198 PASTEURISED OR HEATED MILK 



does not necessarily signify that the milk was particularly rich 

 in bacteria of putrefaction. If curdling is accompanied by an 

 offensive odour or, if the coagulum is peptonised, the presence of 

 putrefactive bacteria is inferred. He continues, " by boiling 

 milk a short time and then incubating, only spore formers 

 develop, and as these are not checked by the lactic bacteria, 

 they increase rapidly and cause the milk to curdle by the action 

 of ferments. Pasteurised milk does not sour, but no precipitate 

 conclusions should be drawn from the results of this test." 



Peter, 1* DugelU,i^ and Klein '® have used this test for milk 

 examination and find that it gives the prevailing types of micro- 

 organisms with a considerable degree of accuracy. A combina- 

 tion of the fermentation test with the methylene blue reduction 

 test has been recommended by Lohnis and Schroeter,^^ and by 

 Fred and Chappelean.^^ 



In 1914 the author compared the results obtained by this 

 test with the usual bacterial count on agar (forty-eight hours at 

 blood heat) and the B. coli count in rebipelagar. The samples 

 were transferred to sterile tubes plugged with absorbent cotton 

 and incubated at 37° C. (98.5° F.) for 20-24 hours. 787 sam- 

 ples of ordinary raw milk, 98 samples of pasteurised milk, and 

 69 samples of nursery milk were examined in this way and the 

 results recorded according to the classification of Dugelli 

 (vide supra). This classification, together with the bacterial 

 flora which Dugelli states is indicated by each type, is as follows: 



Types of Curd 



Type A 



Liquid. The sample does not show any marked change 

 except perhaps a slight deposit on the bottom of the tube. 



1. Completely liquid, sweet or sour taste. 



2. Somewhat coagulated at, the bottom or on the walls. 



3. A sUght ring of curd under the cream, but otherwise 

 liquid and sour. 



