IOO GROWTH OF BACTERIA IN MILK. 



while those at 96 were taken from a different sample, which 

 at the outset contained about the same number of bacteria. 



F 8 HOURS 19 HOURS 52 HOURS 



AT 96. AT 7 0. AT 5 0. 



No. bacteria 13,000 240,000 34,900 62,000 



The significant conclusion from these figures is that tem- 

 perature is a greater factor in determining the number of 

 bacteria than the extent of the original contamination. In 

 eight hours, if kept at warm temperature, the number of 

 bacteria may be larger than it is in twenty hours at 70 or 

 in sixty hours at 50. At 50, indeed, milk may sometimes 

 be kept for forty hours without any appreciable increase in 

 bacteria, a fact which explains the possibility of transporting 

 milk in ice cars for a distance of 500 miles, and its delivery 

 in a city like New York in a condition, so far as bacteria are 

 concerned, actually better than many a sample of milk which 

 is much fresher but has not had so much attention given 

 to artificial cooling. If kept at 45 F. the milk need not con- 

 tain more than 2,500 bacteria in thirty-six hours. If cooled 

 to this temperature within three hours after milking the 

 number should never be more than 1,000,000 to 2,000,000 

 even in two days, and usually far less. Hence, if milk shows 

 larger numbers than these it indicates that it has not been 

 properly cooled. The lesson to be emphasized by this is that 

 by the use of a low temperature a milk dealer may keep his 

 milk for a long time, and that the secret of furnishing satis- 

 factory milk lies in the matter of temperature. The effect 

 of temperature is shown graphically by Fig. 20. 



There are other factors besides temperature concerned in 

 the increase of bacteria, the nature of which is not under- 

 stood. It may happen that the increase of bacteria shows 

 quite unintelligible variations. Two samples of milk may 

 be placed side by side under identical conditions, and yet 

 the development of bacteria in one will be far more rapid 

 than in the other, and in the end the sample that contained 



