What are the main constituents of cow's milk?

Milk is the normal secretion of the mammary glands of all mammals. Its purpose is to nourish the young of the species. 

The principal constituents of milk –water, fat, protein, milk sugar or lactose and the minerals.

Water 
Water is the medium in which all the other components of milk are dissolved or suspended. Water content varies from 85.4% to 87% in different species of cows.

Protein 
Casein is the major protein, in contrast to lactalbumins in human milk. The major milk proteins are casein, alpha-lactalbumins, beta-lactoglobulin and the relatively smaller amounts are contributed by milk, enzymes. 

Fat
The cream portion of milk is primarily due to the presence of fat. This family occupies about 4% of the total milk space in cow milk. The major lipid content of cow’s milk is triglyceride, which makes up about 98% of milk fat. The other 2% of milk lipids consist of diglycerides, monoglycerides, cholesterol, phospholipids, free fatty acids, cerebrosides and gangliosides.

Carbohydrate 
Milk contains unique sugar lactose. It is responsible for imparting sweetness to milk although it is onw0offth as sweet as sucrose.

Minerals
Average normal is considered to contain 0.70% ash and this amount represents a salt of about 0.90%. Mineral make-up f milk is crucial to the stability of the physicochemical equilibrium in milk.

Vitamin 
Milk is a unique food having at least 10 to 11 vitamins present in different proportion. Milk contains both fat soluble (A, D, E and K) and soluble water soluble vitamins.

Age is not an important factor effecting the composition of milk although there appears to be tendency for the fat content to decrease with increasing age. However, the health of the cow may affect milk composition considerably.

The most important single factor governing the composition of cow’s milk is the breed of the cow.

The principal milk producing breeds are the Ayrshire, Brown Swiss, Guernsey, Holstein and Jersey. Holstein generally produces the most milk, but Guernseys and Jerseys produce milk with the highest fat contents.
What are the main constituents of cow's milk?

Microorganisms in milk

Microorganisms in milk
As drawn from cow’s udder , milk seldom , if ever, is free from microorganisms; bacterial, molds, and yeasts are usually present in small numbers, among which the bacteria are most significant from stand point of quality and the transmission of food borne disease. The control of microbial activity in milk and milk products, especially the control of bacterial and bacterial growth, is the most important function in the handling and manufacturer of dairy products.

Raw milk, when improperly handled, may undergo any of several adverse changes. It may become sour due to the growth of bacteria that produce lactic acid , or it may become foamy due to the growth of gas producing coli form bacteria or yeasts. Raw milk may also be subject to peptonisation (digestion of casein), the formation of rope (viscous polymer of sugars), and sweet curding, when bacterial growth is not controlled.

Dairy herds are tested for tuberculosis and tested for and vaccination for brucellosis, and most milk is pasteurized to prevent the transmission of a variety of food –borne disease. However, some pathogens may be survive the pasteurization process, possibly because they may be protected by fat in which they may become encapsulated.

Most, but not all, certified milk is pasteurized. However, certified milk must be produced from herds that have been inspected, tested, and found to be free form disease, and the milk must be drawn and handled under the best condition of sanitation. There are bacterial standards certified milk that limits the number and types of bacteria that may be present.
Microorganisms in milk