132 THE ENUMERATION OF BACTERIA IN MILK 



relationship between the number of bacteria and the time 

 required for decolourisation. 



The author examined a number of milks by this test in 1914 

 but was unable to find any merit in it. Almost all the samples 

 failed to decolourise in the six hours that were available lor 

 observation under ordinary laboratory conditions, and they had 

 generally showed reduction by the following morning (twenty- 

 one hours). As over 90 per cent of these samples contained 

 less than one million bacteria per cubic centimetre these results 

 are not inconsistent with Fred's (vide supra), but as the time 

 of reduction could only be determined within wide limits no 

 real information could be deduced as to the bacterial condi- 

 tion of the sample, except that it did not contain very excessive 

 numbers. Samples that were allowed to stand and develop 

 large numbers of organisms showed small reduction periods and 

 it would seem that it is in the detection of such milk that the 

 chief value of the test lies. 



A further rapid indirect method that has been suggested 

 for the approximate determination of the bacterial content of 

 milk is the estimation of the acidity. Milk almost invariably 

 contains acid-producing organisms, and as these find milk an 

 excellent mediirai for development it would seem to be logical 

 to assume that the determination of the products of bacterial 

 metabolism would bear some relation to the number of organisms 

 present. Fred (vide supra) determined the acidity of 200 

 samples of milk and arranged the results into groups of 20 

 according to the bacterial count. His results are given in 

 Table LII. 



Fred is of the opinion that the acidity determination serves 

 a useful purpose in indicating to some extent the proper dilu- 

 tions to be used for the bacterial counts, and adds that " the 

 relationship to the number of bacteria is only approximate." 

 Russell and Hastings have also suggested using this test as a 

 guide to the dilutions to be made in the plate method and advise 

 10, 100, and 1,000 dilutions for acidities under 0.2 per cent and 

 1,000, 10,000 and 100,000 for acidities over 0.2 per cent. 



