THE ALCOHOL TEST IN RELATION TO MILK. 



15 



In order to show the effect of rennet of bacterial origin, the action 

 of a pure culture of a rennet-forming organism was studied. Two 

 flasks of sterile skim milk were inoculated with different amounts of a 

 pure culture of a rennet-forming organism. These flasks were 

 incubated at 37° C, and the bacterial increase was determined at 

 definite intervals, together with the alcohol test. The results are 

 shown in Table 11. From a study of the table it is evident that 

 rennet-forming bacteria will cause a positive alcohol test, but there 

 must be a large bacterial increase to produce rennet enough to cause 

 a positive test. 



The acidity was also increased during the incubation, but we believe 

 this acidity played a minor part in causing the positive alcohol test. 



Table 11. — Influence on the alcohol test of rennet produced in milk by the growth of a pure 

 culture of a rennet-forming organism. 



Experi- 

 ment 

 No. 



Age of 

 cul- 

 ture. 



Bacteria 

 per cubic 

 centimeter. 



Acid- 

 ity. 



Alcohol test. 



75 per 

 cent. 



68 per 

 cent. 



44 per 

 cent. 



I 

 II 



Hours. 

 

 2 

 4 

 5 

 6 

 7 

 

 2 

 4 



34,000 



62,000 



4, 700, 000 



9,000,000 



21,000,000 



31,000,000 



147,000 



200,000 



15,000,000 



1.98 



2.02 

 2.06 

 2.10 

 2.11 

 1.94 



2.10 



1 +L 

 + L 

 +L 



+ L 



+ L 

 + L 



+ L 



+ L 



+s 



1 See footnote under Table 1. 



DIFFERENTIATION BETWEEN ACIDITY AND RENNET ACTION. 



The fact that reducing the acidity did not cause a negative alcohol 

 test, as mentioned above, led us to believe that it might be possible 

 to differentiate between a positive alcohol test caused by acidity and 

 one caused by rennet action. In order to determine whether this 

 was true two flasks of sterile skim milk were prepared. One was 

 inoculated with a pure culture of a lactic-acid-forming organism and 

 the other with equal amounts of a pure culture of lactic-acid bacteria 

 and rennet-forming bacteria. The two flasks were then incubated at 

 37° C. As may be seen from Table 12, the milk containing the 

 lactic-acid bacteria had an acidity of 2.23 after 3 hours' incubation 

 and the test was positive with both 75 per cent and 68 per cent 

 alcohol. When the acidity was reduced to 1.49 all the alcohol tests 

 were negative. The milk containing a mixed culture of lactic-acid 

 bacteria and rennet-forming bacteria after 3 hours' incubation had 

 an acidity of 2.32 and the alcohol test was positive with 75 per cent 

 and 68 per cent alcohol. In both cases the coagulation was in the 

 form of large flakes. When the acidity was reduced to 1.70 the 



