THE GRADING OF MILK 169 



when subjected to different conditions. The more favourable 

 the conditions for bacterial development, the shorter will 

 be the reduction time. It follows that not only does the 

 reductase test give an estimate of the number of organisms 

 present, but the result will be influenced to some extent 

 according to the vitality of the organisms, which is a factor 

 as important as any other in determining the keeping power 

 of the milk. As the reductase test will reveal any appreciable 

 bacterial increase before this becomes apparent through the 

 presence of lactic acid, it is not only the most convenient, but also 

 the most sensitive, method for the grading of milk, whether for 

 retail direct or for the making of dairy products. On the other 

 hand, it must be admitted that milk from individual cows or milk 

 that has been subjected to unusual treatment may give divergent 

 results. The translator l has thus found that milk which has 

 been kept for a long time at low temperatures appears to be better 

 than it really is, in the reductase test carried out at 38 C., because 

 the majority of the bacteria in it are greatly weakened at the 

 temperature in question. Such milk is decolorised quicker at 

 28 C. A comparison between the reduction times of the same 

 milk at 28 and 38 C. can thus furnish information as to how this 

 milk has been treated. According to the author's proposal, 

 however, the test should be carried out at 38 to 39 C., as this will, 

 in the majority of cases, give the shortest reduction time. On 

 the other hand, it is a wrong principle to employ a still higher 

 temperature, as was oTiginally done, for then the development of 

 all the most common milk bacteria will be hindered. 



The methods of sampling and the apparatus required for the 

 reductase test are the same as have already been described under 

 the fermentation test. Particular care must be taken to measure 

 out exactly 40 c.c. of milk, either by means of a graduation mark 

 on the tube or by taking the sample in a measure holding exactly 

 40 c.c. when filled to the rim. As the preparations of methylene 

 blue on the market are very different in their properties, and the 

 solutions made from them are not permanent, it is necessary to 

 use a fresh solution of definite strength, made from a standard pre- 

 paration. The tabloids prepared for the purpose 2 are readily 

 soluble in warm water ; each tabloid makes 200 c.c. of solution, 

 1 c.c. of which is required for each test with 40 c.c. of milk. The 

 colour is mixed with the milk by rolling the tube in the hands, 

 then pressing the mouth against a clean portion of the palm of the 



1 "Analyst," 1918, 43, p. 1. 



2 By Messrs. Blauenfeldt and Tvede, of Copenhagen, who also supply all 

 requisites for the reductase test, including complete outfits suitable for 

 dairies dealing with milk from 100 to 400 suppliers. 



