10 
BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE 
countered which may defeat the pri- 
mary 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 mil- 
lions of bacteria. 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. Since bacteria 
can not be seen with the naked eye a 
tank or pipe apparently clean may 
contain many millions. Means must 
be taken to destroy as many of them 
as possible. To do this steam is usual- 
ly 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. 
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 
discussed in a paper entitled " Pasteur- 
ization of Milk," which is a report of 
the Committee on Milk Supply of the 
Sanitary Engineering Section 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 Pasteur- 
izing temperature. 
4. Have accurate recording thermometers 
so arranged as to show the total heating 
period. Recording thermometers should be 
frequently checked against a. standard ther- 
mometer 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 develop- 
ment. 
After proper heating and holding, 
the pathogenic organisms have been 
destroyed and the total number of bac- 
teria reduced to a minimum point. 
The next problem is to cool and bottle 
the milk without reinfection, particu- 
larly with disease-producing bacteria. 
To do this, bacteriologically clean 
coolers, bottle fillers, bottles, and ster- 
ilized caps are necessary : and what is 
of greatest importance is to see that 
the Pasteurized milk does not come 
in contact with human hands, or with 
apparatus, including bottles and caps, 
touched by the hands after being 
sterilized. The hands of milk hand- 
lers constitute perhaps the most dan- 
gerous source of reinfection in the 
plant, for they may convey patho- 
genic organisms. Through such chan- 
nels milk may be contaminated by car- 
riers of many diseases. 
In order to guard against such pos- 
sibilities, all employees who handle 
apparatus or milk in the plant or dur- 
ing delivery should undergo frequent 
medical examination, and any dis- 
eased persons or carriers should be 
prevented from working in positions 
in which they are in even indirect con- 
tact with milk, milk equipment, or 
delivery of the product. 
It is perhaps unnecessary to say 
that flies are also a very serious men- 
ace to the milk supply. They must 
be kept out of milk plants, for it is 
impossible to tell when they may in- 
fect the milk. This infection can oc- 
cur directly by flies getting into the 
milk or indirectly through contamina- 
tion of equipment or containers. 
At every step in the Pasteurization 
of milk, one is compelled to think of 
the process in terms of bacteria in 
order to supervise it intelligently. 
HANDLING MILK AFTER PAS- 
TEURIZATION 
Pasteurization of milk destroys 
about 99 per cent of the bacteria ; con- 
sequently the milk is not sterile. 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 con- 
sumer and the milkman to remember. 
Milk after Pasteurization should be 
cooled to about 40° F. and kept at 
that temperature until delivery. Dur- 
ing warm weather it should be iced 
on the delivery wagons. From a sani- 
tary standpoint all milk, whether raw 
or Pasteurized, should be delivered as 
soon as possible, in order that the con- 
sumer may get it in the best condi- 
tion. In the best Pasteurized milk, 
when held at about 40° F., there is 
only a slight bacterial increase during 
the first 24 hours. In many cases the 
Pasteurization and delivery may be 
so arranged that the consumer gets 
the milk before much, if any. change 
has taken place in the bacterial con- 
tent. For the benefit of the consumer 
the word " Pasteurized " should be 
printed on the cap, as it is only right 
for him to know whether he is using 
raw or Pasteurized milk. Some peo- 
ple object to Pasteurized milk, espe- 
