496 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



the attacks of putrefactive organisms and will remain unchanged for an 

 unlimited time. 



The second r61e 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 

 persistence of lactic acid, or to the formation of volatile acids after the 

 initial fermentation, is sufficient to prevent the growth of the putrefac- 

 tive bacteria 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 re- 

 main in much the same condition as when it was removed from the 

 press. Cheese made from milk containing no acid-forming bacteria but 

 many putrefactive bacteria is likely to undergo putrefaction, since the 

 latter class of organisms finds conditions for growth in the absence of 

 an acid reaction. 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 attacks of putrefactive bacteria; but it is not cer- 

 tain that the cheese will develop a normal flavor when lactic acid is 

 replaced by mineral acids. 



Other Groups of Bacteria in Cheese. It has been shown at the Wis- 

 consin Experiment Station that other groups of bacteria are constantly 

 present in Cheddar cheese. The development of certain members of the 

 Bact. bulgaricum group occurs somewhat later than that of the Bad. 

 lactis acidi group. It occurs largely after the sugar has disappeared. 

 Their numbers approximate those of the Bact. lactis acidi group. 

 Coccus forms are also found in great numbers in the cheese. It is 

 probable that these two groups may be responsible for the ammonia 

 production, since typical cultures of both groups are able to produce 

 small amounts of ammonia in sterile milk. 



Flavor Production in Cheese. The factors that have been discussed 

 are undoubtedly the most important ones concerned in the proteolysis 

 of the curd, and are thus the factors concerned in the changes of texture, 

 solubility and digestibility. The flavor, which develops during the 

 ripening process, has been regarded as due to the proteolysis of the para- 

 casein. A thoroughly ripened cheese contains a large amount of am- 



