12 



BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 



umns. These figures tire based on a number of actual counts and 

 illustrate the effect of a difference of 18° on the multiplication of 

 bacteria. If the milk had contained at the beginning 1,000 bacteria, 

 the part held at the lower temperature would have contained at the 

 end of 24 hours only 4,100 bacteria, while the other would have con- 

 tained at the same stage 6,128,000. Table V, from Bulletin 133 

 (Extension Bulletin S) of the agricultural experiment station of 

 Nebraska, illustrates the importance of holding cream at low tem- 

 peratures. 



Table IV. — Multiplication of bacteria in milk held at different temperatures. 1 



Milk held at— 



Relative number of bacteria held at— 



hours. 



6 hours. 



12 hours. 



24 hours. 



48 hours. 



50° F 



1 



1 



1.2 

 1.7 



1.5 



24.2 



4.1 

 6,128 



6.2 



68° F 



357, 499 







1 Rogers, Lore A. Bacteria in milk. U. S. Department of Agriculture, Farmers' Bulletin 490. 

 Washington, D. CT 



Table V. — The effect of temperature on the growth of bacteria in cream. 



Temperature of cream. 



°F. 



32 



50 



60 



Time 

 held. 



Hours. 

 10 

 10 

 lOi 



Number of 

 bacteria 

 per c. c. 



3,300 

 11,580 

 15, 120 



Temperature of cream. 



Time 

 held. 



Hours. 

 11 

 11 

 lli 



Number of 

 bacteria 

 per c. c. 



188,000 

 2,631,000 

 4,426,000 



THE INFLUENCE OF TIME ON THE BACTERIOLOGICAL FLORA OF MILK. 



The influence of temperature and time bear certain definite rela- 

 tions to each other; hence, a study of one necessarily includes a study 

 of the other. Table VI serves to illustrate the effect of time as well 

 as temperature on the keeping qualities of milk. If the table is read 

 downward we note the effect of temperature and if read across the effect 

 of time. When milk is first drawn from the cow it usually contains 

 bacteria, even though it is produced under sanitary conditions, and 

 if held at the ordinary temperature of the surrounding air in a short 

 while the bacteria will grow and increase in numbers so rapidly that 

 when such milk reaches the consumer it will contain many thousand 

 bacteria per cubic centimeter. 



Conn furnishes an example of milk giving the following results : 



Bacteria per c. c. 



Milk drawn at 59° F 153, 000 



After 1 hour 616, 000 



2 hours 539, 000 



4 hours 680, 000 



7 hours 1, 020, 000 



9 hours 2, 040, 000 



24 hours 85, 000, 000 



