Food Lipids: Chemistry, Nutrition, and Biotechnology

droxy and epoxy fatty acids. In underlying layers, associated with the suberin matrix,
another cutin-like lipid polymer containing aliphatic and aromatic components is
found [12]. In some instances, internal nonsuberin waxes, which are stored in plant
seeds, are the major energy reserves rather than triacylglycerols. In insects, intracuticular
waxes are the major constituents of the inner epicuticular layer [13–15].
A variety of aliphatic lipid classes occur in epicuticular waxes. These include
hydrocarbons, alcohols, esters, ketones, aldehydes, and free fatty acids of numerous
types [16,17]. Frequently, a series of 10 carbon atom homologs occurs, while chain
lengths of 10–35 carbon atoms are most often found. However, fatty acids and
hydrocarbons with fewer than 20 carbon atoms are known, as are esters with more
than 60 carbon atoms. Other minor lipids such as terpenoids, flavonoids, and sterols
also occur in epicuticular waxes. The composition and quantity of epicuticular wax
varies widely from one species to another and from one organ, tissue, or cell type
to another [16]. In insects, wax composition depends on stage of life cycle, age, sex,
and external environment [17].
In waxes, the biosynthesis of long chain carbon skeletons is accomplished by
a basic condensation–elongation mechanism. Elongases are enzyme complexes that
repetitively condense short activated carbon chains to an activated primer and prepare
the growing chain for the next addition. The coordinated action of two such soluble
complexes is plastid results in the synthesis of the 16 and 18 carbon acyl chains
characterizing plant membranes [18–20]. Each condensation introduces a �-keto
group into the elongating chain. This keto group is normally removed by a series of
three reactions: a �-keto reduction, a �-hydroxy dehydration, and an enol reduction.
Variations of the foregoing basic biosynthetic mechanism occur, giving rise to
compounds classified as polyketides. Their modified acyl chains can be recognized
by the presence of keto groups, hydroxy groups, or double bonds that were not
removed before the next condensation took place. It is well established that the very
long carbon skeletons of the wax lipids are synthesized by a condensation–elongation
mechanism. The primary elongated products in the form of free fatty acids are
often minor components of epicuticular waxes. Most of them, however, serve as
substrates for the associated enzyme systems discussed. The total length attained
during elongation is reflected by the chain lengths of the members of the various
wax classes [15–21]. Normal, branched, and unsaturated hydrocarbons and fatty
acids are prominent components of plant waxes, while insect waxes usually lack
long chain free fatty acids [22–26].
II. CHEMISTRY OF STEROLS
A. Introduction
Sterols constitute a large group of compounds with a broad range of biological
activities and physical properties. The natural occurring sterols usually possess the
1,2-cyclopentano-phenanthrene skeleton with a stereochemistry similar to the transsyn-trans-anti-trans-anti
configuration at their ring junctions, and have 27–30 carbon
atoms with an hydroxy group at C-3 and a side chain of at least seven carbons at
C-17 (Fig. 1). Sterols can exhibit both nuclear variations (differences within the ring
system) and side chain variations. The examples of the three subclasses of sterols in
Figure 1 represent the major variations of sterols. Sterols have been defined as hy-
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