324 



MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



and they now grow rapidly, causing the decomposition of the casein. 

 The changes resulting from this type of organisms are of special signifi- 

 cance in cheese making and are discussed more fully in another chapter. 



ABNORMAL FERMENTATIONS IN MILK. 





 GASSY FERMENTATION. It frequently happens that instead of the 



normally rapid development of the Bad. lactis acidi type of organisms 

 in the milk, other acid producers develop rapidly, with the production 

 of more or less gas. The organisms most prominent in this type of 

 fermentation are the B. coll communis and the Bact. lactis aerogenes types. 

 This group of organisms contains a number of varieties, some of which 

 produce little or no gas while others develop large amounts. Their 

 action in milk is usually accompanied by disagreeable odors and flavors. 

 They grow readily in the presence of air and therefore develop abundant 

 colonies on the surface of plate cultures. This distinguishes the members 

 of this group quite clearly from those of the 

 true lactic group which grow chiefly below the 

 surface of the medium. The members of this 

 group do not form spores, but certain varieties 

 are quite resistant to heat and will oft-times sur- 

 vive pasteurizing temperatures which completely 

 destroy the Bact. lactis acidi group. They grow 

 most rapidly at high temperatures between 20 

 and 37. 



SWEET CURDLING FERMENTATION. This 

 phenomenon is caused by a variety of organisms 

 which cause the milk to coagulate without the 

 production of acid. The coagulation is brought 

 about by a rennet-like enzyme produced by this 

 type of bacteria. The resulting milk is either 

 neutral or alkaline in reaction. Usually the 



coagulation of the milk is followed by the digestion of the casein as a 

 result of another enzyme which is also produced by these bacteria. The 

 coagulation caused by these organisms is slower than in the case of the 

 acid formers and the curd is usually soft and mushy as compared with 

 the curd formed in the normal acid fermentation. The members of this 

 group get into the milk from and along with dust and dirt associated with 



FIG. 78.' Ropy cream 

 lifted with a fork. (After 

 Ward.} 



