14 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
steamed coolers, to run into thoroughly cleaned and steamed, or other- 
wise sterilized, bottles through a thoroughly cleaned and sterilized 
bottle filler, then to cap the bottles with sterilized caps and place the 
milk in low-temperature refrigerators. 
This process sounds relatively simple, yet at every step problems are 
encountered which may defeat the primary object. 
First of all, it must be kept in mind that bacteria are too small to 
be seen by the naked eye and that they are distributed in the air of 
the milk plant, upon the equipment with which milk comes in contact, 
and upon the hands of employees. Flies also carry millions of bac- 
teria. When milk comes to the plant to be pasteurized the logical 
thing to do is to see that it comes in contact only with apparatus 
which has been thoroughly cleaned and thoroughly steamed. The 
equipment may appear clean, but since bacteria can not be seen 
with the naked eye, a clean (to the eye) tank or pipe may contain 
many millions. Means must be taken to destroy as many of them as 
possible. To do this steam is usually employed, for steam at 205° F. 
or above for a period of 2 to 5 minutes will destroy disease-producing 
bacteria and all but spores of the harmless types. Equipment so 
treated may be called bacteriologically clean, but must be visibly 
clean before application of the steam if satisfactory results are to be 
expected. 
When the equipment is in this condition, the milk can be pasteur- 
ized. At this point the object is to heat all the milk to 145° F. and 
hold it for 30 minutes. In intelligent supervision many problems 
are encountered at this step in the process. They are well dis- 
cussed in a paper entitled Pasteurization of Milk, which is a report 
of the Committee on Milk Supply of the Sanitary Engineering Sec- 
tion of the American Public Health Association, 1920. Briefly, the 
principal points are: 
1. Heat all the milk to 145° F. 
2. Hold all the milk for 30 minutes. (Some continuous-flow systems do 
not do this.) 
3. Watch for leaking valves, also pipe lines which hold milk below the pas- 
teurizing temperature. 
4. Have accurate recording thermometers so arranged as to show the total 
heating period. Recording thermometers should be frequently checked against 
a standard thermometer of unquestionable accuracy. 
5. Watch for foam on the milk. This may stay in the vats for hours at a 
warm temperature suitable for bacterial development. 
After proper heating and holding, the pathogenic organisms have 
been destroyed and the total number of bacteria reduced to a mini- 
mum point. The next problem is to cool and bottle the milk with- 
out reinfection, particularly with disease-producing bacteria. 
