MICRO-OKGANISMS IN MILK 331 



seems to take place after the lapse of a few days. This at least 

 is the opinion of some investigators. The importance of prompt 

 cooling immediately after milking is emphasized by this fact, so 

 as to prevent initial multiplication. 



Conn and Esten observed that when milk was kept at 1 C. 

 there was no increase of germ life for six to eight days; after that 

 time there was decided growth, but no lactic acid bacteria seemed 

 to multiply. The authors think, therefore, that when milk is 

 kept at low temperature for some time it becomes unfit for food, 

 especially for infant feeding, as the organisms multiplying at this 

 temperature do not change the milk in such manner as to render 

 the altered condition easily detectable. The milk, according to 

 this reasoning, may become a menace. However, it has not 

 been conclusively shown that the products of these bacteria are 

 really injurious. 



Pennington found in milk kept at 29 to 32 F. that both acid- 

 forming and proteolytic bacteria multiply. Ravenel, Hastings, 

 and Hammer held milk at 9 C. and observed no increase in bac- 

 teria that grow on agar and gelatin; they further found that the 

 acidity decreased, while the amount of soluble nitrogen increased. 

 In milk kept at C. there was a marked increase in bacterial 

 content, also an increase in acidity and soluble nitrogen. The 

 period of observation was 203 days. According to the work of 

 these authors increase in soluble nitrogen is not necessarily due to 

 multiplication of bacteria, but possibly to enzyms produced by 

 them. 



Bischoff observed that in milk kept at to 1.5 C. there was 

 a decrease of bacteria for four to seven days, after which period 

 there was an increase in number and in acidity. 



There seems to be sufficient agreement in the work of a num- 

 ber of investigators to assume that at a temperature C. or some- 

 what higher there is an initial decrease of germ content, followed 

 by a marked increase. It is probable that many bacteria cease 

 multiplying at such low temperature and gradually die, similar 

 to conditions observed in ice. But some types are tolerant, in a 

 measure at least, toward low temperatures, and finding suitable 

 food conditions in milk, multiply, at first slowly and later with 

 greater rapidity. The study of the types that are able to mul- 

 tiply in cold milk is still incomplete. 



In this connection Rullmann's investigations are interesting. 

 The following table gives the author's results of bacterial enumera- 

 tions in raw and pasteurized milk kept at 4.5 to 5.5 C. : 



NUMBER OF BACTERIA IN RAW AND PASTEURIZED MILK AT 4.5 TO 5.5 C. 



1 day. 2 days. 3 days. 4 days. 5 days. 6 days. 7 days. 



Raw milk 21,120 23,680 121,080 338,560 Infinite Infinite Infinite 



Pasteurized 60 40 30 360 23,040 209,920 Infinite 



