158 PRACTICAL DAIRY BACTERIOLOGY 



process. Indeed, the milk is commonly heated no longer than 

 one minute. While either method may thoroughly pasteurize 

 the milk, there is a larger chance of error in the continuous 

 pasteurizers, and practical study has shown that, as they are 

 commonly used, the pasteurization by the continuous pasteurizer 

 is less thorough than by the use of the discontinuous machines. 

 As frequently used in the large milk supply companies, the pas- 

 teurization is of little value in improving the healthfulness of 

 the milk. The milk is brought to the required temperature for 

 an instant only, just sufficient to reduce the number of acid 

 bacteria and thus increase the keeping quality of the milk, but 

 not long enough to have much influence on the disease germs. 

 There is, to-day, a move being made to bring this under proper 

 inspection. If pasteurization can be properly controlled and 

 placed under inspection so that its efficiency can be assured, it 

 will be a very simple means of removing from the milk the 

 danger of disease distribution. 



In pasteurization the cooling of the milk after heating is of 

 as much importance in preserving it as the original heating. 

 The process does not kill all bacteria, and those remaining will 

 subsequently grow rapidly unless the milk is cooled at once. 

 From this it also follows that the milk should be used as quickly 

 as possible, for while it will keep longer than unpasteurized 

 milk the bacteria are growing in it all the while, and, since the 

 acid -bacteria are lacking, the putrefactive germs will have a 

 greater chance to grow than usual. Old pasteurized milk is, 

 therefore, worse than sour milk. It should be recognized also 

 that only fresh milk should be pasteurized. After it has begun 

 to sour the heat may curdle it, and it is never wise to use this 

 treatment upon milk that contains more than 2 per cent. acid. 



Pasteurization is sometimes applied to cream to enhance its 

 keeping and enable it to find a market. The cream keeps well, 

 but it loses some of its consistency. Such cream appears thinner 

 than before treatment, and it will not whip as will ordinary 



