Casein is a type of curd that is difficult to digest and break down and has a high molecular weight and dense structure. Casein is the main protein in mammalian dairy products, including cow’s milk, goat’s milk and human milk, and is mainly composed of α, β, γ and κ casein. Casein molecules are large and dense, and are not easily digested and broken down for absorption in the human digestive tract. Due to this property of casein, casein aggregates into clusters in the stomach and slowly passes through the intestinal tract for absorption, ensuring that amino acids enter the bloodstream at a low but steady rate, so as not to cause dramatic fluctuations in amino acid concentrations in the blood. Casein is a phosphorus- and calcium-bound protein that is acid-sensitive and precipitates at low pH. Although casein is not easily digested and absorbed, it has high nutritional value as a supplement of essential amino acids and as a source of calcium and phosphorus. In human milk, β-casein is the main form of casein, and β-casein is more easily digested than α-casein.