METHODS OF REDUCING THE GERM CONTENT 531 



tion of milk. The most commonly advanced one is the supposed 

 destruction of lactic acid bacteria and the survival of spores of 

 peptonizing bacteria which are said to produce poisonous sub- 

 stances. This objection is based largely on the much discussed 

 work of Fliigge, who isolated spore-bearing bacteria from heated 

 milk, and after cultivation injected them into rabbits. Intoxica- 

 tion followed in some cases. It is usually assumed that lactic 

 acid bacteria are "beneficent" or "nature's danger signal," etc. 

 It is true that the acid produced by this group of bacteria restrains 

 many other varieties, and even may destroy them when a suffi- 

 cient amount of acid has accumulated to coagulate the casein and 

 impart a decidedly sour taste to the milk. On the other hand, it 

 has never been conclusively proved that peptonizing bacteria 

 actually produce poison. Flugge's work shows that large num- 

 bers may produce intoxication in rabbits, an observation which 

 cannot be applied to human beings without further evidence. 

 But if peptonizing bacteria do produce poisons they will do so in 

 raw as well as in pasteurized milk, and most bacterial poisons are 

 destroyed at 60 C. 



What changes in the bacterial flora of milk actually take place 

 during pasteurization was a matter of speculation before Ayers 

 and Johnson made their exhaustive study. The results of this 

 study are of great importance, and some space can profitably be 

 devoted to them. The work of Ayers and Johnson is peculiarly 

 exhaustive, and has been confirmed by many observations since 

 it was published. Study of individual types of bacteria in milk 

 is difficult, time consuming, and sometimes misleading. The 

 authors therefore classified milk bacteria into the following five 

 groups: 1, Acid-coagulating; 2, acid, not coagulating; 3, inert; 

 4, alkali forming; 5, peptonizing. Grouping was based on the 

 study of the reactions in litmus milk. Each colony on suitable 

 plates prepared from milk was transferred to a tube of litmus milk, 

 incubated at 30 C., and kept under observation for fourteen days. 

 Plates were prepared from raw milk and from the same milk after 

 heating for thirty minutes at the desired, temperature. The ac- 

 companying charts (Figs. 216, 217) graphically show the results. 



The following important facts are demonstrated by this work: 

 In raw milk the inert group is the largest. After pasteurization 

 at 62.8 C. (145 F.) for thirty minutes the relative number of 

 acid-forming bacteria has greatly increased, while the relative 

 number of alkali-forming and peptonizing bacteria has decreased. 

 At 71.1 C. (160 F.) the percentage of acid-forming bacteria is 

 still the largest, but they have been affected by the heat so that 

 the majority coagulate milk slowly. Alkali-forming and pep- 

 tonizing bacteria represent the smallest groups. After pas- 



