Animal depot lipids

Animal depot lipids are used in edible applications, sometimes being consumed directly but more often in such applications as baking, cooking, and deep fat frying. They are also used in industrial applications, primarily in soap production, as an energy and nutrient source in animal feeds, in lubricants, and as a source of industrial fatty acids. These applications are discussed in detail elsewhere in this series. 

Examination of Table 1 shows that the prevalent fatty acids in animal depot lipids are either 16 or 18 carbons in length and are either fuUy saturated or contain one or two double bonds. Animal lipids generally contain a higher proportion of samrated fatty acids than do the lipids of temperate zone plants. 

Lipids are biological materials that are insoluble in water but soluble in nonpolar organic solvents. Here the term will be used interchangeably with acylglycerol, the primary component of animal lipids. These are classihed as either fats or oils. The former are predominantly solid at room temperature (24°C), and the latter are liquid. The depot lipids of animals are generally fats. The major animal fats (also termed meat fats) of contemporary commerce are produced from pigs Sus scrofa), in which case they are termed lard and rendered pork fat, from the fat of cattle Bos taurus) or sheep Ovis aries) and termed tallow, or from poultry (primarily chickens. Callus gallus) and termed poultry fat. Tallow... 

This chapter will consider contemporary aspects of the classihcation, composition, properties, production, processing, and consumption of the depot lipids of land animals. For a more detailed discussion, the reader is referred to the recent comprehensive animal fats text (1). Additional discussions of the material covered here, and related topics, are found in excellent contemporary texts (2-4). Some consideration of greases will also be included here, because these are largely handled by and traded within the animal fats sector of the lipid industry. Discussions of mUk-fats and hsh oils can be found elsewhere in this series. 

Animal and plant depot lipids consist primarily of triacylglycerols triesters of glycerol and three fatty acids. Lipids differ from species to species with regard to the types and amounts of fatty acids they contain. Table 1 presents the fatty acid... 

The lipid in muscle is composed primarily of triglycerides (depot fats) and of phospholipids (membrane components), and is a constituent which varies enormously not only in amount present, but also in properties such as degree of saturation (species dependent). The ash of lean meat is comprised of various minerals such as phosphorus, potassium, sodium, magnesium, calcium, iron and zinc Carbohydrate was not noted in the proximate composition because while some may be present, it is normally there in low concentration compared to the other constituents. Glycogen is the carbohydrate occurring in greatest concentration in muscle but is normally degraded soon after the animal is sacrificed. 

Comparative biochemistry. Some researchers believe that the proper role of comparative biochemistry is to put evolution on a molecular basis, and that detoxication enzymes, like other enzymes, are suitable subjects for study. Xenobiotic-metabolizing enzymes were probably essential in the early stages of animal evolution because secondary plant products, even those of low toxicity, are frequently lipophilic and as a consequence would, in the absence of such enzymes, accumulate in lipid membranes and lipid depots. The evolution of cytochrome P450 isoforms, with more than 2000 isoform cDNA sequences known, is proving a useful tool for the study of biochemical evolution. 

The bulk of our knowledge regarding thermal oxidation has been derived from studies with model systems of fatty acids and their derivatives, or with individual natural oils (2,3,6,12,13,14,15,16). However, in biological systems as complex as food, lipids usually exist in a complicated environment markedly different from that of the single phase model system. In cell membranes, for example, the lipid molecules are highly ordered, relatively restricted in distance and mobility, and closely associated with different neighboring molecules, e.g., other lipids, protein, cholesterol, water, pro- and antioxidants. What influence does such an environment have on the oxidation of the lipids at elevated temperature Even in less organized systems, e.g., depot fat from animal or plant, the lipids... 

Frank (1991, 1992) has shown that squirrels (,Spermophilus beldingi) capable of hibernation contain lipids with melting temperatures approximately 25°C lower than those of nor-mothermic mammals. He hypothesized that the animals selectively fed on plants containing high levels of polyunsaturated fatty acids, and that the composition of the resulting depot... 

