130 Dairy Bacteriology. 



from which it flows over the coolers. Such an apparatus 

 is called a " holding" machine, and is probably the most 

 feasible type of pasteurizer now on the market, when 

 all factors are considered. In some of the continuous 

 machines, an attempt is made to accomplish the same re- 

 sult, by building the machine so that the milk requires 

 fifteen to twenty minutes for passage through the ma- 

 chine, but in all such cases the same disadvantage of 

 variation in rate of flow, as in other continuous flow type 

 of machines obtains. 



Tests of pasteurizing machines. It is possible for 

 the operator to test the rate of flow in a machine, so as to 

 determine whether all of the milk is heated for a uniform 

 time. This is done most easily in the following manner : 

 The machine is first filled with water, heating the same 

 to the desired temperature, and regulating the rate of 

 flow as it would be if milk was used. The flow of water 

 is then turned off, and a stream of milk containing a 

 known per cent of fat admitted to the machine. The 

 time elapsing between the admission of milk to the ma- 

 chine, and that at which the first sign of turbidity is 

 noted at the outlet, will be the minimum period necessary 

 for any portion of the milk to flow through the machine. 

 At frequent intervals thereafter, samples of the out- 

 flowing liquid may be collected, noting the time at which 

 each sample is taken. The percentage of fat in the 

 various samples is determined by the Babcock test; at 

 the moment when all of the water has been removed, the 

 sample taken will show the same fat content as the milk 

 used. The samples taken previous to this will show a 

 lower fat test, dependent upon the relative amount of 

 water and milk. In this manner, the minimum, the 

 maximum, and the average period of exposure of milk 

 in the machine tested, can be determined with exactness. 



