•GROWTH OF BACTERIA IN NORMAL MILK. 57 



III. THE COMPARISON OF THE GROWTH OF BACTERIA AT 



20 DEGREES CENTIGRADE AND AT 1 3 DEGREES 



CENTIGRADE. 



Twenty degrees is a temperature higher than that at which 

 milk is usually kept when it is intended for market. The milk 

 dealer recognizes the value of low temperatures in preserving 

 milk and commonly endeavors to reduce the temperature as 

 rapidly and as low as possible to check the growth of bacteria, 

 and hence to delay the spoiling of the milk. Our next series 

 of experiments was designed for the purpose of determining 

 with more accuracy, not only the value of a moderately low 

 temperature in reducing the number of bacteria, but also the 

 influence of such low temperatures upon the variety of bac- 

 teria which develop in the milk or which remained alive in 

 the milk after exposure to low temperatures. For this pur- 

 pose we have compared two samples of milk placed at different 

 temperatures, and have studied in each case the development 

 of the different species of bacteria. The method of experi- 

 mentation was as follows: 



The milk was obtained from the same barn as that from 

 which most of the other samples described have been obtained. 

 It was drawn directly from the cow into a sterilized vessel, 

 brought to the laboratory and plates made from some of it 

 immediately. The rest was then divided into two lots, one 

 of which was kept at the ordinary temperature of 20° C, the 

 temperature varying slightly in successive hours, and the other 

 placed at a temperature of 13° C. This temperature was chosen 

 as being approximately that which the milk dealers frequentl}^ 

 attempt to obtain in their milk for the purpose of preserving 

 it. A bacteriological examination w^as made of each of these 

 samples at different intervals. The milk kept at 20° was 

 plated, as in the above experiments, at intervals of about .six 

 hours for a period of twenty-four hours. Milk kept at 13° was 

 not, however, tested so frequently. At the outset of the 

 experiments we made the tests of this sample at the same in- 

 tervals as the sample kept at 20°, but it was soon found that 

 the development of bacteria was so ver}^ slow at the lower 

 temperature that the series of plates showed practically no 

 change from one test to another, the milk at the end of twenty- 

 four hours being almost like the milk at the start. To decrease 