Brockerhoff, H., Hoyle, R.R., Hwang, P.C. and Litchfield, C. (1968) Positional distribution of fatty acids in depot triglycerides of aquatic animals. Lipids, 3, 24. 

The question of the origins of milk fat, whether wholly from the diet or synthesized by the animal, was an early topic of debate (Jordan and Jenter, 1897). A leading theory, put forth by the eminent lipid chemist, Hilditch (1947), was that the unique short-chain fatty acids arise from the degradation of oleic acid. This conclusion was based upon empirical observations on the composition of ruminant depot and milk fats. 

The use of animal fats by humans may well predate civilization. As the depot fats of animals are readily noticed during the butchering of a slaughtered animal, are easily harvested, and are available in the absence of plant domestication and the adoption of established agriculture, it is probable that animal fats were the hrst lipids employed as industrial and as distinct edible lipids by humans. This is evidenced by the fact that the paints used in prehistoric cave paintings were animal fat-based, as were the fuels in the lamps that illuminated the cave artists at their work. Despite a tremendous diversihcation to include other lipid types over the intervening centuries, animal fats still play a prominent role in our diets, industry, and commerce. 

CiaHJ3Oa mol wl 304.46. C 78.89%. H 10.60%, O 10.51%. An essential fatty acid, q.v, and a precursor m the biosynthesis of prostaglandins, thromboxanes, and leukotrienes, q.q. v. Structure Mo wry et al.. J. Biol. Chem. 142, 679 (1942) Arcus, Smedley-Maclean. Biochem- J. 37, 1 (1943). Occurs in liver, brain, glandular organs, and depot fats of animals, in small amounts in human depot fats, and is a constituent of animal phospha tides. Isolation from Liver lipids Brown, J, Biol. Chem. 80, 455 (1928) from beef... 

Lipids occurring in plant and animal materials consist of structural lipids, which build the cell membranes, and depot fats. The cell and organelle membranes of animal organisms are made of phospholipids and non-esterified cholesterol, whereas in plants they consist of phospholipids and glycolipids. The latter are also found in the central nervous systems of some animals. If a muscle tissue like that in lean fish contains only 0.3% w/w of lipids, they consist almost entirely (90%) of phospholipids. Galactoglycerols and phospholipids serve as important factors in nutrient and antioxidant delivery systems (Herslof, 2000). 

Differences between the animal and vegetable lipids in relation to composition, digestion, absorption, and in the composition and concentration of sterols incorporated in ingested fat affect the metabolism and composition of fat depots. 

TAGs serve as food stores and are therefore the most abmidant lipid in animals. They exist in most cell types but are especially prevalent in adipose tissue when they are known as depot fat. The hydrolysis of the ester bonds of TAGs and the release of glycerol and fatty acids from adipose tissue is referred to as fat mobilization. Depot fat is a water-free mixture of TAGs that differ from each other in the nature of the three fatty acyl groups which they contain. 

Triglycerides are the most abundant lipids in animals. They form depot fat, and hydrolysis of the ester bonds releases fatty acids and glycerol from adipose tissue. Phospholipids are the main stmctnral lipids in cell membranes, while glycoglycerolipids exist in cell membranes they are much less abimdant. 

Since the general problem has largely become one in which the level of lipid-soluble autoxidizable material in the tissues is paramount in practical importance, more attention has been paid to the factors which affect tissue autoxidizability. In animals, as indicated above, attempts are being made to relate the rate at which polyunsaturated fatty acids accumulate in the various tissues. In man, one is practically limited to examination of the components of plasma, erythrocytes, and depot fat samples if multiple examinations of large groups are to be made. 

Branched-chain acids of this kind have been found in the depot, milk and blood lipids of animals, including humans. They are, for example, significant components of the acids (and alcohols) present in human skin lipids. 

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