16 BULLETIN 342, XJ. S. DEPARTMENT OF AGRICULTURE 
The primary object is to kill any disease-producing bacteria which 
may be in the milk, and to handle the pasteurized milk in such manner 
that it will not be infected before bottling. Pasteurization at a 
temperature not lower than 142° F. maintained for a period of 30 
minutes kills a large percentage of the bacteria in the milk and the 
keeping quality of the milk is greatly improved. The milk after being 
heated should be immediately cooled, bottled, mechanically capped, 
and placed in a refrigerator the temperature of which should never 
exceed 50°. All coolers, bottlers, bottles, and other milk-contact 
surfaces must be thoroughly cleaned and treated to kill bacteria. 
This process seems relatively simple, yet problems that may 
defeat the primary object are encountered at every step. 
First of all, bacteria are distributed in the air of the milk plant, 
upon the equipment with which milk comes in contact, and upon the 
hands of employees. Also, flies carry millions of bacteria. 
When milk comes to the plant to be pasteurized the logical thing 
to do is to permit it to come in contact only with apparatus which has 
been thoroughly cleaned and thoroughly steamed or treated to kill 
bacteria. As bacteria can not be seen with the naked eye, a tank or 
pipe that is apparently clean may contain many millions. As many 
of the bacteria as possible must be killed. To do this steam is usually 
employed, for heat at 200° F. or above, applied for a period of 5 min- 
utes, will kill disease-producing bacteria and all but the spores of the 
harmless types. Equipment must be visibly clean before treating to 
kill bacteria if the results are to be satisfactory. 
The United States Department of Agriculture recommends that the 
following points in the pasteurizing process be carefully looked after: 
(1) Heat all milk to a temperature not lower than 142° F., and hold at 
this temperature for not less than 30 minutes. (2) Watch for leaking 
valves. (3) Avoid exposed outlet pipes, valves, and pipe lines which 
hold milk below the pasteurization temperature. (4) Use accurate 
recording thermometers, with the chart spaced in single degrees 
throughout the pasteurization range. Check these frequently against 
a standard thermometer of unquestionable accuracy. (5) Take tem- 
peratures near the bottom of the pasteurizer. (6) Watch for foam on 
the milk, and do not let foam remain in the vat. 
It is of the greatest importance that the hands of milk handlers do 
not touch the pasteurized milk, the apparatus, the bottles or the caps, 
after these have been treated to kill bacteria. The hands are perhaps 
the most dangerous source of reinfection in the plant, for through such 
means milk may be contaminated by persons who are carriers of dis- 
ease. In order to guard against such possibilities, all employees who 
handle apparatus or milk in the plant or during delivery should under- 
go frequent medical examination, and any diseased persons or carriers 
of disease should be prevented from working where they are even in 
indirect contact with milk or milk equipment, either in the plant or on 
the delivery route. 
It is perhaps unnecessary to say that flies are also a very serious 
menace to the milk supply. They must be kept out of milk plants, for 
it is impossible to tell when they may infect the milk. Such infection 
can occur directly through flies getting into the milk, or indirectly 
through contamination of equipment or containers. 
