BACTEEIA OF MILK 83 



from the milk, otherwise the leuco-methylene blue quickly goes 

 back to the methylene blue. 



This action should not be confused with the ability of niUk 

 to decompose hydrogen peroxide or to reduce " Schardinger's 

 Reagent" (5 cc. sat. ale. solution methylene blue plus 5 cc. 

 formalin plus 190 cc. distilled water) at the time of milking. 

 This reducing substance is designated aldehyde-reductase, or 

 simply reductase. This reductase is destroyed if milk be heated 

 to 80° C. or higher — a fact upon which the Storch test for 

 heated milk depends. 



The reduction of methylene blue free from formaldehyde is 

 due to the development of bacteria and not to the reductase, 

 which works only when aldehyde is present. This power is quite 

 closely proportionate to the number of bacteria present, and 

 hence may be used as a rapid and simple method of determining 

 the bacterial content of milk. There are two methods for deter- 

 mining the reducing power: the first consists in recording the 

 length of time necessary for disappearance of the blue color ; the 

 second consists in determining the amount of methylene blue 

 remaining unchanged at the end of two to four hours by titrating 

 it with titanium chloride. 



Barthel. Zeitschr. Nahr. u. Genussmit. 15 : 385. 1908. 



WicHERN. Zeitschr. physiol. Chem. 57 : 865. 1908. 



Jensen. Rev. gen. du lait 7 : 308. 1909. 



Fred. Centralbl. f. Bakt., 2te Abt., 35 : 391. 1912. 



LoiiNis. Handbuch d. landw. Bakteriologie, p. 170. Berlin, 1911. 



1. Prepare a standard solution of methylene blue by dissolving 

 one part of methylene blue in 1000 parts of water. 



2. Procure several milk samples of various ages and of various 

 grades of cleanliness. ^lake plates on whey agar for counting 

 the number of organisms, as directed in Exercise 110. 



3. Place in test tubes 10 cc. of each sample of milk to be 

 tested, and add to each tube 0.5 cc. of standard methylene blue 

 solution ; shake until mixed, then cover the Uquid in the tube 

 with a layer of paraffin oil 2 cm. deep. The oil layer excludes 

 atmospheric oxygen. If whole milk be used, the rising cream 

 will exclude atmospheric oxygen, and the oil is unnecessary. 



