134 THE BACTERIAL CONTENT OF MILK 



The results may be briefly summarised thus : — 



(1) Milk at 37° after i8 hours was sour to taste. 



„ „ 24 „ curdled in flask. 



(2) Milk at 15° after 66 hours was sour to taste, 



„ on 7th day was curdled in flask. 



(3) Milk at 10° after 120 hours was sour to taste. 



„ on 7th day was curdled in flask. 



These results are of interest, and compare closely with the 

 results obtained during the bacteriological experiments above re- 

 corded. Thorner has investigated the acidity requisite to coagulate 

 milk, and fixed it at 22 degrees or 0-207 P^'' cent, of lactic acid. 

 We have found the coagulating point somewhat higher than this, 

 and would suggest that when the acidity is above 0-25 per cent. 

 lactic acid the milk should be considered unfit for dietetic use. 



Fifthly, as the flasks of milk were kept intact, we were able to • 

 repeat the experiment in every particular after the lapse of exactly 

 two years (April 1902) from the commencement. The milk was 

 the same milk, and the experiment was repeated as at 30 days. 

 During the intervening period the flasks had been kept, hermeti- 

 cally sealed, at the three temperatures. The result was that the 

 flasks were found to be germ free with the exception of an abundant 

 growth of Oidiuin lactis and other moulds. {^See Plates 14-17.) 



As far back as 1878 Lister pointed out the marked inhibitory 

 effect which the presence of a high degree of lactic acid had upon 

 common moulds and ordinary saprophytic bacteria.^ When the 

 lactic acid declines, these other forms commence growth, and j 

 eventually enormously preponderate. ; 



■ The question naturally arises, What is the explanation of these j 

 facts ? We believe the answer to this question will be found in a \ 

 glance at the life-history of such a milk as that under consideration. ; 

 At the time of milking there is, as we have seen, an introduction j 

 into the warm milk of vast numbers of common saprophytic and | 

 parasitic bacteria. Finding themselves in an ideal nidus, they 

 multiply with almost incredible rapidity. Hence the first rise in 

 numbers of bacteria. Competition and exhaustion of pabulum 

 soon produce inevitable effects, and we obtain the first decline. At 

 this stage it may be said that the common extraneous bacteria, 

 whether putrefactive or simple saprophytes, practically die out. 

 and that for a very simple reason, namely, that they cannot live in 

 the presence of the new tide of acid-forming bacteria. Although 



1 Path. Soc. Trans., 1878, p. 440. 



