CHAPTEE VI. 



PREPARATION OF KEEPING MILK, FERMENTED MILK, AND THE 

 BYE-PRODUCTS OF MILK. 



129. Keeping Milk. By keeping milk, is understood milk which 

 by heating, or by heating and other suitable treatment, possesses 

 the property of being able to keep, without becoming decomposed, 

 for a longer time than ordinary milk. As long as milk stands boiling 

 without coagulation, and possesses no other foreign flavour than a 

 slight taste of cooked milk, it may be regarded as a good keeping 

 milk. The keeping qualities of milk, on the other hand, may be 

 increased to such an extent, that it will keep for days, or months, or 

 for a much longer period. In such cases the milk may possess its 

 original percentage of water, or it may lose a portion of it by 

 becoming thick. 



130. Pasteurized Milk. By such milk is meant that which has 

 been heated, for a shorter or longer period, to a temperature under 

 the boiling point of water, but high enough, as experience has 

 shown, to kill most of the microscopic fungi. The temperature 

 which meets these conditions, and which is consequently commonly 

 used in Pasteurizing, lies between 56 and 80 C. Within these 

 limits, the higher the temperature, the shorter is the period in which 

 a distinct effect is produced. It would be very extraordinary, indeed, 

 if milk were rendered free from spores by Pasteurizing. Since 

 Pasteurized milk is scarcely ever kept free from spores, it possesses 

 only, as a rule, a slightly increased keeping property. This is 

 explained by the fact that the lasting spores of certain kinds of 

 bacteria, which are not uncommonly present in milk, can withstand 

 for a long time the application of such heat as is applied in Pasteur- 

 izing, and that there are bacteria which only begin to develop at 

 temperatures over 50 C. ; indeed, there are some which even rapidly 

 increase at temperatures of from 70 C. to 75 C. Fortunately 

 such bacteria as agree with these high temperatures are generally 

 uncommon, and are only very rarely found in milk. Experiments 

 have shown that in Pasteurizing, the vegetative forms of nearly all 

 bacteria, and especially, also, of the most dangerous pathogenic germs, 



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