16 
BULLETIN 202, U. S. DEPARTMENT OF AGRICULTURE. 
alcohol test remained positive^ although the size of the flakes was 
reduced. This milk after oh 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- 
peri- 
ment 
No. 
Pure culture 
of— 
After incubation at 37° C for 3 hours. 
After incubation at 37° 
C. for 5i hours. 
Acidity. 
Alcohol test. 
Acidity. 
Alcohol test. 
75 per 
cent. 
68 per 
cent. 
44 per 
cent. 
75 per 
cent. 
68 per 
cent. 
44 per 
cent. 
I. 
Lactic - acid 
bacteria. 
Mixtureof 
rennet- 
forming 
and lactic- 
acid bacte- 
2.23 
Neutralized to 
1.66. 
2.32 
!+L 
+L 
+L 
+L 
- 
3.21 
Neutralized to 
1.49. 
4.38 
+ L 
+L 
+L 
+L 
+L 
+L 
II. 
ria. 
Lactic - acid 
bacteria. 
Neutralized to 
1.70. 
+s 
+VS 
- 
Neutralized to 
1.49. 
2 2.70 
+L 
+L 
+L 
+M 
+s 
Mixtureof 
rennet- 
3 2.00 
+L 
+M 
- 
2 3.42 
+L 
+L 
+L 
forming 
and lactic- 
acid bacte- 
t 
ria. 
Neutralized to 
1.80. 
- 
- 
- 
Neutralized to 
1.60. 
+L 
+M 
+VS 
Note.— Acidity of normal milk in experiment I, 1.75; in experiment II, 1.78. 
1 See footnote under Table 1. 
2 After 4J- hours incubation at 37° C. 
3 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. 
