Clotting process, milk

Two biologically related processes which have received attention for many years are the clottings of milk and blood. 

Chymosin is secreted by the abomasum in the form of an intially inactive precursor, prochymosin, which is converted autocatalytically to chymosin (see below, p. 176). Many reviews on the properties of the zymogen and the enzyme have been published. One of the most recent, by Foltmann (J), also contains references to earlier reviews. The action of chymosin on K-casein is primarily responsible for the milk-clotting process. The numerous studies of this reaction have been reviewed recently by Mackinlay and Wake (32). Because of the number of diflFer-ent milk proteins and the complexity of their interactions, especially in the casein micelle, the full details of the coagulation process are not yet understood. 

None of the models satisfy all of the above requirements for micelle structure. However, a discussion of the mechanism of milk coagulation in terms of some of the models may prove illuminating (2). A number of models for micelle structure have been proposed four models requiring various clotting mechanisms are discussed here. Since any mechanism of milk clotting must be predicated on the correct micelle model, it will become evident from the ensuing discussion that much additional research is necessary to fully understand the clotting process. 

Cheese production is a traditional and effective method of preserving much of the food value of milk. There are at least 400 varieties of cheese but the process of cheese making always involves the clotting of milk by rennin at some stage. 

Cheese making—During this process, milk is clotted by either acid or rennet (an enzyme-containing substance derived from animal stomachs) so that it separates into cheese and whey. Usually, much more calcium and phosphorus are lost in the whey from acid-clotted items like cottage cheese than from the rennet-clotted cheeses like Cheddar and Swiss. 

The use of chymosin in cheesemaking has been known long before the terms ferment or enzyme were conceived. It was also one of the first enzymes to be named (5). Extensive literature exists, especially about the milk clotting process the biochemistry of chymosin has been reviewed by one of the authors (6). 

Activation of prochymosin involves the splitting of peptides from the N-terminal end of prochymosin with simultaneous reduction in molecular weight from about 36,000 to 31,000. The rate of conversion increases markedly with decreasing pH below 5.0 (Rand and Emstrom 1964). At pH 5.0, NaCl concentrations up to 2M increase the rate of activation. Milk-clotting activity plotted against activation time at pH 5.0 shows the course of activation (Fig. 12.1) to be autocatalytic. If activation is carried out in the presence of preformed chymosin, the S-shape disappears and the initial rate of the activation process increases with increasing concentration of preformed chymosin. Folt-... 

Milk-clotting is a complex process, involving a primary enzymic phase in which K-casein is altered and loses its ability to stabilize the remainder of the caseinate complex, a secondary non-enzymic phase in which aggregation of the altered caseinate takes place, a third step where the aggregate of casein micelles forms a firm gel structure and a possibly separate fourth step where the curd structure tightens and syneresis occurs (McMahon and Brown 1984B). 

The most widely used animal enzyme is chymosin which is used for milk clotting in the production of cheese. Well known plant enzymes include papain, bromelain and cereal malt. Microbial enzymes have been used in the fruit and cereal processing industries since the 1950 s and offer a less expensive source. Eor example, chymosin (a relatively expensive enzyme found in the stomach of calves) have been replaced by the microbial rennet in the production of cheese. 

Such processes have existed for many thousands of years in the manufacture of cheese from milk and in the brewing of beer from barley. In the former process the enzyme chymosin catalyses the hydrolysis of one peptide bond in casein, causing the milk to clot, and in the latter the amylases from malted barley (the diastases of Peyen and Persoz) catalyse the hydrolysis of starch. While these remain important processes the application of enzymes such as these to industrial chemistry is quite recent, dating back only to about 1960. 

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