India is now the largest producer of milk in the world with 74 million tonnes in 1998-99 (Indian Agriculture, 1999). In the first decade after independence (1950-60), the production was 17 to 20 million tonnes. There was no drastic increase in its production in the next 2 decades. The production was accelerated to reach 58.6 million tonnes in 1992-93. Success in raising the level of milk production is ascribed to the operation flood project. More than 68,000 Dairy co-operative societies have been organised in 170 milk sheds, involving about 8.8 million farmers by 1999 (Indian Agriculture,2000. World wide production of milk, in thousand metric tonnes, is given below (FAO,2000).
|Cow’s milk, whole fresh||474,960||68,823||72,200|
Milk from different sources, regardless of bread or even species, will contain the same classes of constituents. They are milk fat (3-6%), protein (3-4%), milk sugar (5%) and ash (0.7%). Water accounts for the balance of 85.5-88.5%. All the solids without fat is known as total solids (11.4-14.5%) and the total solids without fat is known as milk solids-non-fat (MSNF) or solids-non-fat (SNF). The price of milk depends on its fat content and to a lesser extent, on its SNF content.
There are quantitative differences in the constituents of milk from different sources; widest variations occur with fat, next with protein, followed by milk sugar and minerals (ash). The yield of milk and its composition, from the same source, vary depending on many factors. These include the breed of the animal, its age, the stage of lactation, time of milking, time interval between milking, season of the year, feed of the animal, condition of the animal and so on.
The main protein in milk is casein and it constitutes 3.0-3.5% of milk. It is present as calcium caseinate in colloidal suspension. When milk is converted into curd by lactic acid bacteria, a fine precipitate of casein is formed. When milk is curdled by addition of lime juice, casein is precipitated as flocculent precipitate. When milk is acted upon by rennin or pepsin in presence of calcium salts, a thick curd of calcium paracaseinate is formed. Besides casein, milk contains albumin (lactalbumin) at a level of 0.5% and globulin (lactoglobulin) in small amount (0.1).
The fat content of milk varies from about 3.5% in cow’s milk to about 8% in buffalo milk. Fat is present in the form of fine globules varying in diameter from 1-10 microns, the major portion having diameter of 3 microns. Milk also contains small amounts of phospholipids and cholesterol.
The chief carbohydrates of milk is lactose. It is present up to 4.4-4.8%. When milk is autoclaved, the colour becomes light brown. This is due to reaction between the reducing group of lactose and the end amino group of lysine residue in casein. This reaction is known as maillard reaction.
The important minerals present in milk are calcium, phosphorous, sodium and potassium. The salts of these minerals function as buffer maintaining the pH of milk at a constant level of 6.5-6.6. At this pH, casein exists mostly as calcium salt in colloidal suspension. Calcium is essential for the formation of curd from milk by the action of rennin.
Milk is a good source of both fat-soluble and water-soluble vitamins. The concentration of fat soluble vitamins except vitamin K, depends on concentration of these vitamins in the feed consumed. Vitamins K is synthesized in the cow’s rumen or tissues.
Milk is especially rich in riboflavin but this vitamin is lost rapidly on exposure to light and may produce an oxidized off-flavour involving both riboflavin and protein. The concentration of niacin and ascorbic acid is relatively low in milk.