172 DAIRY BACTERIOLOGY 



used in the reductase test, in the case of milk containing a greater 

 number of bacteria. Conversely, milk which is very rich in 

 bacteria generally shows good results in the fermenting test, for 

 even if it may contain millions of gas-producing bacteria, it will 

 generally contain still greater numbers of true lactic acid bacteria, 

 and therefore become sour so quickly that the former type will 

 not be able to gain predominance. No error will be committed in 

 ignoring the results of the fermenting test in the cases mentioned, 

 for the small numbers of bacteria in the best milk, whatever their 

 nature, will not be able to exert any influence on the mixed milk 

 of the dairy, while the worst milk will already have been placed in 

 the lowest class, and cannot therefore be degraded any further. In 

 the combined test, the fermentation need not be observed until 

 after the lapse of twenty to twenty-four hours as the reductase 

 test gives a far more accurate estimate of the number of bacteria 

 present than the fermenting test after twelve hours. It should 

 be clearly understood that while the indications afforded by the fer- 

 menting test are purely qualitative, those afforded by the reductase 

 test are purely quantitative, and it is only by combining the two tests 

 that any real insight will be obtained into the nature of the bacterial 

 contents of the milk. 



In large dairies it will only be possible to take an average sample 

 of each supplier's milk. If the number of suppliers is not large, 

 there is no reason why the morning and evening milk should not 

 be tested separately provided that the cans are properly marked 

 according to the respective meals as they always should be. 

 When the milk only comes to the dairy in the morning, it will 

 generally be found that the morning milk will be better than the 

 evening milk, though if the milk comes from a long distance, the 

 reverse may be the case during warm weather if the morning milk 

 has not been cooled. In such cases it will also be necessary to 

 cool the morning milk. The more frequent the test the juster will 

 be the impression formed as to the relative goodness of the milk 

 from different suppliers. Large dairies which of course should 

 keep a well-equipped laboratory under the guidance of a chemist, 

 who has been trained in bacteriology, will do best to test each 

 supplier's milk daily. In the co-operative dairies, samples from 

 all the suppliers should be tested once a week, and one of the 

 members of the association should be present in turn, as an 

 impartial witness. The co-operation of expert milk tasters is no 

 longer absolutely necessary in view of the more objective methods 

 which are now at our disposal ; the examination of samples in the 

 reductase test requires no particular scientific knowledge, and may 

 be performed by any reliable boy or girl. Samples which need 

 not be subjected to the fermenting test should be removed from 



