168 DAIRY BACTERIOLOGY 



explanation, however, hardly covers the case of the obligate 

 anaerobic bacteria, but, as far as the aerobic organisms are 

 concerned, the reducing action of the milk itself is, no doubt, a 

 contributing factor, so that in the reductase test the degree of 

 aeration of the milk is a condition which must not be overlooked. 

 Milk will already begin to absorb oxygen freely as it comes in 

 a fine stream from the udder, and its oxygen content will naturally 

 be increased on pouring from vessel to vessel, and especially during 

 any process of aeration to which it may be submitted. The 

 dissolved oxygen will gradually be consumed as the micro- 

 organisms increase in number ; the less the milk is shaken, and 

 the deeper the vessels in which it is kept, the quicker will the 

 oxygen content fall off. The temperature at which the milk 

 is kept is also an important factor in this connection, for not only 



FIG. 70. Apparatus for Reductase and Fermenting Test to take 200 Samples. 



will the bacteria develop more rapidly at higher temperatures, 

 but the individual cells will consume more oxygen at higher than 

 at lower temperatures 1 . In order to equalise these conditions, 

 it will be advisable always to shake the milk well before subjecting 

 it to the reductase test. 



It will be seen that the theory of the reductase test is not 

 so simple as was formerly supposed ; nevertheless, numerous 

 experiments with mixed milk of commerce have shown that the 

 reduction time, as determined by the reductase test, furnishes 

 just as satisfactory a measure of the bacterial contents as the 

 troublesome method of plate counts, which, as a matter of fact, 

 is by no means less subject to error than the reductase test. 

 Moreover, the differences between the reducing powers of the 

 different species of bacteria are not greater than the differences 

 displayed in this respect by the members of the same species 



1 This is clearly shown by C. Lind's work, " Reduktaseproven sammen- 

 io-net med Bakterietaellingsmetoden " (" Ma-elkeritidende," 1915, p. 921), 



