GROWTH OF BACTERIA IN NORMAL MILK. 77 



J. The question of temperahwe is a viattei'' of more signifi- 

 cance in determini^ig the nnmber of bacteria %vhich are present at 

 any stage than the question of the original contamination of the 

 milk. 



^. In the general changes zvhich occur in the species present in 

 milk tivo stages are clearly seen, which are not, hozvever, sharply 

 ma?'ked off front each other. The first is the period of the first 

 few hours, when the mimber of bacteria is less than abozit 10,- 

 000,000. This period lasts, in milk kept at 20° , for about tiventy- 

 four hours, but if kept at rj° it lasts for aboiU fifty hours or even 

 77iore. The second period is that in zvhich the total number of 

 bacteria is over 10,000,000, and from this up to the time of curd- 

 ling. 



FIRST PERIOD. 



5. For a number of hours after the milk is drawn from the 

 cow there is 710 multiplicatiofi of bacteria, but, on the contrary, 

 frequently an actual reductio7i i7i 7iu77ibers. This has bee7i 

 poi7ited out before and has bee7i called a ''''ger7nicide prope7'ty,'''' 

 lasting for a varying le7igth of time up to forty hours, the le7igth of 

 ti77te dependi7ig so7newhat iipon te77iperat7ire. After the first six 

 hours at 20° most species of bacteria begi7i to increase m absolute 

 niu7nbers; so77ie, hozvever, re77iai7i without a7iy appreciable growth 

 and others see77i to disappear, not bei7ig found i7i the later a7ialy- 

 ses. This 77iea7is, at all eve7its, that they do 7iot 77iaterially develop 

 i7i 7iu77ibers, although they m,ay not actually be destroyed. 



6. At 20° the7^e is a u7iifor77i increase hi B. acidi lactici dur- 

 i7ig the first pe/iod of tzve7ityfiou7 hours. The total 7iumbers 

 i7icrease reg7da7dy a7id the perce7itage also i7ic7'eases, sometimes 

 7'eaching ^o per ce7it. i7i the course of twe7ityfiour hours, although 

 more C077i77i07ily it is far less, f 7^07/1 10 to jo per ce7it. 



y. At 20° the other lactic bacteria are, i7i 77iilk i7i this r-egio7i, 

 irregular i7i their develop77ie7it. B. acidi lactici //. occurs in 

 fresh milk only ra7-ely. It is occasionally found i7i so7ne qua7itity 

 at the e7id of tzve7ityfiour hours but is 7iot usually of 7nuch i7npo7'- 

 tance. B. aeroge7ies is foimd zvith 77iore or less regula7'ity, but 

 never i7i very la7ge nu77ibers. hi the 7iu77ierous sa7nples of /7iilk 

 which zve have tested this orga7iis77i is 7'a7'ely p7'ese7it i7i p7'opo7'- 

 tio7is greater tha7i 2 to j per ce7it. 



