RELATION OF MICROORGANISMS TO CHEESE. 355 



acid-forming bacteria are essential factors in cheese ripening. The 

 pepsin of the rennet extract and the galactase suffice to account for the 

 initial proteolysis of the paracasein. Since neither of these enzymes 

 forms ammonia, which is always found in ripe cheeses, the origin of this 

 simple decomposition product of protein is still unexplained. It may 

 owe its origin to microorganisms not yet discovered. 



Prevention of Putrefaction. The various stages in the decomposition 

 of milk have been outlined in a previous chapter. Briefly they are as 

 follows: The first evident change is the curdling due to the acid-forming 

 bacteria. Succeeding this, the acid, semi-solid mass or curd is a favorable 

 substratum for the characteristic mold of milk, Oidium lactis, which soon 

 forms a white, velvet-like layer over the surface of the milk. Like other 

 molds, this form can use acids as a source of energy. The acid is then 

 oxidized to carbon dioxide and water, and thus the reaction of the milk 

 is slowly changed until a point is reached which allows the putrefactive 

 bacteria, that have remained dormant during the period of unfavorable 

 environment, to develop. The curd is accordingly peptonized and 

 putrefaction occurs. If the acid reaction is maintained through the 

 prevention of mold growth, the milk will be preserved from the attacks 

 of putrefactive organisms and will remain unchanged for an unlimited 

 time. 



The second role of the acid-forming bacteria in cheese is to protect 

 it against the putrefactive organisms that are constantly present in milk 

 and hence in cheese. The acid reaction of the cheese, due to the per- 

 sistence of lactic acid, or to the formation of volatile acids after the initial 

 fermentation, is sufficient to prevent the growth of the putrefactive bac- 

 teria within the cheese. If the cheese is made from milk which contains 

 no acid-forming bacteria and few putrefactive ones, or if the sugar is 

 removed from the curd by washing it with water, the cheese will not 

 ripen since there is no acid to activate the pepsin; the curd will remain in 

 much the same condition as when it was removed from the press. Cheese 

 made from milk containing no acid-forming bacteria but many putre- 

 factive bacteria is likely to undergo putrefaction, since the latter class 

 of organisms finds conditions for growth in the absence of an acid re- 

 action. Such a condition is rarely noted in a hard cheese under normal 

 conditions, but may be produced experimentally. The biological acid 

 may be replaced by other acids added to the curd in appropriate amounts, 

 since these will activate the pepsin and protect the cheese against the 



