Casein is the predominant phosphoprotein found in fresh milk. When coagulated with rennet, casein is sometimes called paracasein. British terminology, on the other hand, uses the term caseinogen for the uncoagulated protein and casein for the coagulated protein. As it exists in milk, it is a salt of calcium.
Casein is not coagulated by heat. It is precipitated by acids and by rennin, a proteolytic enzyme obtained from the stomachs of calves. The enzyme trypsin can hydrolyze off a phosphate-containing peptone. In addition to being consumed in milk, casein is used in adhesives, binders, protective coatings, and other products such as knitting needles. The isoelectric point of casein is 4.6. The purified protein is water insoluble. While it is also insoluble in neutral salt solutions, it is readily dispersable in dilute alkalis and in salt solutions such as sodium oxalate and sodium acetate.
Casein is a mixture of phosphoproteins found in milk to the extent of about 3%. It contains all of the common amino acids an is high in the essential ones. Casein is often used as a dietary base for use in evaluation of vitamins, since it is easy to prepare casein as a pure protein. Casein is usually prepared by acid precipitation. It exists in milk as the water soluble calcium salt of a phosphoprotein. Acid treatment removes the calcium cation, leaving a water insoluble phosphoprotein. Generally acetic acid is used because it is less harsh than hydrochloric acid. Most people extracting casein in the form of a school experiment do so with the solution cold. Such practice leads to the formation of casein with poor physical properties. Softness and rather incomplete separation of the casein are problems encountered.
However, if the precipitation is performed warm, the casein separates as a single large colloid, with complete separation, leaving a yellow solution of whey behind. As a side note, whey primarily consists of milk-sugar, lactose. Decent casein precipitation is delicate. If too much acid is added, or if the acid is added too quickly, or if the acid is too strong, part of the casein will redissolve. Casein is amphoteric, that is, it forms salts with both acids and alkalines.