Synthesis of milk fat and opportunities for nutritional manipulation
Authors: Murphy J.J. 
Published: June 2019

Milk fat consists of approximately 960-980 g of triacylglycerol, 20-25 g of 1,2-diacylglycerol, 10 g of phospholipid, 5g of cholesterol and very small quantities of free fatty acids and monoacylglycerol per kg. There are three stages in milk fat biosynthesis: the accumulation of fatty acids in the mammary cells through de-novo synthesis or absorption from the blood stream, triacylglycerol construction and fat globule assembly and secretion. Fatty acids in mammary secretory cells arise from two sources. Those having between 4 and 14 carbon atoms are synthesised de-novo in the mammary gland whereas those with 18 carbon atoms are of dietary origin and are absorbed from the blood stream. Palmitic acid (16 carbon atoms) is supplied almost equally from the diet and de-novo synthesis. In ruminants the principal sources of carbon for fatty acid synthesis are acetic acid and b-hydroxybutyrate.

Alteration of milk fat concentration is achieved by changimore spreadable butter. Monounsaturated fatty acids in the diet have been shown to have beneficial effects on the plasma lipoprotein indicators of coronary heart disease risk. From a human nutrition point of view it could be beneficial to incorporate the long chain omega-3 fatty acids, eicosapentanoic (EPA, C20:5) and docosahexanoic (C22:6) acids, into milk fat. The principal source of these fatty acids is fish oil but research to date indicates that their transfer into milk fat is inefficient. Conjugated linoleic acid (CLA) is a collective term describing one or more positional and geometric isomers of linoleic acid (cis-9, cis-12 C18:2). CLA has been shown to have anticarcinogenic activity, antiatherogenic activity, an ability to reduce the catabolic effects of immune stimulation and an ability to enhance growth promotion and reduce body fat. It is present in ruminant milk and meat as a result of biohydrogenation in the rumen where it is an intermediate. Its concentration in bovine milk fat is influenced by dietary factors such as pasture feeding and supplementation with full fat oilseeds. Two other components of bovine milk fat which have been shown to have anticarcinogenic properties are butyric acid and sphingomyelin and their concentration warrants further study. It is likely that research will continue into means of manipulating both the content and composition of milk fat but ultimately the adoption of any of the strategies in practice is likely to depend on strong economic or consumer imperatives.ng either the level of de-novo synthesis in the mammary gland or the supply of long chain fatty acids in the diet. Dietary factors that affect the supply of acetic acid from the rumen for de-novo synthesis include fibre quantity and quality, forage to concentrate ratio, buffer inclusion, concentrate composition and concentrate feeding frequency. The effects of fat supplements on fat concentration are variable. In general, feeding rumen protected fat increases milk fat concentration whereas moderate amounts of unprotected unsaturated fat tend to decrease it.

Most nutritional manipulation has been directed at increasing the proportion of unsaturated fatty acids in milk fat in order to enhance its appeal to the consumer and to produce a softer fat. A more spreadable butter could be produced from such fat thus overcoming a major criticism of conventional butter.

If unsaturated fatty acids are fed to ruminants in an unprotected form rumen microbial digestion can be impaired and the unsaturated fatty acids are extensively saturated in the rumen. One strategy to overcome this is to include unsaturated fatty acids in a form protected from microbial digestion in the rumen. This resulted in the production of polyunsaturated milk fat from which a low melting point butter was produced. This product was predisposed to oxidative deterioration. More recently whole oilseeds have been fed to dairy cows. The unsaturated 18-carbon fatty acids in these seeds are hydrogenated in the rumen but the activity of a D-9 desaturase in the mammary gland and to a lesser extent the intestine converts the stearic acid (C18:0) to the monounsaturated fatty acid, oleic acid (C18:1). Milk fat rich in oleic acid is softer than conventional milk fat allowing the manufacture of a more spreadable butter. Monounsaturated fatty acids in the diet have been shown to have beneficial effects on the plasma lipoprotein indicators of coronary heart disease risk.

From a human nutrition point of view it could be beneficial to incorporate the long chain omega-3 fatty acids, eicosapentanoic (EPA, C20:5) and docosahexanoic (C22:6) acids, into milk fat. The principal source of these fatty acids is fish oil but research to date indicates that their transfer into milk fat is inefficient. Conjugated linoleic acid (CLA) is a collective term describing one or more positional and geometric isomers of linoleic acid (cis-9, cis-12 C18:2). CLA has been shown to have anticarcinogenic activity, antiatherogenic activity, an ability to reduce the catabolic effects of immune stimulation and an ability to enhance growth promotion and reduce body fat. It is present in ruminant milk and meat as a result of biohydrogenation in the rumen where it is an intermediate. Its concentration in bovine milk fat is influenced by dietary factors such as pasture feeding and supplementation with full fat oilseeds. Two other components of bovine milk fat which have been shown to have anticarcinogenic properties are butyric acid and sphingomyelin and their concentration warrants further study.

It is likely that research will continue into means of manipulating both the content and composition of milk fat but ultimately the adoption of any of the strategies in practice is likely to depend on strong economic or consumer imperative.