THE FERMENTATION TEST 229 



requirement is made that the milk shall withstand the 

 test of boiling it in a test tube without the separation of 

 flakes or curds, and be able to withstand the, so-called, 

 alcohol test, which is made as follows : 56 



Exactly equal parts of milk and alcohol (68 per cent.) 

 are carefully mixed in a tube and observed closely. 

 Fresh milk shows no precipitation ; in that which is be- 

 ginning to sour, fine flocules are deposited on the walls 

 of the glass and, with greater acidity, flakes and lumps 

 are separated. 



c. The fermentation test is used to show whether there 

 is an excess of bacteria of putrefaction in the milk. It 

 consists simply in incubating a sample of milk at body 

 temperature for 8 to 16 hours, followed by an examina- 

 tion as to its appearance, odor and flavor. The examina- 

 tion is easily made as follows: Into large test tubes 

 holding at least 25 c.c., that have been carefully cleaned 

 and sterilized, the milk samples, warmed to about 36 C. 

 (97 F.), are poured. The tubes are closed by cotton 

 and placed in an incubator or closet, where they are kept 

 at 30 to 35 C. (86 to 95 F.). In the course of eight 

 or ten hours, and again later, the contents of the tubes 

 are examined. Good, undeteriorated milk is then sour 

 and curdled and forms a homogeneous coagulum, without 

 much separation of whey or formation of gas. Fre- 

 quently, gas bubbles have split the coagulum and con- 

 siderable fluid has separated. This change, however, 

 does not necessarily signify that the milk was particu- 

 larly rich in bacteria of putrefaction. On the other 

 hand, if the milk curdles and has an offensive odor, or 



56 As a result of boiling, separation occurs not only where the 

 milk has reached a certain degree of acidity but also in cases where, 

 as a result of disease of the cow, abnormally large quantities of 

 albumin and of globulin are present, or when there is admixture of 

 colostrum. 



