In many cheese factories, chymosin is added to the milk immediately after or nearly at the same time as the culture is added. Some cheese makers allow a pre-ripening period to give the culture a brief opportunity to produce a small amount of acid and a slight lowering of the milk pH. Since chymosin is an acid protease (its optimum activity on K-casein occurs at pH 5.5), it will be more active as milk pH decreases.The solubility of calcium also increases as the pH decreases. Thus, with pre-ripening, less chy-mosin can be used to give the same clot firmness. For similar reasons, it is also common to add calcium chloride to the milk to promote coagulation (and yield).
In any event, the amount of chymosin added, and the length of the setting period prior to cutting, depends on the cheese being made and the curd firmness desired. Usually, about 200 ml of single-strength chymosin per 1,000 kilograms of milk will give a suitable coagulation within about 30 minutes. For many large, automated manufacturers, the point at which the curd is sufficiently firm and ready for cutting is based strictly on the clock (but also on a priori knowledge of what times give the best cheese). Although specialized instruments are available for this purpose, curd firmness is more often than not determined on a subjective basis, i.e., when the operator deems it ready based on a simple cutting method.
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