140 Report oF THE BACTERIOLOGIST OF THE 
Steam pressure.— The pasteurizer was connected by a 4-inch 
pipe directly to the high-pressure steam pipe. As will be noticed 
in the diagram Plate II, the steam had a free outlet at a point 
nearly opposite to the inlet, so that there was at no time any ap- 
preciable pressure on the pasteurizer itself. On the contrary, the 
steam was nearly all condensed and there was rarely any waste, 
even when the steam valve was opened to its full capacity. 
The steam pressure given in the tables is that in the 30 horse- 
power boiler and was noted as one of the possible factors in the 
great variation in rate of operation on different days. 
Rate.— The amount of milk which a pasteurizer will heat to a 
given temperature in a given time is important from the practical 
standpoint. 
In order to minimize the variation in temperature to which the 
milk was exposed and to determine the rate more accurately, it 
was our custom to add water to the supply tank and when every- 
thing was running at full speed and the last of the water was leay- 
ing the tank to add the milk and note the time. After all of 
the milk had been added and just as the last was leaving the tank 
the time was again noted. The interval was taken as the time 
required to handle the milk. Since the amount of milk at any 
one time between the milk valve and the milk outlet was only 
about 5 pounds, the error was not great. 
As one would expect, the rate varies with the pasteurizing tem- 
perature. The machine was expected to handle 2,500 pounds an 
hour at 70° C. (158° F.), and would do even more under favorable 
conditions. In our experience, it did not much exceed 2,100 at 
80° C. (176° F.), and handled less at 85° C. (185° F.). It will 
be noticed that the rate at 80° C. ranged from as low as 900 to a 
little over 2,100 pounds per hour. This extreme variation is due 
to a number of factors, among which are variations in steam 
pressure and initial temperature and the cooking of the milk on to 
the walls of the pasteurizer. This layer of cooked material not 
