Principles of Milk Preservation. 103 



With either condition fixed, the same result is accom- 

 plished more rapidly by an increase either in temperature 

 or duration of exposure, so that an exposure for a longer 

 time at a lower temperature is quite as effective as a 

 shorter exposure at a relatively higher temperature. 



With the great majority of bacterial species that have 

 been individually tested as to their thermal death-point, 

 140 F. for ten minutes has been found fatal. This de- 

 gree of heat suffices to kill all the disease-producing bac- 

 teria that are found in milk, with the exception of the 

 tubercle bacillus. 



According to Forster, heating thirty minutes at 149 F. , 

 fifteen minutes at 155 F., or ten minutes at 167 F. 

 suffices to destroy this germ. 



The maximum temperature that can be employed is just 

 below the point at which the milk will permanently ac- 

 quire a cooked flavor. The appearance of this peculiar 

 flavor cannot be detected with absolute accuracy, but it 

 is not far from 158 F. It appears in milk to some ex- 

 tent before this temperature is reached but upon chilling 

 disappears. It is not enough to heat milk to this tem- 

 perature, but it should be maintained for the requisite 

 length of time. 



1 04. Subsequent chilling". The heating process de- 

 stroys only the vegetative organisms, the spores resisting 

 this temperature. To prevent the germination of these 

 latent forms the milk should be quickly chilled, for if ger- 

 mination once occurs, they can develop' at even low tem- 

 peratures. 



The following experiments by Marshall 1 are of interest 

 as showing the influence of refrigeration on germination 

 of spores. 



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



