278 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
bator for at least one-half hour. Centrifugalize, prepare 
a slide from the sediment, and stain as described for the 
microscopic method. 
FERMENTATION TEST 
The fermentation test has been in use in cheese fac- 
tories for a long time to detect milk which is unsuitable 
for cheese-making. It can be used in milk control work 
to discover the presence of the more objectionable milk 
bacteria, such as the gas-formers and peptonizers, and 
thus ascertain what form of decomposition the milk is 
likely to undergo with age, especially if improperly cared 
for. It is also of assistance in determining the source of 
excessive bacterial contamination. 
The test is very simple and does not require any spe- 
cial apparatus. In cheese factories, bottles holding from 
120 to 140 c.c. or test tubes of 40 to 50 c.c. capacity are 
used for the milk samples. After the milk is introduced, 
they are closed with a rubber stopper and are held at a 
temperature of 37 to 38° C. ina water bath. Ina labora- 
tory, it is more convenient to use the test tubes ordinarily 
used for bacteriological cultures and to place the tubes 
in an incubator after the milk has been placed in them. 
The test tubes are washed and cleansed in the usual 
manner, plugged with cotton and sterilized by heating in 
a hot-air sterilizer for 2144 hours at 150 to 160° C. It is 
important that the tubes be sterilized, since any organ- 
isms in the tubes will develop in the milk and may influ- 
ence the result. Each tube is numbered with a paraffine 
pencil to correspond with the sample of milk and is then 
filled with milk to within a finger’s breadth of the bottom 
of the cotton plug, closed with the cotton plug and placed 
in the incubator. In transferring the milk from the ves- 
