THE GROWTH OF BACTERIA IN MILK 8l 



PHASES OF BACTERIA GROWTH IN MILK 



Germicidal Phase. For some time after milk has been drawn 

 from the cow bacteria fail to develop; indeed, many tests have 

 shown that for the first few hours there is an actual reduction 

 in^e numbers of bacteria. 1 The exact reason for this reduc- 

 tion has been the subject of some discussion, but the different 

 ways of explaining it need not occupy our attention. The 

 general feeling has been that there is present in milk some sub- 

 stance which is inimical to some kinds of bacteria and actually 

 destroys them. For this reason this period has been called the 

 germicidal phase. It lasts for a varying time, depending largely 

 upon temperature. It commonly continues only a few hours, 

 sometimes not more than two. At other times it may last for 

 several hours, especially if the milk is kept cold. (See experi- 

 ment on page 297.) The practical value of this fact is simply that 

 for a period of a few hours after milk is drawn the bacteria 

 do not develop, and the handler of the milk has, thus, a short 

 period in which he need not fear much bacteria growth, and 

 during which he can bring the milk into conditions for further 

 preservation. 



First Period of Growth. Following the germicidal phase the 

 bacteria grow with a rapidity depending, as we have seen, on 

 temperature. 2 For a number of hours there is an increase of 

 most species. Most of the kinds of bacteria present in the milk 

 find it a favorable medium, and multiply with varying degrees 

 of rapidity according to the nature of the species. But some 

 multiply more rapidly than others, so that the proportion of 

 the different types will constantly change as the hours pass. 

 For a period of 12 hours, at ordinary temperatures, there is 



* Hunziker. Bui. 197, Cornell Agri. Exper. Sta., 1901. 



Koning. Milch. Zent., I., p. 99, 1905. 



Mayer. Rev. Gen. d'Lait, II., p. 381, 1903. 

 2 Koning. Rev. Gen. d'Lait, 1904. 



Gissier and Gasching. Ann. d. 1'Inst. Past., xvii., p. 540, 1903. 



