70 DAIRY TECHNOLOGY 



bacteria inhibit the growth of the putrefactive organisms, 

 but in pasteurized milk the spore-producing putrefactive 

 bacteria have a clearer field for growth, and may develop 

 poisonous substances without changing the appearance or 

 physical condition of the milk. 



Ayers and Johnson 1 have found that milk pasteurized 

 under commercial conditions sours because of the develop- 

 ment of lactic-acid bacteria which, on account of their 

 high thermal death point, survive pasteurization; and per- 



FIG. 2. Combination heating, holding, and cooling tank. 

 (Circular 184, Dairy Division, U. S. Dept. of Agr.) 



haps, in some cases, because of subsequent infection with 

 acid-forming bacteria, during cooling and bottling. The 

 nature of the souring of efficiently pasteurized milk (140 

 F. for 30 minutes or 160 F. for a few seconds) is similar 

 to that of clean raw milk. In both classes of milk, accord- 

 ing to the same authorities, the alkali or inert bacteria 

 constitute the largest group, the lactic acid next, and the 

 peptonizers the smallest group. As these milks are held, 

 the lactic-acid group gains ascendency over the other two 

 classes, while the peptonizers remain the smallest group. 

 1 U. S. Dept. of Agr., Bu. An. Ind., Bui. 126. 



