MILK 39 
observers report that no increase occurs duing cestum, 
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- 
