Milk is a nutrient-rich liquid food produced by mammals, such as cows, goats, and sheep. It is composed of several constituents, including casein (a type of PROTEIN), LACTOSE (a type of sugar), and PROTEIN. Casein is a phosphoprotein found in milk, making up about 80% of the proteins in cow's milk. LACTOSE is a disaccharide sugar found in milk, which makes up around 2-8% of milk's solids. PROTEIN is an essential macronutrient found in milk, making up around 3.2-3.5% of milk's solids.