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Coagulation of milk is a fundamental process in cheesemaking and coagulating enzymes have been utilised for centuries in cheese manufacture. Traditionally, enzyme coagulants were derived from animal sources, principally calf stomachs, although coagulants produced from a range of plant species also appear to have been used for nearly as long. In more recent times an inadequate and decreasing supply of calf coagulant stimulated interest in alternative milk-clotting systems. Consequently a range of substitute milk clotting enzymes from other animals, plants and microbial sources have been developed together with biotechnologically derived coagulants. However under cheese manufacturing conditions the performance of each coagulant will vary slightly and directly affect cheese quality. In this article various coagulants are assessed and aspects of their use in cheesemaking examined.
Cheese manufacture is basically a dehydration process in which milk is acidified by starter organisms, coagulated and subjected to a cutting, cooking, stirring, salting and pressing regime to encourage moisture expulsion. The essential step in the manufacture of all cheese varieties is coagulation, in which casein, the major milk protein, forms a gel and entraps the milkfat if present. Coagulation can be initiated by protein breakdown through the action of specific enzymes, by acidification to about pH 4.6 or by acidification to a higher pH but in combination with a heating step (Fox 1993).
Rennet
The majority of cheeses produced around the world are manufactured using an enzymic coagulant traditionally extracted from the abomasum or fourth stomach of 10 to 30 day old, milk-fed calves. This extract, known as rennet, consists of two proteolytic enzymes: chymosin, the major component (88-94%) and bovine pepsin (6-12%). The relative proportion of these two enzymes varies with the age of the animal from which the rennet is extracted. Mature animal rennet extracts are made up of 90-94% pepsin and only 6-10% chymosin. Chymosin is classified as an aspartic protease and exhibits maximum activity under acidic conditions at around a pH of 3-4. Previously, chymosin was refered to as rennin, but to avoid any confusion between rennin and renin, a protease secreted by the kidney, the term chymosin has been adopted.
The major proportion of the protein in bovine milk consists of thousands of individual casein molecules interacting with calcium phosphate and arranged...