The composition of milk fat is somewhat complex. Although dominated by triglycerides, which constitute some 98% of milk fat (with small amounts of diglycerides, mono-glycerides, and free fatty acids), various other lipid classes are also present in measurable amounts. It is estimated that about 500 separate fatty acids have been detected in milk lipids; it is probable that additional fatty acids remain to be identified. Of these, about 20 are major components; the remainder are minor and occur in small or trace quantities (7,8). The other components include phospholipids, cerebrosides, and sterols (cholesterol and cholesterol esters). Small amounts of fat-soluble vitamins (mainly A, D, and E), antioxidants (tocopherol), pigments (carotene), and flavor components (lactones, aldehydes, and ketones) are also present.

The composition of the lipids of whole bovine milk is given in Table 2 (7,8). The structure and composition of the typical milk fat globule is exceeding complex. The globule is probably 2 to 3 n in diameter with a 90-A-thick membrane surrounding a 98 to 99% triglyceride core. The composition of the milk fat membrane is quite different from milk fat itself in that approximately 60% triglycerides are present, much less than in the parent milk fat (Table 3) (9,10).

It has been generally recognized that butterfat consists of about 15 major fatty acids with perhaps 12 or so minor (trace quantity) acids. Triglycerides are normally defined with respect to their carbon number (CN), that is, the number of fatty acid carbon atoms present in the molecule; the three carbon atoms of the glycerol moiety are ignored. Because the fatty acid spectrum of milk fat is dominated by acids containing an even number of carbon atoms, so is the triglyceride spectrum. However, the proportion of triglycerides with an odd carbon number is about three times greater than the proportion of odd-numbered fatty acids.

Although obvious correlations exist between fatty acid composition and triglyceride distribution, detailed information is lacking that would enable the triglyceride distribution to be predicted from the fatty acid composition. Much more needs to be understood of the strategy used in the bovine mammary gland in assembling a complex array of fatty acids into triglycerides. This is not an arcane study; it is necessary if processes such as fractionation are to yield products with consistent qualities throughout the year. In effect, the detailed structure of milk fat is not yet understood. Perhaps this is not surprising if we consider only the 15 major fatty acids; there are 153 (3375) possible triglyceride structures using a purely random model.

The concentration of milk fat results in a dairy product called butter. The concentration is a two-stage process with the first stage being effected in a mechanical separator, which causes a 10-fold concentration of the fat into an intermediate product, cream. The second stage is the churning process, which results in a further twofold concentration of the fat.

In butter manufacture, the preparation of the cream is effected quite independently of the butter-manufacturing stage. Today most butter plants receive their cream from other operations rather than directly from the farm, as was the case in the past. Because of general improvement in sanitation practices, the receipt of fresher milk and cream, and advances in the knowledge and understanding of deleterious handling conditions, the quality of cream is constantly improving.

The composition of milk fat is the most important factor affecting the firmness of butter and, therefore, its spread-ability. The composition of milk fat changes primarily according to the feed; therefore, the entire problem is connected to the animal's diet. Today, the fatty acid composition of milk fat produced in various countries is rather accurately known, along with its seasonal variations. In Europe the amount of saturated fatty acids is generally highest in winter and lowest in summer or fall (Table 5) (11). Green fodder decreases the amount of saturated fatty acids and correspondingly increases the amount of unsaturated fatty acids (12). The differences between the maximum and minimum values can be fairly large. For palmitic and oleic acids, the quantitatively most important fatty acids, a difference of more than 10% between the maximum and minimum values was found in some cases. This makes it understandable that there are also significant differences in the physical characteristics of the butter. The structure of the triglycerides in the milk fat, along with the fatty acid composition, is important in determining the physical characteristics of the fat, because the softening point of fat has been found to rise as the result of interesterification (13).

Protected oils are hydrolyzed in the abomasum, and the fatty acids are absorbed in the small intestine, thereby avoiding hydrogenation. The 18:2 content in the milk fat was increased about fivefold, and the 14:0, 16:0, and 18:0 were decreased accordingly. Plasma and depot fats were also increased in 18:2 content by this program (14).

Results at the USDA are similar: cow's milk can be increased in 18:2 acid from 3 to 35% by feeding protected safflower oil (15,16). However, at high 18:2 levels, milk develops an oxidized off-flavor, usually after about 24 h, and creams require a longer aging time for satisfactory churning. As expected, butter that contains more than 16% lin-oleic acid is soft and sticky (8).

Table 5. Compositional Characteristics of Summer and Winter Milk Fat, 1970

Samples investigated (N = 140)

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