PRESENT STATUS OF THE PASTEURIZATION OF MILK 17 
HANDLING OF MILK AFTER PASTEURIZATION 
Although pasteurization kills all pathogenic bacteria some harmless 
organisms remain. On account of this fact pasteurized milk is still 
a perishable product, and must be handled with the same care as raw 
milk. This is a point for both the consumer and the milkman to 
remember. 
After pasteurization milk should be cooled to from 33° to 40° F. 
and kept at that temperature until delivery. In warm weather it 
should be iced on the delivery wagons. From a sanitary standpoint 
all milk, whether raw or pasteurized, should be delivered without 
delay, in order that the consumer may get it in the best condition. 
In milk held at about 40° there is only a slight bacterial increase dur- 
ing the first 24 hours. Pasteurization and delivery can be so arranged 
that the consumer gets the milk before any appreciable change has 
taken place in the bacterial content. 
COST OF PASTEURIZING MILK 
The cost of pasteurization in 1922 was estimated by Bo wen from 
the cost given in his earlier paper (15). He obtained information in 
five establishments which were considered to represent the average 
city milk plant. The pasteurizing equipment in each plant consisted 
of a heater, a holding tank, a regenerator, and a cooler. The cost of 
operation was based on the pasteurizing cycle, starting with the initial 
temperature of the raw milk and raising it to the pasteurizing tem- 
perature, then cooling to the initial temperature of the raw milk. 
Bo wen based the costs on interest at 6 per cent per annum on capital 
invested in all equipment used in the pasteurizing process. He 
allowed 25 per cent per annum for depreciation and repairs on the 
pasteurizing equipment proper, and 10 per cent on other mechanical 
equipment, such as engines, boilers, etc. Other costs figured were 
labor; coal, estimated at $8.16 a ton; cooling water, estimated at 
$0.75 per 1,000 cubic feet; and refrigeration, estimated at $2 a ton. 
With later estimates substituted for his older figures, Bowen calcu- 
lated that the average cost of pasteurizing 1 gallon of milk was 
approximately $0.0049, or a little less than one-half cent. 
BACTERIA THAT SURVIVE PASTEURIZATION 
About 1 per cent of the bacteria in the milk remain alive after 
pasteurization, and the kinds left depend entirely on the temperature 
to which the milk was heated and the number of heat-resistant 
bacteria in the milk. From studies of the bacteria which survive 
pasteurization, it is possible to show graphically the hypothetical 
relations of the bacterial groups in raw milk and in milk pasteurized 
<#> by the holder process at various temperatures under laboratory 
conditions (3, 4). 
The bacterial flora of the various kinds of milk is represented in 
Figure 1 by columns of equal length divided into sections, which, in 
a general way, show the relative proportions of the bacterial groups. 
From the illustration it may seen that raw milk contains four 
principal groups of bacteria — the acid, inert, alkali, and peptonizing. 
The acid group is divided into two types — the acid-coagulating, which 
