During the first two or three days watery yellowish fluid that comes from the mammary gland, which differs from the regular milk is called Colostrum. It is secreted in small quantity of about 10-40 ml. It is rich in protein. Total fat content is less than mature milk. The concentration of lactose is less. Levels of niacin, pantothenic acid, biotin, riboflavin are low. Vitamin C is secreted at about the same level as in mature milk. Zinc content of colostrum is 20 mg/l, whereas mature milk has 2.6mg/l of zinc.
Colostrum gives the first immunization to the infant. It contains interferon like substance which possesses strong antiviral activity. Colostrum contains vitaminB12 binding protein, which renders vitaminB12 unavailable for growth of E.coli and other bacteria. It also contains antibodies against viral diseases like small pox, measles and influenza. Enzymes like lysozyme etc that promote cell maturation are found to be more in colostrum.