278 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



bator for at least one-half hour. Centrif ugalize, 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. in a water bath. In a 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 2% 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- 



