CASEIN AND SOURING OF MILK

There are three kinds of proteins in milk: casein, lactalbumins, and lactoglobulins. All three are globular proteins, which tend to fold back on themselves into compact, nearly spheroidal units and are more easily solubilizied in water as collodial suspensions than fibrous proteins are. They are “complete proteins”, so called because they contain all the amino acids essential for building blood and tissue, and they can sustain life and provide normal growth even if they are the only proteins, but they can contain greater amounts of amino acids than the proteins in egg and meat.

Casein, the main protein in milk, is a phosphoprotein. The phosphate groups are attached to the hydroxyl groups of some of the amino acid side chains. Casein exists in milk as the calcium salt, calcium caseinate. It is actually a mixture of at least three similar proteins which differ primarily in molecular weight and the amount of phosphorus groups they contain ( α, β and κ caseins), they form a micelle, or a solublized unit. Neither the α nor the β casein is soluble in milk and neither is soluble either singly or in combination. If κ casein is added to either one, or to a combination of the two, however, the result is a casein complex that is soluble owing to the formation of the micelle.

  

Calcium caseinate has an isoelectric point of pH 4.6. Therefore, it is insoluble in solutions of pH less than 4.6. The pH of milk is about 6.6; therefore, casein has negative charge at this pH and is solubilized as a salt. If acid is added to milk, the negative charges on the outer surface of the casein micelles are neutralized (by protonation of the phosphate groups) and the neutral protein precipitates, with the calcium ions remaining in solution:

Ca-caseinate + 2H⁺  → casein + Ca²⁺

A natural example of this process occurse when milk sours. The souring of milk is an intricate process started by the action of microorganisms on the principal carbohydrate in milk, lactose. The microorganisms hydrolyse the lactose into glucose and galactose. Once galactose has been formed, lactobacilli, a strain of bacteria present in milk, convert it to the sour-tasting lactic acid. Since the production of the lactic acid also lowers the pH of the milk, the milk clots when it sours due to the precipitation of casein. The structure of lactic acid is shown below 

Many "cultured" milk products are manufactured by allowing milk to sour before it is processed. For instance, milk or cream is usually allowed to sour somewhat by lactic acid bacteria before it is churned  to make butter. The fluid left after the milk is churned is sour and is called buttermilk. Other cultured milk products include sour cream, yogurt, and certain types of cheese.