Fat and associated lipid materials are present in milk as an emulsion of small individual globules in an aqueous phase. These fat globules are predominantly in the liquid state at 37 degree. The fat globules vary in size from about 0.1 to 10 um in diameter.
Surrounding each milk fat globule is a membrane composed of cholesterol, phospholipids, proteins, and other surface-active molecules. The fat globules membrane appears to have two layers: an inner layer that contains a phospholipids-protein complex, and an outer layer that contains enzymes originating in the mammary gland.
The membrane serves to prevent individual fat globules from combining with other to form a larger globule. Mechanical agitation can lead to a disruption of the membrane and thus cause the individual fat globules to coalesce to form larger globules and eventually to form butter. Disruption of the globule membrane may also contribute to the development of rancid flavor in milk.
Milk fat is made up of approximately 12.5% glycerol and 85.5% fatty acids by weight. The predominant glycerides are triglycerides (8-99%); diglycerides account for 0.05%; and monoglycerides for 0.04% of the total. These glycerides contain more than 16 different fatty acids combined with glycerol in a number of different ways.
The principal fatty acids contain an even number of carbon atoms. Milk fat is unique in that it is the only natural food fat that contains butyric acid in its glycerides structure. This fatty acid has a strong characteristics odor. Their release from triglycerides during lipolysis is the cause of rancid odor and flavor in milk.
Milk fat contains a number of minor components. Among these are free fatty acids, cholesterol, phospholipids, and the fat soluble vitamins.
Fat in Milk
