Long Chain Fatty Acids
The long-chain fatty acids of milk fat are extracted from blood lipoprotein triglycerides, predominantly derived from the diet (Palmquist and Mattos, 1978). This is demonstrated dramatically by changes in milk fat composition within 24 hours of changing the amount or nature of dietary fat. Supplementing the cow's diet with fat has become common practice, with the result that the long chain fatty acids have been increased often to more than 60% of the milk fat (Palmquist et al., 1993).
The fatty acids of milk fat triglycerides are less saturated than those of blood triglycerides (Morales et al., 2000). Stearoyl-CoA desaturase activity in the mammary gland converts stearic acid (18:0) to oleic acid (cis )9-18:1); the same enzyme also desaturates 14:0, 16:0, and trans )-11 18:1 (the product of the latter is CLA) (Griinari et al., 2000).
A particularly unique aspect of ruminant milk fat is the abundance of short- and medium-chain length fatty acids. No doubt this evolved as a compensation for the large amount of saturated fatty acids arising from ruminal biohydrogenation (see below). The regulation of the chain length of these fatty acids remains mostly a mystery. In rodents, short fatty acyl chains are cleaved from the fatty acid synthetase enzyme complex by thioesterase II. Although both rodents and ruminants contain a thioesterase-I, specific for 16-C fatty acids, (Smith, 1994), there is no thioesterase-II in ruminants; rather chain length seems to be determined by a multiple complex which includes the rate of chain-lengthening (largely determined by the supply of malonyl-CoA) and competition between enzymes that load and unload the acyl chain from the fatty acid synthetase complex (Knudsen and Grunnet, 1982; Smith, 1994). Thus, to study regulation of chain length of milk fatty acids in ruminants is a quite difficult task.
There seems to be some compensation among groups of fatty acids in milk fat, so that the melting point of the fat remains in a range that keeps it liquid at body temperature of the cow (39°C). For example, Jersey cows tend to have a higher proportion of short-chain fatty acids than other breeds (Beaulieu and Palmquist, 1995), which would decrease the melting point; however, the oleic acid (18:1) content is lower and stearic acid (18:0) is higher. We suggested that Jersey cows have lower activity of stearoyl-CoA desaturase, causing the melting point to be higher. In balance, it has long been known that milk fat from Jersey cows tends to be harder than milk fat from other breeds, an important factor in milk processing.