534 MILK 



(145 F.) for thirty minutes is kept in an ice-chest at 10 C. 

 (50 F.) the changes produced by bacteria are different. Pep- 

 tonizing bacteria multiply slowly, if at all; the relative number of 

 lactic acid bacteria remains the same for some time and alkali- 

 forming bacteria may increase after five days. Ultimately the 

 acid group predominates. 



The authors determined the thermal death-points of 64 acid- 

 forming bacteria. One culture in broth required a temperature 

 of 79.4 C. (175 F.) for thirty minutes for its complete destruc- 

 tion. In milk it was necessary to expose a certain strain of 

 Streptococcus lacticus to a temperature of 75.6 C. (168 F.) for 

 thirty minutes to accomplish destruction. 



Ayers thinks there are two classes of streptococci which sur- 

 vive pasteurization, namely: 1, Streptococci which have a low 

 majority thermal death-point and among which there are a few 

 individuals that survive pasteurization temperature, and 2, 

 streptococci which have a high majority thermal death-point and 

 survive because this point is above the temperature of pasteuriza- 

 tion. 



Prolonged heating at 54.4 C. (130 F.) increases the relative 

 destruction of bacteria. However, in milk pasteurized at 62.8 C. 

 (145 F.) application of heat for six hours does not produce greater 

 destruction of bacteria than heating for thirty minutes. 



This work of Ayers and Johnson has thrown the first light on 

 the changes which take place in the bacterial flora during pas- 

 teurization. It shows clearly that some lactic acid bacteria sur- 

 vive, with the result that pasteurized milk usually sours nor: lally. 

 In exceptional cases efficiently pasteurized milk does not turn 

 sour, but is peptonized. This same phenomenon happens in very 

 clean raw milk. 



The comprehensive investigations of Ayers and Johnson were 

 made under laboratory conditions. Therefore only tentative con- 

 clusions can be drawn as to the effect of commercial pasteurization 

 upon bacteria. However, the results are so clear cut that it seems 

 reasonable to assume that they apply, in a measure at least, to 

 efficient pasteurization. 



In a more recent paper the same authors have published 

 further investigations on the ability of streptococci to survive 

 pasteurization. Strains, 139 in number, were isolated from cow 

 feces, from the udder and mouth of the cow, and from milk and 

 cream. At 60 C. (140 F.) 64.03 per cent, survived; at 62.8 C. 

 (145 F.) 33.07 per cent, survived, and at 71. l b C. (160 F.) 

 2.58 per cent, survived. All were destroyed at 73.9 C. (165 F.). 

 The strains from milk and cream were more resistant than strains 

 from other sources. Of the milk and cream strains, 100 per cent. 



