METHODS OF REDUCING THE GERM CONTENT 519 



but under commercial conditions there are some difficulties which 

 have to be taken into account. When large quantities of milk 

 are handled the temperature is not always consistent throughout 

 the tank, running lower in some parts and higher than is desired 

 in others. Even on the assumption that all parts of the milk 

 have at some time reached the desired temperature, there may be 

 some parts which have not been held at the right temperature for 

 the right length of time. Shorer and Rosenau have investigated 

 temperature conditions in connection with a large pasteurizer in 

 Boston, and have concluded that in order to have a margin of 

 safety the temperature should be 145 F. (62.8 C.), and that 

 the milk should be held at this temperature for thirty minutes. 



A second difficulty in operating a holding pasteurizer is the 

 formation of foam on the milk. A stirring device is usually in- 

 stalled in the holding tank to insure reasonable distribution of 

 heat, and consequently more or less foam will form according to 

 the kind and intensity of the stirring. This foam loses some of 

 the heat, and therefore has a protective influence on bacteria, 

 similar to that of the membrane which forms on heated milk. It 

 is desirable, therefore, to have a stirring device which moves the 

 mass of milk quietly, with as little foam formation as possible. 



There are chiefly two kinds of pasteurizing machines in which 

 the milk is held for a definite period at relatively low tempera- 

 tures. The one type is the absolute holder, the other the con- 

 tinuous flow system. The difference between the two lies in the 

 fact that in the absolute type the milk is held in an insulated tank 

 after the proper temperature has been attained, while in the con- 

 tinuous-flow type the milk flows through the system. After hav- 

 ing been heated milk enters the pasteurizer and flows at a rate 

 which insures its being held at the proper temperature for the 

 requisite length of time. 



In the absolute-holder type there is an arrangement by which 

 the milk is automatically discharged into the cooling system after 

 the period of pasteurization has elapsed. The tanks must be 

 insulated and the milk gently agitated in order that all parts of 

 it may be held at the right temperature for the right length of 

 time. Leaky valves which admit or discharge the milk too early 

 defeat the purpose of pasteurization. The stirring device should 

 excite as little foam as possible. 



Another type of holder consists of a tank in which there is a 

 coil of pipe (Figs. 207, 208X This coil contains the hot water or 

 steam- and revolves, with the result that the milk is kept in mo- 

 tion. Usually several tanks are used in a single plant in order 

 that there may be a continuous flow of heated milk through the 

 cooling system. 



