Milk Fat Composition
Of the greater than 400 fatty acids which have been identified in milk fat, fewer than a dozen contribute at least 1% to the total weight of milk fatty acids (Jensen and Newburg, 1995). These include the fatty acids synthesized de novo in the mammary gland from acetate and β-hydroxybutyrate, and ranging in chain length from 4 to 16 carbon atoms. Part of the palmitic acid (16:0) and all 18 carbon chain fatty acids are derived from blood lipids, which are predominantly of dietary origin, with minor amounts (except in early lactation) from mobilized adipose tissue fatty acids. Considerable desaturation of 14, 16, and 18 carbon fatty acids by mammary delta-9 desaturase enzyme may occur. These fatty acids are responsible for the concept of milk fat as a saturated fat, and also contribute importantly to the physical structure of the fat.
However, the much greater number of minor fatty acids in milk fat are gaining an increased amount of attention. Most of these fatty acids arise from ruminal microbial metabolism and thus are unique to ruminant fats. Among these is the recently-described anticancer CLA. Others include ruminal metabolites that contribute to the unique flavor characteristics of milk fat. Recently there has been intensive research by a New Zealand laboratory (Joblin and Hudson, 1997) to characterize the ruminal microbes responsible for synthesizing various hydroxy fatty acids, which are precursors of delta-(δ) and gamma-(γ) lactones, flavor agents that Stuart Patton characterized early in his research career (Patton et al., 1954).