Fatty acids in milk fat

Milk fats, especially ruminant fats, contain a very wide range of fatty acids more than 400 and 184 distinct adds have been detected in bovine and human milk fats, respectively (Christie, 1995). However, the vast majority of these occur at only trace concentrations. The concentrations of the principal fatty acids in milk fats from a range of species are shown in Table 3.6. Notable features of the fatty acid profiles of milk lipids include ... 

Table 4.3. Composition and Stereospecific Distribution of Fatty Acids in Milk Fat Trigylcerides from Bimonthly Samples of Maleny Butter.

Parodi, P. W. 1976. Distribution of isomeric octadecenoic fatty acids in milk fat. J. Dairy Sci. 59, 1870-1873. 

It is important to bear in mind when discussing the effect of dairy fat in association to heart disease that dairy products contain many different saturated fatty acids that do not exert the same biological response in terms of, for example, cholesterol levels. The saturated fatty acids in milk fat include shorter and medium chain fatty acids (2 0-10 0), lauric acid (12 0), myristic acid (14 0), palmitic acid (16 0), and stearic acid (18 0). Other fatty acids in milk fat are oleic acid (18 1) and linoleic acid (18 2n-6) as indicated in Table 1.2. 

TABLE 1.2 Percentages of the different fatty acids in milk fat... 

The fatty acids of bovine milk fat arise from two sources synthesis de novo in the mammary glands and the plasma lipids originating from the feed. The fatty acids from these two sources differ in their structure. The fatty acids that are synthesised de novo are short-chain and medium-chain length acids, from 4 0 to 14 0 and also some 16 0, while the Cis fatty acids and some 16 0 arise from the plasma lipids. De novo fatty acid synthesis accounts for approximately 45% (w/w) of the total fatty acids in milk fat, while lipids of dietary origin account for the rest (Moore and Christie, 1979). 

The presence of Qg tram-fatty acids in milk fat is the result of incomplete biohydrogenation of the unsaturated dietary lipids in the rumen. These fatty acids have attracted attention because of their adverse nutritional affects. Clinical trials have shown that traus-octadecenoic acids, relative to the cis isomer, can increase the LDL-cholesterol and decrease the HDL-cholesterol, thus, producing an unfavourable affect on the LDL HDL ratio (Mensink and Katan, 1993). 

The quantitative determination of individual isomers of tram-18 1 fatty acids in milk fat is not straightforward. It involves a multi-stage analytical procedure (i.e., transesterification of milk fat, argentation TLC of the fatty acid esters to separate the civ-isomers and tram-isomers, followed by capillary GC). This method gives an almost complete separation of the 13 individual tram-18 1 isomers, from A4 to A16 (Precht and Molkentin, 1996). 

There are about 200 minor monoenoic, dienoic and polyenoic fatty acids in milk fat ranging in chain length from C10 to C24, and consisting of both positional and cisltrans isomers. A number have considerable nutritional significance for example, eicosapentaenoic acid (20 5,0.09%) and docosahex-aenoic acid (22 6,0.01%) are present in the metabolic pathway of the n-3 fatty acids, while arachidonic acid (20 4, 0.14%) is part of the n-6 pathway. 

As noted earlier, there are some 400 fatty acids in milk fat, which means that theoretically milk fat could contain many thousand triacylglycerols. Even if one considers only the 15 or so fatty acids that are present at concentrations above 1% (Table 1.2), and ignores the placement of these fatty acids at specific positions on the triacylglycerol molecule, there are still 680 compositionally different triacylglycerols. 

The fatty acids in milk fat are derived from two sources, de novo synthesis of fatty acids in the mammary gland and plasma lipids (see Pal-quist, Chapter 2). De novo synthesis generally involves short-chain and medium-chain fatty acids and some 16 0. The proportions of various fatty acids depend on the specific balance between enzymatic chain elongation and chain termination. The plasma lipids are derived from the diet and also from storage in the body tissues. For non-ruminants, the diet has a large influence on the fatty acid composition but for ruminants, biohydrogenation in the rumen results in much less impact of diet on the final fatty acids absorbed into the bloodstream. 

More than 95% of Ci8 and longer-chain fatty acids in milk fat are derived from the blood TAG-rich lipoproteins. Non-esterified fatty acids are... 

Figure 2.6. Proportions of individual fatty acids in milk fat at 1,4,8 and 12 weeks of lactation relative to their proportions at 16 weeks (from Palmquist et al., 1993. J. Dairy Sci. 76, 1753-1771).

Changes in the proportions of fatty acids in milk fat by supplementation of various oils and oilseeds are summarized in Figure 2.8 (Grummer, 1991). Hermansen (1995) developed a set of regression equations to predict the composition of milk fat based on the proportions of lauric, myristic and... 

Glascock, R.F., Duncombe, W.G., Reinius, L.R. 1956. Studies on the origin of milk fat. 2. The secretion of dietary long-chain fatty acids in milk fat by ruminants. Biochem. J. 62, 535-541. 

Donovan, D.C., Schingoethe, D.J., Baer, R.J., Ryali, J., Hippen, A.R., Franklin, S.T. 2000. Influence of dietary fish oil on conjugated linoleic acid and other fatty acids in milk fat from lactating dairy cows../ Dairy Sci. 83, 2620-2628. 

Physical modification of milk fat by fractionating milk fat or by blending milk fat or milk fat fractions with other oils and fats results in products with an altered triacylglycerol composition, but one in which the fatty acids in milk fat maintain their original position in the triacylglycerol molecules (Kaylegian, 1999). 

Penicillium roqueforti and P. camemberti produce very active extracellular lipases, which are the principal lipolytic agents in mold-ripened cheeses. They preferentially hydrolyze the short-chain fatty acids in milk fat. P. roqueforti produces two lipases, one with an alkaline pH optimum and the other most active at pH 6 6.5, with slightly differing fatty acid specificities (Menassa and Lamberet, 1982). P. camemberti secretes a single lipase with optimal activity at pH 9 (Lamberet and Lenoir, 1976). 

Up to one-third of the fatty acids in milk fat have a chain-length of 14 carbons or less. Because these acids are oxidized rapidly in the liver, have a lower energy value and are oxidized more readily than long-chain fatty acids, it follows that milk fat should contribute less to overweight than an equivalent amount of other dietary fats (Parodi, 2004). A study by Schnee-man et al, (2003) showed that milk fat is a more potent stimulator of cholecystokinin than a blend of non-milk fat with a similar ratio of polyunsaturated to saturated fatty acids. Cholecystokinin is a satiety hormone released into the blood stream by the intestine during feeding and acts to suppress further eating. 

As discussed in section 3.6, the fatty acids in milk fat are not distributed randomly and the melting point may be modified by randomizing the fatty acid distribution by transesterification using a lipase or chemical catalysts. 

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