PASTEURIZATION OF MILK 193 



more difficult to digest and assimilate than raw milk, and 

 frequently children with weak digestive powers do not 

 flourish when fed upon such milk. This is a serious 

 matter and has prevented the widely extended use of 

 sterilized milk. Because of these objections the practice 

 of sterilizing has not increased in the last few years as it 

 was once believed it would. 



6. Pasteurization. — The objections to boiling have led 

 to the adoption of a different method of treating milk for 

 the purpose of accomplishing the same results. Pasteur- 

 izing is also dependent upon the use of heat, but a lower 

 degree than that of boiling is used. The temperature 

 adopted for this purpose is commonly between 155° and 

 170°, sometimes running slightly above or below these 

 limits, and the milk should be kept at this temperature 

 from ten minutes to half an hour. It must then be cooled 

 rapidly. 



It may seem strange that the use of a lower tempera- 

 ture than boiling should be more satisfactory than boiling. 

 The reasons, however, are simple enough. The bacteria 

 which sour the milk do not produce spores and are, there- 

 fore, nearly all killed at a temperature of 155°. Conse- 

 quently pasteurized milk will keep much longer than 

 unpasteurized milk. Furthermore, all disease germs that 

 are most liable to be present are also destroyed by this 

 low temperature. The diseases ordinarily distributed by 

 milk are produced by bacteria that do not develop spores, 

 and a temperature of 160° is sufficient to destroy such 

 bacteria. Moreover, the disagreeable changes produced 

 by boiling do not appear at the pasteurizing temperatures. 

 Pasteurized milk does not have the taste of boiled milk, 



