Food Lipids: Chemistry, Nutrition, and Biotechnology

B. Agricultural Products
The extended metabolism of 18-carbon PUFAs to longer chain, more unsaturated
fatty acids [frequently referred to as highly unsaturated fatty acids (HUFAs)] in
animals means that even though animals and plants may contribute similar families
of PUFAs to the diet, the precise form of these fatty acids will differ. As a result,
an important consequence of consuming animal in contrast to vegetarian foods is
that in the latter, linoleic and linolenic acids are the fatty acids ingested from the n-
6 and n-3 families, whereas in animal foods, their metabolic products, preformed
arachidonic, eicosapentaenoic, and DHA, are also ingested. It is now clear that these
are significant nutritional, biochemical, and physiological differences.
Furthermore, whereas animal sources of fat are often grouped as similar, avian,
aquatic, and ruminant or nonruminant mammalian storage lipids are very different
in the quantity of depot triacylglycerols, their distribution and their fatty acids, as
well as their composition and arrangement on the glycerol. The final content of fatty
acids in storage triacylglycerols is the result of diet, metabolism, and de novo synthesis.
In this respect, each of the major animal fat sources differs in important ways,
which tends to distinguish each as a fat-rich commodity. These differences have
important effects on the texture, flavor, and caloric density of the muscles as consumed
directly [79–81] and also on processed foods prepared from them [82].
Although the differences among species in the quantity and distribution of fat
are associated with the particular commodities, they are not necessarily all innate to
them. These differences also reflect the historical development of the particular muscle
food as a commodity. Even among ruminants, the fat content of modern beef
muscle is higher and more saturated than that of comparable wild ruminant muscle
[83]. Breeding and feeding practices allow for the production of meat at a specific
fat concentration [84]. If different properties were perceived to be beneficial, the fat
content could arguably be altered to various extents accordingly. Thus, when examining
the content of storage fat in muscle tissue that is used as food, one is looking
at a rather narrow window of a wide range of possibilities. As commodity needs
become more defined and the fat functionality better understood, the means to arrive
at these targets will need to be explored.
In addition to the differences in total quantity of fat and its tissue distribution,
the composition of storage triacylglycerols in animal species differs as well. Red
meats tend to be relatively higher in saturated fatty acids and lower in PUFAs than
poultry or fish. Poultry and fish differ significantly in the chain length of monounsaturated
fatty acids and in the content of n-3 PUFAs.
Once again, a consistent observation of the lipid content in different animal
tissues is the variability within species. Even within ruminant animals in which the
dietary PUFAs are largely hydrogenated by rumen flora, there is a significant range
of composition. Among monogastric animals, the variability in fat composition
within species due to muscle type, diet, environment, and age is typically greater
than the differences noted among species [81].
An important question becomes, what unique properties of the metabolism of
the three animal types lead to the observed or apparent differences in lipid composition
and behavior? In all animals, the storage triacylglycerols both in adipose and
individual muscle cells can be assembled from both dietary fatty acids and fatty
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