354 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



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 gram 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 con- 



A B 



FIG. 83. Proteolytic action of rennet extract in the absence and in the presence of 



acid-forming bacteria. A, sterile milk agar; a strip of filter paper treated with rennet 



was allowed to remain on the medium for i hour at 37. No digestion of the casein. 



B, milk agar inoculated with Bad. lactis acidi; incubated for 24 hours at 37, then 



reated as A. True digestion of the casein is indicated by the clearing. (Original.) 



tains the enzyme pepsin, which for its action outside the body requires con- 

 ditions similar to those which obtain in the stomach; in other words, the 

 presence of sufficient acid to activate it. The hydrochloric 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 decomposition products such as albu- 

 moses and peptones. In the absence of acid-forming bacteria no acid is 

 formed; consequently the pepsin does not become active and no proteo- 

 lytic effect is produced. Under these conditions the curd remains tough 

 and elastic and the solubility is not increased. It is thus evident that 



