THE RELATION OF MICROORGANISMS TO MILK 463 



mately of the total number in the milk. From the standpoint of the 

 milk consumer milk ceases to be of value when the end of this period is 

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

 certain lines of dairy manufactures, notably cheese making. 



THIRD STAGE. PERIOD FROM TIME or 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 millions 

 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 remained more or less dormant in the milk during 

 the earlier stages now begin to grow. The organisms especially impor- 

 tant in this stage are Oidium lactis, 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 decomposi- 

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

 mentation are the B. coli communis and the Bact. lactis aero genes 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 mem- 



