METHODS COMMONLY USED IN HEATING MILK 299 



' flash process.' Both systems are used in the United States, but 

 in this country the ' flash ' process appears to be more generally 

 used. 



1. The 'Holder Process.' In this process the milk is gradually 

 brought up to the temperature which is intended to be the maximum 

 temperature to which it shall be exposed, and is held at that tempera- 

 ture for a given time. There is no definite temperature to which 

 the milk is raised in this process, different dairies employing various 

 temperatures. 



Ayers and Johnson, in their investigations carried out for 

 the Department of Agriculture in Washington, made enquiries 

 of the pasteurising firms in nearly all the American cities having 

 populations of over 25,000. The replies showed that the average 

 temperature with this process was 62*8 C. (145 F.). The actual 

 temperatures used, however, varied very considerably. Out of 

 219 plants, 75 used the holder process, and 62 of these employed 

 a temperature of from 6o-65'5 C. (i4o-i5o F.). One used a 

 temperature somewhat below this, and 12 employed tempera- 

 tures ranging from 667-767 C. (i52-i7o F.). 



2. The 'Flash Process.' This process consists in heating the 

 milk up to a particular temperature for one minute and then rapidly 

 cooling it. Out of the 219 milk plants concerning which information 

 was obtained, 144 used the flash process, the average temperature 

 being 71-1 C., or 160 F. Of the 144 plants, 61 used the average 

 temperature, 61 used temperatures below the average (being in 

 some cases as low as 140 F.), and 22 used temperatures above the 

 average, in some cases as high as 82-2 C., or 180 F. No reasons 

 appear to be given for these great divergences in practice. It is 

 probable that the lower temperatures are in some of the cases 

 too low to be of any appreciable value, since they hardly touch 

 the thermal death-point of a considerable number of organisms. 



The cream-line is said to be affected when the temperature is 

 raised above 150 F. Hence the object of many of the pasteurising 

 firms is to keep the temperature about this point. 



The cost of pasteurising must be borne in mind, and the holder 

 process will involve a longer heating and hence be more expensive 

 than the flash process. 



Some authorities consider that the holder process is a safer 

 method than the flash process, but this probably depends very 

 largely upon care exercised by the individual dairyman. 



A pasteurising apparatus is of necessity a somewhat compli- 

 cated one, and the problem of keeping it clean cannot be regarded 

 as a simple matter. Plate VII shows the exterior of a pasteurising 

 plant. The milk after heating is conducted to the cooler, over 

 which it passes on its way to the tank, and thence to the bottles 

 The figures which have been already given show the differences 

 of temperature which obtain in different parts of even a small 

 amount of milk when it is being heated. In order to ensure that 



