Milk has a high nutritional value, the composition is as follows.1, protein cow's milk protein content of 2.8% to 3.3%, mainly by 79.6% of the casein, 11.5% of the whey (albumin) protein and 3.3% of the composition of lactoglobulin, and a small amount of other proteins, such as immunoglobulins and enzymes. Where 20 ℃ in pH 4.6 precipitation of cow's milk protein is called casein. Casein is a heat-resistant protein, but can be precipitated under acidic conditions, yogurt and cheese is made on this principle. In milk casein combines with calcium and phosphorus to form casein gels, which are present as a colloidal suspension in buttermilk. Whey proteins are thermally unstable and coagulate and precipitate when heated. Cow's milk protein digestion and absorption rate of 87% to 89%, the biological value of 85, is a high-quality protein. 2, fat milk fat is about 2.8% to 4.0%, in the form of particles of fat globules dispersed in the emulsion, is a very good emulsification, easy to digest and absorb, the absorption rate of 97%. Lipids in cow's milk are mainly triglyceride-based, a small amount of phospholipids and cholesterol, milk fat in the fatty acid composition is complex, oleic acid accounted for 30%, linoleic acid and linolenic acid accounted for 5.3% and 2.1%, respectively, the short-chain fatty acids (such as butyric acid, has been the acid, octanoic acid) content is higher, about 9%, is the reason why the milk fat has a good flavor and is easy to digest. 3, carbohydrates milk carbohydrates are mainly lactose, lactose, milk lactose content is about 3.8%, lactose content is about 3%, and the absorption rate is as high as 97%. Milk lactose content of about 3.4% to 5.4%. Lactose in the intestinal tract can promote the absorption of calcium, iron, zinc and other minerals, improve its bioavailability; promote intestinal lactic acid bacteria, especially bifidobacteria reproduction, improve the micro-ecological balance of the human body, and promote intestinal bacteria synthesis of B vitamins. Some people do not drink cow's milk for many years after adulthood, the lactase activity in the body is very low, can not decompose lactose, lactose in the intestine by intestinal microbial decomposition and fermentation, resulting in flatulence, diarrhea and other symptoms, known as lactose intolerance. This part of the population can consume lactase-treated milk powder, or drink yogurt. 4, vitamins cow's milk contains almost all kinds of vitamins needed by the human body, and its content is related to the way of feeding, season, processing methods. Such as grazing period in cow's milk vitamin A, vitamin D, carotene and vitamin C content than winter and spring in the shed feeding significantly more. Vitamin D content in cow's milk is low, but there is some increase in summer when there is more sunshine. Overall, cow's milk is a good source of B vitamins, can provide a considerable amount of riboflavin, vitamin B12, vitamin A, vitamin B6 and pantothenic acid. 5, minerals cow's milk in the mineral content of about 0.7% to 0.75%, rich in calcium, phosphorus, potassium, sulfur, magnesium and other macronutrients and copper, zinc, manganese, and other trace elements. 100mL cow's milk contains 110mg of calcium, three times that of human milk. It is a good source of calcium with high absorption rate. The ratio of calcium and phosphorus in cow's milk is 1.2:1. Cow's milk is also rich in potassium and magnesium, which is good for controlling blood pressure and has become the only alkaline food among animal foods. But the copper, iron content is low, must be obtained from other foods. 6, other substances in the milk there are a large number of physiologically active substances, of which the more important are lactoferrin, immunoglobulin, biologically active peptides, *** conjugated linoleic acid, tyrosine, hormones, growth factors and a variety of active peptides and so on. Bioactive peptides are produced by protease hydrolysis of milk proteins during digestion, including sedative and tranquilizing peptides, anti-hypertensive peptides, immunomodulatory peptides and antimicrobial peptides. The content of lactoferrin in cow's milk is 20~200ug/mL, which has the function of regulating iron metabolism, promoting growth and antioxidant, etc. The peptide fragments formed by protease hydrolysis have certain immunomodulatory effects.