Food Lipids: Chemistry, Nutrition, and Biotechnology

olive (Olea europea), and avocado (Persea americana)] and seed lipids account for
over 80% of world production. In the United States, plant oils account for over 94%
of the annual demand of edible oils (8). Furthermore, the plant sources included in
Table 1 are useful because of their suitability for organized agriculture (5) and acceptible
yield of biomass (and, therefore, lipid). Less than 0.1% of about 300,000
known plant species are cultivated in organized agriculture, and fewer than 20 species
account for the principal lipid products of commerce (5). However, the great diversity
among the plant kingdom offers potential for exploitation in view of the current
ability to genetically transfer and manipulate specific plant traits. For example, a
species that produces an unusual, precious, or otherwise noteworthy fatty acid or
assembled lipid but is not suitable for commercial cultivation can serve as germ
plasm for incorporating these valuable traits into species that are subject to commercial
culture. Thus, in view of the relative contribution to commerce and exploitable
diversity of plant lipids, it seems appropriate to restrict this chapter to a discussion
of storage lipid biosynthesis in plants. Reviews on fatty acid and glycerolipid
biosynthesis in animal and microbial systems are available elsewhere (9–12).
II. LIPID DIVERSITY AMONG PLANTS
As diverse as lipids are in plants (and other living organisms), they can be easily
classified into two categories based on their physiological role. Functional lipids are
those involved in vital and metabolic processes. Typically, these are isoprenyl-derived
lipids (e.g., sterols, carotenoids), and polar glycerolipids (e.g., galactosyl- and phospholipids),
components that comprise, and contribute to the functioning of,
(sub)cellular membranes (10,13). In view of the role of functional lipids in membranes,
their fatty acid composition in plants is essentially restricted to palmitic
(16:0),* palmitoleic (16:1�9), trans-�3-hexadecenoic (16:1�3t), hexadecatrienoic
(16:3�7,10,13), stearic (18:0), oleic (18:1�9), linoleic (18:2�9,12), and linolenic
(18:3�9,12,15) acids (10,13,14). Functional lipids are not discussed in this chapter, except
as they relate to the biosynthesis of fatty acids and the assembly of triacylglycerols
(Sec. V). (Some functional lipids, such as soybean lecithin, are products of
commerce, but many products of these types are simply by-products of the edible
oil industry and would probably not alone justify the use of the host plant tissue in
commerce.) The interested reader is encouraged to consult reviews on biosynthesis
of polar lipids (15,16) and isoprenyl lipids, including sterols, carotenoids, tocopherol,
and lower isoprenoids (17–19).
Storage lipids are deposited as oil bodies in fruit or seeds and are intended to
serve as a carbon (energy) source for growth during germination of seeds. In fruit
tissues, the raison d’être of the copious quantities of oil may be to attract predators
and facilitate seed dispersal. Invariably, storage lipids predominantly take the form
of triacylglycerols (20), and their fatty acyl groups are first mobilized for catabolic
processes by endogenous lipases (21) and, ultimately converted into metabolic carbon
(acetate) via � oxidation in peroxisomes (22). There is a much greater diversity
in fatty acid composition (range of 2–24 acyl carbons) of storage triacylglycerols
*Fatty acid (or acyl group) identification is formalized throughout the chapter as X:Y, where X is the
acyl carbon number and Y is the number of double bonds, all of which are cis unless otherwise noted.
Subscript � i identifies position of the double bond, counting from the carboxy terminus.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.