CAUSES OF RIPENING CHANGES 367 



(i) Rennet-extract contains an enzym which has 

 the power of digesting or dissolving the insoluble 

 protein in cheese. 



(2) Such digesting action by rennet-extract does 

 not take place in cheese which has been made without 

 any acid or acid salt in the milk and curd. 



(3) The digestive action of the enzym contained 

 in rennet-extract exerts its digesting power only in 

 the presence of acids or acid salts. In the case of 

 normal cheese, the acid formed in the cheese-making 

 process is lactic acid, which, however, does not act 

 as free acid, since it reacts with calcium salts, form- 

 ing neutral calcium lactate and calcium acid phosphate 

 and, probably, citrate. The acid salts enable the ren- 

 net-enzym to exert its digesting power. The same 

 general result may be accomplished by adding a free 

 acid or an acid salt to milk during the cheese-making 

 process. 



(4) The extent to which the digesting enzym of 

 rennet-extract can act depends largely upon the degree 

 of acidity developed in the cheese-making process. It 

 is probable that no action begins until the equivalent 

 of 0.30 per cent of lactic acid has been formed. 



(5) The products formed by rennet digestion of 

 cheese proteins are largely confined to the bodies 

 known as caseoses and peptones, only small 

 amounts of amino acids being formed and little or 

 no ammonia. 



(6) Increased use of rennet-extract in cheese- 

 making results in a more rapid formation of water- 

 soluble protein compounds. This is not due, as some 

 formerly thought, to an increased amount of water in 

 cheese, which was supposed to be a necessary result 



