428 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



Babcock and Russell demonstrated the presence of an inherent 

 proteolytic enzyme in milk, to which the term galactase was appUed. 

 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 investi- 

 gators to consider this inherent enzyme of milk an important factor 

 in cheese, ripening. 



Present Knowledge of Causal Factors* — The r61e of certain factors 

 in the ripening of Cheddar cheese has been established beyoiid 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. 



As has been shown the growth of acid-forming bacteria is rapid 

 during the making of Cheddar cheese. The growth continues during 

 the pressing and subsequent thereto; the maximum number of lactic 

 bacteria is found when the cheese is one to five days old. As many as 

 1,500,000,000 per g. of the moist cheese have been demonstrated. 



Causes of Proteolysis. — ^The proteolytic action of rennet extract on 

 the paracasein of cheese was demonstrated by Babcock and Russell, 

 and by Jensen. This property is due to the fact that rennet extract 

 also contains the enzyme pepsin, which for its action outside. the body 

 requires conditions similar to those which obtain in the stomach; 

 in other words, the presence of sufficient acid to activate it. The hydro- 

 chloric acid secreted by the walls of the stomach acts as the activating 

 agent in the body. The acidity resulting from the fermentation of 

 the sugar in the curd is sufficient to activate the pepsin. Under its 

 influence the paracasein is partially converted into soluble decomposi- 

 tion products such as albumoses and peptones. In the absence of 

 acid-forming bacteria no acid is formed; consequently the pepsin does 

 not become active and no proteolytic effect is produced. Under these 

 conditions the curd remains tough and elastic and the solubility is 

 not increased. It is thus evident that acid-forming bacteria are essen- 

 tial factors in cheese ripening. The pepsin of the rennet extract and 



* Cheddar cheese. 



