128 Dairy Bacteriology. 



trol of the operator and may. be thoroughly and effi- 

 ciently treated. 



As pasteurization was introduced for the treatment of 

 market milk, and for the preparation of cream for but- 

 ter, machines have been devised which permit large quan- 

 tities, as thousands of pounds, to be handled per hour. 

 It is evident under these conditions that the milk must 

 be heated for only a short time, and hence a higher tem- 

 perature must be employed. These machines are called 

 "continuous flow" pasteurizers since the milk passes 

 through them in a constant stream. The period of ex- 

 posure is very short, in some only a few seconds ; hence, 

 they are sometimes called " flash" pasteurizers. 



All machines of this type possess the obvious disad- 

 vantage that it is impossible to heat all of the milk for 

 a uniform period. The milk in contact with the walls 

 of the machine flows much more slowly than in the mid- 

 dle of the stream, just as the current near the bank is less 

 rapid than in mid-stream. In none of the machines yet 

 devised have the designers been able to overcome this 

 disadvantage. In a test of one of the most widely used 

 pasteurizers of this type, it was found that some of the 

 milk passed through the machine in 15 seconds, while the 

 larger part of it was held for about 30 seconds, and some 

 as long as forty-five to sixty seconds. If the tempera- 

 ture employed had been such as to destroy the bacteria 

 in that part of the milk heated for the minimum time, 

 hygienic safety would be assured, but in order to avoid 

 injuring the* physical properties of the milk, the tendency 

 is to use as low a temperature as possible, so that the 

 milk heated for the minimum time may often contain 

 organisms that have passed through the machine un- 

 injured. 



