
MILK PRESERVED AT SEVENTY AND FIFTY DEGREES. 49 
their chief effect upon lactic bacteria, which do not grow so 
rapidly as the other forms, and thus at the end of 50 hours are 
not only less in absolute numbers, but also less in relative 
abundance than in milk kept at 70°. 
3. Lhe effect of straining.—These tables show. as little value 
in straining as was shown in Tables 3 and 4. If Tables 5 
and 6 are compared with each other, it will be seen that the 
strained samples contained at 50 hours slightly larger average 
numbers of acid bacteria than did the unstrained samples; and 
also that the per cent. of acid was very slightly lower in the 
strained than in the unstrained samples. ‘The difference, how- 
ever, is too slight to have any meaning. 
4. Lhe development of actd.—In the column giving the de- 
velopment of acid it will be noticed that during this 50 hours 
growth at 50° there was only a very slight increase in the 
amount of acidity. The acidity in the original milk averaged 
about .18 per cent. In the 50-hour milk kept at 50° the aver- 
age, it will be seen, is only .20 per cent., there being thus 
almost no increase in the amount of acidity during this time. 
This is extremely interesting, taken in connection with the 
fact that there has been such an immense increase in the num- 
ber of acid bacteria. Table 5, for example, shows that in the 
fresh milk there was only an average of 1,903 acid bacteria, 
whereas in the 50-hour milk the number of acid bacteria aver- 
aged 3,000,000. In spite of this immense increase in num- 
bers the increase in acidity was almost inappreciable. The 
suggestion from this is that the development of the acid in 
milk is not the direct result of bacterial growth, but occurs only 
later, after the acid organisms have taken full possession of the 
milk and have come to compose a large part of the organisms 
present. | 
5. The effect of 50° upon time of curdling.—The most re- - 
markable and significant result shown by this table is the effect 
of the comparatively slight lowering of temperature upon the 
keeping property of the milk. In Tables 3 and 4, with the 
temperature at 70°, it required 87 and 97 hours respectively 
to produce a sufficient acidity of the milk to curdle it. A 
lowering to 50° produced a very unexpected increase in the 
keeping power. It will be seen from Tables 5 and 6 that 
