THE GRADING OF MILK 1(57 



It must not be assumed that the time taken to decolorise 

 methylene blue (reduction time) under standard conditions is 

 an accurate measure of the number of microorganisms in the 

 milk ; for, in the first place, all microorganisms do not reduce with 

 equal rapidity, and, second, the milk itself, as obtained from the 

 cow, contains reducing substances. Of the milk bacteria examined 

 by the author Streptococcus liquefaciens appears to have particu- 

 larly marked reducing powers, while the true lactic acid bacteria 

 are among the organisms which reduce slowly. The obligate 

 anaerobic bacteria reduce rapidly, a fact easily explained on 

 considering that they derive their energy from reduction processes. 

 The reduction time is shortened by the addition of a little alkali, 

 and lengthened by the addition of a little acid. As regards the 

 reducing substances natural to milk, the most important is 

 aldehyde reductase, an enzyme which appears to be associated with 

 the fat globules 1 , but which has no significance in the present 

 connection, as it only decolorises methylene blue in presence of 

 formaldehyde. Greater interest attaches to the leucocytes, which, 

 like other living cells, are able to reduce methylene blue 2 . They 

 will, however, only exert an appreciable effect on the reduction 

 time if present in large numbers, while it can hardly be considered 

 a drawback to the test that milk rich in leucocytes should appear 

 to be worse than its bacteriological condition would warrant, 

 inasmuch as such milk should always be regarded with suspicion. 

 Of still greater importance is the fact that milk contains substances 

 other than enzymes which exert a reducing action in the absence 

 of oxygen. Thus Burri and Kursteiner have shown 3 that newly- 

 sterilised milk which has been prevented from absorbing oxygen 

 decolorises methylene blue rapidly, and Barthel 4 has shown that 

 raw milk behaves similarly when the dissolved oxygen is expelled 

 by a current of hydrogen or carbon dioxide, from which he 

 concludes that the decolorisation of methylene blue in milk is 

 due to the action of the milk itself, and that the microorganisms 

 only act indirectly by consuming the dissolved oxygen. This 



1 Orla Jensen, " Uber den Ursprung der Oxydasen und Reduktasen der 

 Kuhmilch," Det Kgl. Danske Videnskabernes Selskabs Oversigter (Danish 

 Academy of Sciences), No. 5, 1906, and Centralblatt f. Bakt. II. Abt. 

 1907, XV1IL, p. 211. 



2 Olav Skar, " Skandinavsk Veterinaertidsskrift," 1913, p. 51. 

 " Milchwirtschaftliches Zentralblatt," 1912, p. 269. 



4 " Skandinavisk Veterinaertidsskrift," 1916, p. 155. The author has 

 been able to confirm BartheVs results, and has found that milk with a low 

 bacterial count which is kept free from atmospheric oxygen by passing a 

 current of hydrogen through it reduces methylene blue in forty-five 

 minutes at 40 C., no matter whether raw or sterilised. (On the addition 

 of a little formaldehyde decolorisation was complete in ten minutes.) 

 As pure lactose solutions were not found to decolorise methylene blue 

 under similar experimental conditions, the reducing action of milk itself 

 cannot be due to the lactose. 



