THE RELATION OF MICROORGANISMS TO CHEESE 429 



the galactase suffice to account for the initial proteolysis of the para- 

 casein. Since neither of these enzymes forms ammonia, which is 

 always found in ripe cheeses, some other factor must be responsible 

 for the production of this compound. It may owe its origin to micro- 

 organisms 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 curdhng due to the acid-forming 

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

 able substratum for the characteristic mold of miik,Oidiumlactis,w]Ach 



A B 



Fig. 136. — 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 one hour at 37°. No digestion of the 

 casein. B, milk agar inoculated with Bact. lactis acidi; incubated for twenty-four 

 hours at 37°, then treated as A. True digestion of the casein is indicated by the 

 clearing. (Original.) 



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 putre- 

 factive bacteria, that have remained dormant during the period of un- 

 favorable environment, to develop. The curd is accordingly pepton- 

 ized and putrefaction occurs. If the acid reaction is maintained 

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



