Rennin, an enzyme secreted by the young calves, brings about the coagulation of milk.The coagulaion is due to the change in the physico-chemical status of casein, as a result of the cleavage of a particular peptide bond (between phenylalanine and methoionine) in the K-casein fraction. This results in the release of a peptide. The remaining protein is no longer soluble and it cannot act as a stabilising agent for micellar proteins. Hence, an insoluble casein gel is formed. Rennin acts as a general proteolytic enzyme on all the protein components.
When casein is precipitated by the action of rennet, the calcium is not released to the whey but remains attached to the casein. Therefore, cheese made with rennet is a much better source of calcium than cheese made by acid precipitation alone. Cottage cheese is often made by acid precipitation.
Factors affecting rennin coagulation
Temperature: The optimum temperature is 40-42°C. Low temperature retards the reaction and produces a more tender coagulum. Higher temperature within the range of activity tends to toughen the coagulum. No action occurs below l0oC or above 650C.
Heating milk before enzyme action: Heating milk to temperature over 65°C prior to the addition of rennin retard or may entirely prevent the formation of firm clot. By heating, the casein is made more resistant to coagulation by rennin. Pasteurisation temperatures do not affect the action of rennin .
pH of the milk: Rennin acts best in a faintly acid medium at 5.8 pH but milk that is acidic enough to curdle is not coagulated by rennin. Rennin action does not occur in an alkaline medium.
Concentration of constituents: Diluting milk, dilutes the casein and calcium. Rennin reacts with diluted milk to form less firm clots and the reaction is noticeably retarded. An excess sugar may tend to prevent the formation of clot by rennin.
Salts: Monovalent ions reduce clotting tendencies and divalent and trivalent cations hasten coagulation.
Agitation: The clot formed by rennin is easily broken by stirring and cause sineresis or separation of the watery portion of the milk.
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