Milk is nutritionally one of the best food items. This high nutritional value of milk is quite useful to mammals because milk remains the only food for the young mammal for long period. The chief essential dietary ingredients which are less or absent in milk are the two minerals name iron and copper both needed for red blood cell formation.

Physical Characteristics: Milk is a liquid a white to yellowish white color due to substances such as carotenes and xanthophylls dissolved in milk fat. Its specific gravity ranges from 1.026 to 1.036. Milk is slightly acidic with a pH between 6.6 and 6.9. Fresh milk does not  coagulate on boiling but a surface film is produced that contains casein and calcium salts.
Chemical Constituents of Milk

   Chemical Constituents of Milk

The exact composition of milk varies with the breed of cattle, the feed used and the period of lactation. Milk has proteins, lipids, carbohydrates, vitamins enzymes and many organic constituents.

 Milk Proteins

These are casein, lactalbulmin and lactoglobuline.

Casein is the characteristic and most abundant protein of the milk. It occurs as it calcium salt, i.e. calcium caseinate. It is a phosphoprotein and its isoelectric pH is 4.6. Casein, however, is not a single protein but there are several types of casein which differ from each other mainly in respect to phosphorous content. Casein is low in methionine and threonine.
Casein can be precipitated by adding an acid to milk. The reaction is given below.

    Ca-caseinate + 2HCL -------------------> Casein + CaCl₂

The proteolytic enzymes, rennin, pepsin and chymotrypsin, can produce curdling of milk which takes place by following mechanism for which Ca⁺⁺ ions are essential. In the first step, casein is partially hydrolyzed giving rise to paracasein.

    Casein --------------> Paracasein (Partial hydrolysis)

Paracasein then combines with Ca++ ions to form calcium paracaseinate which gives rise to curds.

    Paracasein + Ca⁺⁺ --------------------> Ca^- paracaseinate

The cow’s milk has relatively more casein than human milk and therefore it makes tough curds as compared to human milk.

Lactalbumin is also not a single protein but there are at least three types of this protein.
Lactoglobulin is least abundant milk protein; this fraction contains antibodies as well.

 Milk Lipids

Milk is an oil in water emulsion which is made stable by phospholipids and proteins of milk which are absorbed on the surface of the small-sized fat globules. Cow’s milk triglycerides contain fatty acids ranging in chain length from 4C to 20C. But in human milk, fatty acids are of a chain length above 10C. The most abundant fatty acid occurring in milk is oleic acid. Milk has also essential fatty acids and cholesterol. Cow’s milk has about 11 mg cholesterol per 100 ml, all of which is free.

 Milk Carbohydrates
Lactose is the chief sugar of milk; two other sugars namely gynolactose and allolactose are also present. If due to bacterial contamination milk lactose gets fermented, the milk becomes sour. This process involves the conversion of lactose to lactic acid; ethyl alcohol and CO2 are also produced. A factor named bifidus occurs in milk which favors the growth of Lactobacillus bifidus in the intestine of the breast-fed infants. This factor has complex polysaccharides in its structure. Lactose increases the intestinal absorption of Ca and P.

 Vitamins in Milk
Milk is a good source of Vitamin A and its precursors. The vitamin D content of milk is very small. Similarly the Ascorbic acid concentration is also low. Of the B vitamins, milk is rich in riboflavin and Pantothenic acid but the other B vitamins are in a low concentration. However, this low concentration of many vitamins in milk can be compensated if children drink large quantities of milk.

 Enzymes in Milk
Many enzymes occur in milk; these include phosphatase, catalase, lipase, xanthine oxidase, amylase and aldolase.

 Milk Ash
Milk is very rich in Ca and P. As already mentioned, iron and copper are found in milk in negligible amounts only.

   Comparison of Human and Cow’s Milk

The comparison is shown in the following Table:

Milk Component	Human Milk	Cow's Milk
Proteins %	1.4	4.0
Lactose %	7.0	5.0
Fat %	4.0	4.0
Ash %	0.25	0.75

It can be seen from the table that human milk has more lactose but less protein and as. The caloric value of human and cow’s milk is 67 and 65 Cals/100 grams respectively.

 Colostrums

The milk which is secreted during the first few days after parturition is termed as colostrums. It is more viscid than milk secreted later and is alkaline in reaction. Colostrums has a higher concentration of milk solids and protein but less lactose and fat as compared to milk. A characteristic property of colostrums is that it coagulates on heating. The secretion of colostrums decreases gradually till it completely stops by the end of first or second week after parturition.