Food Lipids: Chemistry, Nutrition, and Biotechnology

production of commercially viable oil sources, wax esters and eicosapentaenoic acid
have been produced at a commercially relevant scale in bacteria [70,71].
Microbes as a broad class of single-cell (or relatively undifferentiated collections
of cells) organisms have the obligate biosynthetic capability of generating fatty
acids necessary for membrane synthesis and other processes. Historically these have
not been considered an important source of fat in the diet, although it is recognized
that these organisms add small quantities to certain foods, whose presence may be
an important contribution, e.g., for flavor. Nevertheless, even for microbes that produce
large quantities of storage triacylglycerides, the economics of growing and
obtaining the oils has been noncompetitive with the traditional sources of edible oils.
However, recently this area has seen a considerable resurgence in interest both academically
and in commercial application [72]. This change is largely due to the
improved efficiency and capabilities of large-scale microbial fermentation, to the
identification of therapeutically useful edible oils, and to the capability of microbes
to produce unusual fatty acids or unusual concentrations of fatty acids and glycerides
[73]. Fatty acids that have raised the ante, as it were, for edible oils include dihomo-
�-linoleic acid, eicosapentaenoic acid, and DHA, due to their ability to alter arachidonic
acid metabolism and hence thrombosis, inflammation, cancer, and autoimmune
diseases; DHA for inclusion in infant formulas; nervonic acid for its potential
in treating neuropathies; long chain monounsaturated fatty acids for adrenoleukodystrophy;
and stearculic acid as a possible treatment for bowel cancer [74,75].
Several factors mitigate in favor of microbial production for high-value lipids. The
greater potential for aggressive recombinant approaches to manipulate microbial lipid
metabolism to obtain novel fatty acids is likely to increase the growth of this cottage
industry for fatty acid production. Higher plants and animals are also somewhat
limited in the glyceride forms that they will produce. For example, most plants do
not place a saturated fatty acid in the sn-2 position of a triglyceride. Similarly, fish
tend to place virtually all of the long chain n-3 PUFAs in the sn-2 position. This
both limits the total range of glycerides available using these plants and animals as
sources of lipids and imposes structural effects on digestion and absorption of the
fatty acids in nutritional applications. These limitations are both less well defined
and more mutable in microbial fermentation applications. This area, though coming
under intense regulatory scrutiny, may reach a significant segment of the food industry,
at least in the short term [76,77].
Single-cell eukaryotes have, as a class, a remarkably wide variety of lipid
metabolic capabilities. For example, some yeast produce only a single desaturase,
the stearoyl or �9 enzyme, and neither produces or requires PUFA for growth. At
the other end of the spectrum, some fungi and algae can produce very high amounts
of arachidonic, dihomo-�-linolenic, eicosapentaenoic, and DHA [78]. An additional
and synthetically useful attribute of these organisms is their ability to take up fatty
acids from the medium and either to incorporate them into triacylglycerides and
phospholipids (even with unusual stereospecificity) or to further metabolize them
prior to esterification [78]. These various properties were known previously but were
not thought to warrant commercialization. This is changing. Already, microbial lipid
sources are proving to be a cost-effective feedstock for shrimp and fish aquaculture.
The ability of the microbial feedstock to also elaborate valuable pigments and antioxidants
is used to advantage, so this entire technology and its biotechnological
elaborations are likely to increase in impact in the future.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.