction) [13]. The structure depicted
in Figure 6 illustrates a bilayer arrangement of the fatty acyl chains, which is the
common packing structure for natural fats. However, this bilayer structure does not
exist in all triacylglycerols [18–20]. Indeed, a trilayer structure has been demon-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.