96 SCIENCE AND PRACTICE OF DAIRYING. 



enzymes formed by bacteria are destroyed. Perfect sterilization 

 can only be effected by submitting the milk to the action of 

 continuous heating for two hours at a temperature of 120 C., or 

 for 30 minutes at a temperature of 130 C., or when it is submitted 

 to intermittent heating at different high temperatures. The latter 

 method of treatment, the so-called intermittent sterilization, avoids 

 the heating of milk at temperatures over 100 C., and consists in 

 heating the milk for two hours at a time at a temperature of from 

 70 to 75 C., then keeping it for several days at a temperature 

 suitable for germ development, about 40 C., in order to permit 

 the spores which are left behind to germinate and to form vegetative 

 cells, then in order to destroy these to submit the milk for two 

 hours at a time to a temperature of 70 to 75 C., then again to 

 allow the milk to stand for several days at the same favourable 

 temperature, viz., 40 C. These consecutive changes of temperature 

 are repeated five times, one after the other, and at last the milk is 

 brought to a temperature of 100 C. 



In the above-mentioned treatment of milk, however, its proper- 

 ties undergo considerable changes. Among these changes is the 

 conversion of its soluble lime salts into an insoluble condition. The 

 result is that the milk no longer forms, when treated with rennet, a 

 cohesive coagulation; while it coagulates under the action of acids in 

 a fine, flocculent form. As a further result of this treatment, the fine 

 condition of division of the milk-fat is somewhat altered. A large 

 number of the fatty globules of the milk come together, and after 

 a time there collects on the surface of the milk a cream which 

 resembles butter, and which can no longer be uniformly broken up. 

 Finally the milk assumes a dirty brown yellowish colour and a 

 strong taste of boiled milk. All these undesirable changes, which 

 affect the keeping properties of milk, take place in different cases 

 more or less markedly, according to the method of sterilization, most 

 markedly in the case where milk is heated for a longer period at 

 120 C., and least markedly in the case where it has been subjected 

 to intermittent sterilization. For this reason the latter method of 

 sterilization is to be preferred to all other methods of sterilization 

 Unfortunately, however, it is such an inconvenient method, and 

 requires so much time, that it is not well suited for general 

 application. No other course, therefore, is at present open than to 

 dispense with perfect sterilization, and to be content with milk 

 which has been temporarily sterilized. 



