74 EXPERIMENT STATION EECOED. 



" Four distinct groups of bacteria, tlie acid-forming, the inert, the alkali- 

 forming, and the peptonizing, survive pasteurization, as differentiated by their 

 reactions in litmus milk after 14 days' incubaton at 30° C. (86° F.). 



" The percentage of the acid group is increased by pasteurization while the 

 other groups are decreased in their percentage of the total bacteria. The aver- 

 age percentage of lactic acid bacteria in raw milk which survive pasteurization 

 for 30 minutes at C2.S° C. ranges from 1.27 per cent to 4.55 per cent in the 

 various grades of milk. 



" When the temperature of pasteurization is below 76.7° C. (170° F.) the bac- 

 terial flora of milk is composed for the most part of organisms of the acid group. 

 At temperatures above 76.7° C. the acid group is largely destroyed and the 

 majority of the bacteria which survive are of the peptonizing type. These bac- 

 terial group relations are clearly shown. 



'* When different grades of milk are pasteurized at 62.8° C. in the laboratory 

 and held at room temperature the bacterial flora may undergo three distinct 

 changes. I'irst, when a fair quality of milk is pasteurized the acid group may 

 develop at once and overgrow all the other groups, forming acid and producing 

 a normal curd. Second, when a poor quality of milk is pasteurized the peptoniz- 

 ing group may grow rapidly at first along with the acid group, which later over- 

 grows them. In this case the milk will first become curdled with a rennet curd 

 due to the peptonizing bacteria, then later will become sour from the develop- 

 ment of the lactic-acid group of organisms. Third, when a good grade of milk 

 is pasteurized the peptonizing bacteria may overgrow the acid group of organ- 

 isms so that the milk becomes peptonized without the development of any acid. 

 These same grades of milk, treated in the same manner but held in an ice chest 

 at 10° C, show entirely different changes in their bacterial contents. The 

 growth of the peptonizing group is restrained so that they are of little impor- 

 tance. The percentage of the acid group remains about the same through a 

 long period. Occasionally the percentage of the alkali group may Increase 

 after five days, but eventually the acid group forms the major group. These 

 results were obtained from laboratory experiments and only indicate possible 

 changes in the bacterial flora of pasteurized milk when held at different tem- 

 peratures. They show the delicate balance between the bacterial groups, but 

 can not be applied to indicate the bacterial changes in milk pasteurized and 

 handled under commercial conditions. 



"A qualitative study of the acid group shows that many organisms giving 

 characteristics of the typical lactic-acid bacteria survive pasteurization for 30 

 minutes at 62.8° C. Other lactic-acid forming bacteria survive which may form 

 pigment or vary from the typical lactic types in the fermentative reactions. 



" The inert group was made of organisms which produce no change in lit- 

 mus milk during 14 days' incubation, and therefore include many which did not 

 grow, but which according to the method of differentiation would be included 

 as inert forms. The only truly inert form recognized was a yellow pigment- 

 forming organism. The inert group is probably of little importance. 



" The alkali group which survive pasteurization is of importance. Bacteria 

 of this group can not be differentiated from those of the inert group or from 

 slow acid-formLng organisms by plating, but are easily determined by the milk- 

 tube method. They grow at low temperatures and produce a strong alkaline 

 reaction in milk. In pure cultures enough alkali is produced to dissolve the 

 casein. No acid is produced in fermentation broths, but often an alkaline reac- 

 tion is found. 



"The peptonizing group includes those organisms which peptonize casein. 

 A number of the organisms isolated peptonized casein, but did not liquefy gela- 



