Milk is the nutritional fluid secreted by the mammary gland. The cow's milk contains significant amounts of carbohydrate (approx. 4.6%), fat approx. 4.3%) and protein (approx. 3.3%) and also represents an important source of calcium. Water is the main constituent of milk and much milk processing is designed to remove water from milk or reduce the moisture content of the product. Proteins are an extremely important class of naturally occurring compounds that are essential to all life processes. They perform a variety of functions in living organisms ranging from providing structure to reproduction. Milk proteins represent one of the greatest contributions of milk to human nutrition. Milk is not only a source of nutrient lactation, but also the product that provides the body a wide spectrum of bioactive components, with multiple physiological activities in the gastrointestinal tract.