GROWTH OF BACTERIA IN NORMAL MILK. 47 



5. The liquefying bacteria show the greatest fluctuation. 

 In all plates they increase in actual numbers and in some 

 experiments they have increased in relative abundance for 

 twenty hours. In others there is, however, a decline of per- 

 centage and there is thus a general indication that they do not 

 multiply so rapidly as the B. acidi lactici and the Streptococcus 

 group, although their increase is quite appreciable in all of 

 these experiments. 



6. The miscellaneous bacteria may be two or three species 

 or half a dozen, or even more; but they are present in small 

 numbers. They do not greatly multiply during the first 

 twenty-four hours, and at the close either have disappeared 

 or are present in such small numbers as not to be noticeable in 

 the tests. They cannot, therefore, be of much significance in 

 the changes that occur in the milk. 



7. In general it will be seen that in these experiments the 

 bacteria which are chiefly concerned in producing the changes 

 which must take place in the milk are the B. acidi lactici, the 

 Streptococcus group, and the liquefiers. The slow liquefiers in 

 all cases were not only much more abundant than the rapid 

 liquefiers but they multiplied more rapidly in the milk. 



II. THE EFFECT OF ICING THE MILK UPON THE DEVELOPMENT 

 OF BACTERIA. 



The experiments described below were part of a series de- 

 signed to determine the effect of the preliminary icing of the 

 milk upon the growth of bacteria. Several experiments were 

 performed, but only these two are given here, since these are 

 the only ones in which the results were satisfactory. The 

 method of experimentation was as follows: 



Experiment No. 8. October. The milk was drawn directly 

 from the cow into sterilized vessels, taken immediately to the 

 laboratory, and a number of plates were made from it at once. 

 The rest of the milk was then divided into two lots, one of 

 which was placed upon ice for fourteen hours, and the other 

 was kept for the same time at the normal room temperature 

 of 20° C. without the preliminary icing. At the end of four- 

 teen hours some plates were made from both lots of milk, and 

 from that time plates were made every twelve hours from both 

 samples until the milk was curdled. 



