16 



BULLETIN 202, U. S. DEPARTMENT OP AGRICULTURE. 



alcohol test remained positive, although the size of the flakes was 

 reduced. This milk after 5h hours' incubation had an acidity of 4.38 

 and the milk coagulated with large flakes with each percentage of alco- 

 hol. When the acidity was reduced to 1.49 the alcohol test remained 

 positive, the only change being with the 44 per cent alcohol, in which 

 case the size of the flakes was reduced. This experiment was re- 

 peated, as will be seen from Table 12, and the results confirmed 

 those of the first experiment. These results indicate that it may be 

 possible to differentiate between an acid and an acid-and-rennet 

 fermentation in milk, provided the acidity is not high. 



Table 12. — Differentiation between an acid and a mixed acid-and-rennet fermentation by 

 neutralizing the acidity and using the alcohol test. 



Ex- 





After incubation at 37° C. for 3 hours. 



After incubation at 37° 



C. for 5$ hours. 















peri- 



Pure culture 





Alcobol test. 





Alcohol test. 



ment 

 No. 



of— 



Acidity. 







Acidity. 

























75 per 



68 per 



44 per 





75 per 



68 per 



44 per 









cent. 



cent. 



cent. 





cent. 



cent. 



cent. 



I. 



Lactic - acid 

 bacteria. 



2.23 



l +L 



+L 



- 



3.21 



+L 



+L 



+L 







Neutralized to 



— 



— 



_ 



Neutralized to 



— 



— 



— 







1.66. 









1.49. 











Mixtureof 



2.32 



+L 



+L 



— 



4.38 



+L 



+L 



+L 





rennet- 





















forming 





















and lactic- 





















acid bacte- 





















ria. 























Neutralized to 



+s 



+VS 



— 



Neutralized to 



+L 



+L 



+s 







1.70. 









1.49. 









II. 



Lactic - acid 

 bacteria. 











2 2.70 



+L 



+M 



— 













Mixtureof 



3 2.00 



+L 



+M 



_ 



2 3.42 



+L 



+L 



+L 





rennet- 





















forming 





















and lactic- 

















i 





acid bacte- 





















ria. 























Neutralized to 



— 



— 



— 



Neutralized to 



+L 



+M 



+VS 







1.80. 









1.60. 









Note.— Acidity of normal milk in experiment 1, 1.75; in experiment II, 1.78. 



1 See footnote under Table 1. 



2 After 4J hours incubation at 37° C 



s Acidity after adding pure cultures to milk and before incubation. 



EFFECT OF HEAT COMBINED WITH RENNET ACTION. 



Earlier in this paper we have shown the effect of heat on the alcohol 

 test with milk of high and low acidity, and as a matter of general 

 interest the effect of heat on the alcohol test produced by rennet 

 action may now be considered. The results of two experiments 

 shown in Table 13 explain themselves clearly. Sufficient rennet was 

 added to two samples of milk to cause a positive alcohol test with 75 

 and 68 per cent alcohol. The milk was then heated, and it was found 

 that at 90° C. the milk no longer gave a positive alcohol test. Both 

 experiments showed the same results. 



