Fats Are the Most Important Energy Source in Milk
Of the components of milk, fat is the most important energy source. Milk fat is composed of a number of lipids, including monoglycerides, diglycerides, triglycerides, free fatty acids, phospholipids, and steroids; triglycerides are the main component of milk fat.
The types of Iipid synthesized are complex, with great variations in both chain length and saturation of fatty acids observed on the basis of species. The amount of fat produced varies greatly both within and among species (Table 39-2). Marine mammal milk has a high fat content, with values of about 40% to 50% in seals, 40% in dolphins, and 30% in whales. In these species the high energy content of the milk through fat helps offset the heat loss of the young.In domestic animals, sheep, swine, dogs, and cats have milk that ranges from 7% to 10% in fat content. Dairy cattle have values that range from 3.5% to 5.5%; goats are similar to cows (3.5%), and mares have lower values (1.6%). In the past, milk was sold on a butterfat basis, and breeds that had a relatively high butterfat content of milk (e.g., the Jersey with 5% butterfat) found more acceptance in dairy operations than is currently the case. Small farms produced mainly cream (for butter manufiicture); the fat-concentrated portion of milk was produced by use of a separator that separated cream on the basis of specific gravity and centrifugal force. Because milk is now sold on a solids, not fat, basis, breeds that produce more milk (and protein) are favored, even though the fat content of the favored breed, Holstein-Friesian, is lower (3.5%).