THE GRADING OF MILK 163 



% 



are used, 2 c.c. of milk and 2 c.c. of alcohol will suffice for this test. 

 It is recommended that the dairies should apply it to each can of 

 milk and reject all milk which shows a precipitate or an abnormal 

 colour. 



THE FERMENTATION TEST 



This test shows if the milk has become infected with an undue 

 proportion of gas-producing organisms, which first and foremost 

 include the pseudo lactic acid bacteria. We have already seen 

 that these are among the most objectionable organisms that can 

 be met with in dairy practice, as they cause trouble in various ways, 

 including the spoiling of milk for the purpose of cheese making. 

 As the pseudo lactic acid bacteria are usually brought into the 

 milk with the cow dung, and are particularly plentiful when the 

 cows are suffering from diarrhoea, the fermentation test will give 

 evidence of undue contamination and thus give warning that the 

 milk may possibly be dangerous for human consumption. This 

 test is of value not only in judging of the suitability of milk for 

 cheese making, but also in the laboratory control of retail milk, 

 particularly that which is to be used for infant feeding. While 

 most of the milk bacteria (excepting the thermobacteria, which, 

 however, are rare in fresh milk) grow best at about 30 C., many of 

 them ceasing to develop at temperatures above 38 C. , the pseudo 

 lactic acid bacteria, being typical intestinal organisms, have their 

 optimum at blood heat, and will gain predominance most readily 

 at a slightly higher temperature, for which reason the fermentation 

 test is carried out at 38 to 40 C. It is important that the tem- 

 perature should not be allowed to vary beyond these limits, as at 

 a higher temperature good milk may appear to be bad, while at a 

 Lower temperature bad milk may appear to be good. If fine 

 distinction is made between the different types and degrees of 

 fermentation, these temperature limits are too wide, and the 

 temperature should be kept constant at 38 C. In order that the 

 results of the test may be strictly comparable, it is advisable 

 always to use tubes of a certain diameter (about 2 cm.), into which 

 40 c.c. of milk are introduced. The shape of the tubes is shown 

 in the accompanying illustration ; they should be strongly made 

 and provided with a graduation mark at 40 c.c., and with a frosted 

 square so that they can be marked in pencil. An average sample 

 of each supplier's milk should be taken, preferably from the 

 weighing or measuring vessel, by means of a small measure 

 furnished with a pointed spout. The measure should be rinsed 

 several times with the milk which is being sampled before taking 

 the actual sample. The tubes are marked, covered with a small 

 cap of aluminium or zinc, placed in stands and brought into the 



