CAUSES OF RIPENING CHANGES 361 



come the water-soluble proteins and protein deriva- 

 tives that are found in cheese. Much work yet re- 

 mains to be done before all the details of the action 

 of acid in cheese-making are fully understood. 



ACTION OF RENNET-ENZYM IN CHEESE- 

 RIPENING 



For a long time, there was doubt as to whether 

 rennet-extract had anything to do with cheese-ripen- 

 ing. It may be now regarded as definitely settled 

 that rennet-extract contains a peptic ferment which 

 has a curd-dissolving power. This fact has nothing 

 necessarily to do with the question as to whether the 

 peptic enzym is the same as the coagulating enzym, 

 or whether two different enzyms, each with a dif- 

 ferent function, are present. The action of rennet 

 in cheese-ripening is quite similar to that of a pepsin 

 digestion. There is one important condition for the 

 peptic action of the rennet-enzym the presence of 

 an amount of acid or acid salts, corresponding to 

 about 0.3 per cent of lactic acid. The acid produced 

 in cheese-curd and cheese furnishes the needed con- 

 ditions. Whether this is the chief function of acid 

 in connection with the formation of water-soluble 

 proteins and derived proteins in cheese-ripening, or 

 whether the salts formed by lactic acid exercise some 

 influence apart from rennet action, may not be re- 

 garded as satisfactorily determined at the present time. 



In order to study the effect of rennet-enzym in dis- 

 solving the insoluble protein of cheese-curd, it is 

 necessary to destroy the enzyms and micro-organisms 

 present in milk. This is done by heating the milk 

 to a temperature of 185 to 208 F., after which the 



