RELATION OF MICROORGANISMS TO CHEESE. 353 



in cheese is not sufficiently large, nor is their presence so constant that 

 any importance can be attached to them. Any agent to be considered 

 as a factor in the ripening process must be present in every cheese in 

 sufficient numbers to account for the change for which it is considered 

 responsible. Such agents should be capable of demonstration. It should 

 be remembered that, by following the rules laid down by the practical 

 maker, a normal cheese can invariably be made, hence the factors of 

 importance in the ripening must be constantly present in the milk or 

 rennet. It is doubtful whether the liquefying bacteria will satisfy this 

 requirement. It has been shown by de Freudenreich that such organisms, 

 even when added to milk in large numbers, exert no influence on the 

 ripening of hard cheese, since the conditions within the cheese are not 

 such that growth can occur. 



De Freudenreich, a Swiss microbiologist, by the aid of modern methods, 

 demonstrated the constant presence of certain classes of acid-forming 

 bacteria in Swiss cheese, and to them ascribed an important role in the 

 ripening of this hard cheese. He was led to this conclusion by their great 

 numbers in the fresh cheese, and by the fact that cheese made from milk 

 drawn under aseptic conditions, which thus contains no lactic bacteria, 

 do not ripen; through the discovery, also, that certain of the lactic bacteria 

 predominating in Swiss cheese, those of the Bact. bulgaricum group, exert 

 a solvent effect on the casein of milk, although they are devoid of action 

 on gelatin. 



Babcock and Russell demonstrated the presence of an inherent proteo- 

 lytic enzyme in milk, to which the term galactase was applied. This 

 enzyme can be demonstrated by preserving a sample of fresh milk with 

 chloroform or other mild antiseptic. At 37 curdling occurs in three to 

 four weeks; the content of soluble nitrogen in the milk is slowly augmented. 

 The presence of this proteolytic enzyme, together with the fact that a 

 normal cheese cannot be made from milk in which this enzyme has been 

 destroyed by heat, led these investigators to consider this inherent enzyme 

 of milk an important factor in cheese ripening. 



Present Knowledge of Causal Factors* The role of certain factors in 

 the ripening of Cheddar cheese has been established beyond doubt by the 

 chemical and bacteriological investigations of recent years. It is certain 

 that acid-forming bacteria are essential factors in the ripening of this 

 kind of hard cheese, and probably all kinds of rennet cheeses. 



* Cheddar cheese. 

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