396 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



reached, but there are further developments which are of importance in 

 certain lines of dairy manufactures, notably cheese making. 



Third Stage. Period from Time of Curdling until Acidity 

 IS Neutralized. — At the time milk curdles it contains enormous num- 

 bers of the lactic bacteria. The number usually runs into the miUions 

 and may be even higher than one thousand million per c.c. By the 

 time the coagulation takes place the acidity of the milk is so high that 

 the growth of the lactic organisms is checked and from this time on 

 their number decreases with more or less rapidity. 



During the period following the curdling certain other types of 

 organisms which have existed in the milk during the earlier stages now 

 begin to grow. The organisms especially important in this stage are 

 Oidium lacUs, certain species of molds, and yeasts. These organisms 

 are able to grow in a highly acid medium, and as a result of their 

 development the acid is decreased until the milk finally presents a 

 neutral or alkaline condition resulting from the decomposition of the 

 proteins in the milk. 



Fourth Stage. Final Decomposition Changes. — The reduction 

 of the acidity affords favorable conditions for the growth of certain types 

 of organisms which have remained in the milk during the earlier stages 

 but have been practically dormant. In this fourth stage the conditions 

 are suitable for the growth of the liquefying and peptonizing bacteria 

 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 nlore or less gas. The organisms most prominent in this t3rpe of fer- 

 mentation are the B. coli communis and the Bad. ladis aerogenes types. 

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

 produce Uttle 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 mem- 



