Preservation of Milk. 123 



the milk with the least acid was, as a rule, the freest from 

 spore-bearing bacteria. 



This acid determination can be made at the weigh-can 

 by employing the Farrington alkaline tablet which is used 

 in cream-ripening. Where milk is pasteurized under gen- 

 eral creamery conditions, none should be used containing 

 more than 0.2 per cent acidity. If only perfectly fresh milk 

 is used, the amount of acid will generally be about 0.15 per 

 cent with phenolphthalein as indicator. 



Emphasis has already been laid on the selection of a 

 proper limit of pasteurizing (p. 114). It should be kept 

 constantly in mind that the thermal death-point of any 

 organism depends not alone on the temperature used, but on 

 the period of exposure. With the limits given, 140 to 155 

 F., it is necessary to expose the milk for not less than fif- 

 teen minutes. If a higher heat is employed (and the 

 cooked flavor disregarded) the period of exposure may be 

 curtailed. 



Chilling the milk. It is very essential in pasteurizing 

 that the heated milk be immediately chilled in order to 

 prevent the germination of the resistant spores, for if ger- 

 mination once occurs, growth can go on at relatively low 

 temperatures. 



The following experiments by Marshall ' are of interest 

 as showing the influence of refrigeration on germination 

 of spores : 



Cultures of organisms that had been isolated from pas- 

 teurized milk were inoculated into bouillon. One set was 

 left to grow at room temperature, another was pasteurized 

 % and allowed to stand at same temperature, while another 

 heated set was kept in a refrigerator. The unheated cul- 



Marshall, Mich. Expt. Stat., Bull. 147, p. 47. 



