Milk is a complex fluid containing lipids, carbohydrates, proteins and many other organic and inorganic salts dissolved or dispersed in water. The most variable component of milk is fat followed by protein.
The composition of milk varies with the species, breed, diet, lactational period and interval between milking. There is also individual variation among various factors.
- Milk Fat: Buffalo’s milk contains 6.5 % fat. Cow’s milk contains 4.1 %t fat. Milk fat or butter fat is of great economical and nutritive value. The flavour of milk is due to milk fat. Milk is a true emulsion of oil-in-water. Fat is present in the form of the fine globules with diameter varying from 1 to 10 microns. Each globule of fat is surrounded by a thin layer which is composed of a lipid-protein complex and a small amount of carbohydrate. The lipid portion includes both phospholipids and triglycerides.
Other lipid materials present in milk are phospholipids, sterols, free fatty acids, carotenoids and fat-soluble vitamins. Carotenes are responsible for the yellow colour of milk fat. Gerber test is used to know the percentage of fat present in food.
- Milk Proteins: The main protein in milk is casein and it constitutes about 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 the addition of lemon juice, casein is precipitated as a flocculent precipitate. When milk is acted upon by rennin or pepsin in the presence of calcium salts, a thick curd of calcium paracaseinate is formed. This is the basis of manufacture of cheese. In addition to casein, milk contains an albumin( lactalbumin) at a level of 0.5 % and a globulin ( lactoglobulin) in small amounts ( 0.1 %). The normal acidity of fresh milk is about pH 6.6. Casein is a good source of essential amino acids. Casein contains 8.2 % calcium and 5.7 % carbohydrates.
- Whey proteins: Whey proteins are made up of α- lactalbumin and β-lactoglobulin, serum albumin, immune globulins, enzymes and proteose-peptones. β-lactoglobulin accounts for about 50 % of total whey proteins. These are not precipitated by acid or rennin. They can be coagulated by heat. Whey also contains small amounts of lactoferrin and serum transferrin. By a process of ultra-filtration, whey protein concentrate and isolates were produced. Whey protein isolate can be given in lactose intolerance.
- Carbohydrates: Milk contains 4-5 % carbohydrate. The chief carbohydrate present in milk is lactose. It is present to the extent of about 4.4 to 4.8 % . When milk is autoclaved, the colour becomes light brown. This is due to reaction between the reducing group of lactose and the amino group of lysine residue in casein. This reaction is known as Maillard reaction.
- Minerals: The important minerals present in milk are calcium, phosphorus, sodium and potassium. The salts of these minerals function as buffers maintaining the PH of milk at a constant level at about 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.
- Enzymes: The enzymes found in milk can originate from the mammary glands or may be released by contaminating bacteria. Alkaline phosphatase exists as lipoprotein and is distributed between the lipid and aqueous phases. This enzyme is inactivated by normal pasteurization procedures and its activity is tested to determine the effectiveness of pasteurization.
Milk lipase is responsible for the development of rancid flavours in milk. Lipases may be important in the development of desirable flavours in some cheeses.
- Colour: White colour of milk is caused by the reflection of light by the colloidally-dispersed casein, calcium and phosphorus. Yellowish colour of milk is due to the presence of carotene and riboflavin. Fat-soluble carotenes are found in milk fat; riboflavin is water soluble which can be visible clearly in whey water.
- Flavour and Aroma: Milk is slightly sweet because of its lactose content. Flavour sensation of milk in mouth is due to fat protein and salt calcium phosphate. The slight aroma of fresh milk is produced by a number of low molecular weight compounds such as acetone, acetaldehyde, dimethyl sulphide and short chain fatty acids. Boiling Changes in the flavour of fresh milk is more when milk is boiled than in pasteurized milk.
Off flavour in milk may be influenced by the health of the cow or the feeds that are consumed by the cow, action of bacteria, chemical changes in milk, or the absorption of foreign flavours after the milk is drawn.
Off flavours are also produced when milk is exposed to light. In this reaction, tryptophan and riboflavin may be involved and their content decreases when the off-flavour develops.