Milk is considered to be a complete food as it consists most of the nutrients needed for human body. It is the secretion of lactating mammary gland. The secretion during early stages of lactation is known as colostrums. The pH of milk is 6.5–6.7. Its specific gravity averages 1.03. The chief protein of milk is casein, which constitutes about 80% of the total milk proteins; the remaining are lactalbumin and lactoglobulin. Milk is rich in vitamin A and B2. It is the most important source of calcium and phosphorous; but deficient in iron and copper.