ood MILIk AND MILK PRODUCTS 
the bottle should be first shaken to insure thorough mixing and then the milk 
may be poured into the sample bottle, although it is better here to use a sampling 
tube. 
If the temperature of the milk is desired, it should be taken from a separate 
sample that should then be discarded. All records shall be made immediately 
after taking the sample. The milk sample shall be placed in a properly labeled 
bottle. The most convenient kind of sample bottles are glass stoppered, or those 
closing with a cork-lined screw cap. Cotton plugs are not a satisfactory method 
of closure. The sample bottles shall be placed at once in a carrying case contain- 
ing cracked ice, so that the milk is cooled at once to near the freezing-point. 
The samples shall be transferred to the laboratory as quickly as possible, and 
shall be plated at once. If the samples can be placed in melting ice and water, 
they may be kept for several hours (12) without an increase in bacteria. If the 
plates are not made within four hours of the time of collection, the number of 
hours that have elapsed should be stated on the report. If the milk is kept at 
40° I’. a slight increase may be found in twelve to twenty hours. Up to twenty 
hours this will not be more than 20 per cent. 
Media. This is essentially identical with that used for water analysis 
and has been fully treated in the chapter on that subject. Sherman 
(1916) has demonstrated that lactose agar gives a much higher count 
than plain agar plates for enumerating bacteria in milk. Lactose agar 
also of some value in differentiating the types of bacteria which are 
present. Sherman (1915) has recommended that the reaction of agar 
media used in the bacterial analysis of milk should be 0.5 per cent plus. 
Plating. Yor miscellaneous samples the character of which is not known, 
three dilutions shall be made 1-100, 1-1000 and 1-10000. Where the character of 
milk is known the number of dilutions may be reduced. If the milk is pas- 
teurized, certified or known to be fresh. and of high grade, the 10,000 and 100 
dilutions may be omitted; if the milk is known to be old, the 100 dilution may be 
omitted. In no case shall less than two plates be made of each sample. Any 
convenient method of making dilutions may be used always using pipettes and 
sterile water blanks. The water for dilutions may be placed in dilution bottles 
(99 c.c. and 9 c.c. are convenient sizes) and sterilized for one hour in an auto- 
clave at 15 lbs. pressure. 
These standard methods do not give the maximum number of bac- 
teria. Some research workers in order to secure the maximum number 
of bacteria incubate their plates at 37° C. for five days and at 20° C. for 
two days. Such an incubation period of seven days’ duration is obvi- 
ously too long for routine work. Slack (1917) has advised the incuba- 
tion of agar plates made from milk in routine analysis for forty-eight 
