ENZYMES. 



185 



5 c.c. of 1 per cent, hydrogen peroxide solution (or 9 c.c. of 

 milk and 3 c.c. of hydrogen peroxide). The tube is now held 

 at /and d and shaken with a pendulum motion and the cover to 

 c screwed rapidly down. The tube is then placed in water at 

 25 C. up to the level of c, and the place where the water stands 

 in the upper tube noted at once, the tube is shaken from time 

 to time during two hours, and the volume read off after two hours ; 

 the difference of the reading gives the liberated 

 gas. A volume of more than 25 c.c. of gas per f d 



100 c.c. of milk is about the limit for normal 

 milk. 



Reductase. Fresh milk contains but little 

 enzyme capable of reducing methylene blue 

 and other colours, while many micro-organisms 

 secrete such an enzyme ; advantage is taken of 

 this fact to test whether milk is fresh or not by 

 measuring the time that is taken to decolourise 

 methylene blue. Jensen's modification of Bar- 

 thel's method consists in adding to 40 c.c. of 

 milk 1 c.c. of a methylene blue solution contain- 

 ing 0-015 per cent, methylene blue (not the zinc 

 salt) in 1 litre ; a few cubic centimetres of 

 liquid paraffin are added, and the tube kept 

 at 38 to 40 in an incubator. Good milk is 

 not decolourised for ten hours, but milk con- 

 taining very many bacteria may be reduced 

 in less than half an hour. Arup has shown 

 that the reductase is proportional approxi- 

 mately to the number of micro-organisms 

 present. It is a valuable test for determining 

 whether milk is stale or not ; as a quantitative 

 test it suffers from the defect that the results 

 are not obtained till a long time after the test 

 is started, but from a practical point of view the tint after a half 

 to one hour's standing will give a fair indication of the staleness 

 of the milk. If the tubes be sterile, the fermentation test (p. 441) 

 can be performed by observing the tube after 12 hours or so. 



Fig. 27. Lobeck's 

 Catalase Tube. 



