MILK 39 



observers report that no increase occurs duing oestum, 

 but others state that when the cow is nervous and excitable 

 the catalase is sometimes increased. Pronounced changes 

 in the feed may affect the quantity of catalase. The 

 first milk drawn at a milking contains less catalase than 

 the end milk. 



When milk is separated, the greater portion of the 

 catalase passes over into the cream. Skim milk, there- 

 fore, has a very low catalase content. After milk has 

 reached a certain degree of acidity (about 0.36 per cent.) , 

 the acid begins to exert an inhibitory influence on the 

 activity of the catalase. Up to this point the catalytic 

 activity is increased because the amount of catalase is 

 increased by bacterial growth. In milk which has under- 

 gone " sour curdling," the catalase is paralyzed by the 

 acid and is inactive. The catalytic activity may be re- 

 stored to such milk by neutralizing it with lime water. 



When milk is exposed to a low temperature in winter 

 or to prolonged refrigeration, the catalase is partially or 

 completely destroyed. Catalase operates best at a tem- 

 perature of 37 C. (98.6 F.) . The lethal temperature is 

 around 68 C. (154 F.), but varies within wide limits 

 according to the source of the catalase. Heated milk 

 may be reactivated, since catalase is a bacterial as well 

 as an original ferment. ( See catalase test on pages 287 

 to 294.) 



Reductase. If a small quantity of methylene blue 

 solution is added to milk, the mixture will be colored 

 blue, but the blue color will disappear after a time because 

 the methylene blue is reduced and converted into its 

 leuco-base. This change is brought about by a ferment 

 in the milk called reductase. If formalin is added to the 

 methylene blue solution, forming what is known as Schar- 



