CHANGES PRODUCED IN MILK BY BACTERIA 375 



governed, in addition to the factors considered above, by age and 

 temperature. The influence of temperature is illustrated by the 

 following: 



Temperature 



maintained for Bacteria per c.c. at Hours to curdle 



twelve hours F. end of twelve hours. at 70 F. 



40 4,000 75 



45 9,000 75 



50 .. 18,000 72 



55 38,000 49 



60 453,000 43 



70 8,800,000 32 



80 55,300,000 28 



All of these samples at first contained the same number of bacteria 

 but were kept for twelve hours at the different temperatures and 

 then all maintained at the high temperature. We find over ten 

 thousand times as many bacteria at the end of twelve hours in the 

 sample kept at a high temperature as the one kept at a low. 

 Although the difference in temperature was maintained for only 

 twelve hours, the milk at 40 kept three times as long as did that at 

 80. 



Changes Produced in Milk by Bacteria. The changes occurring 

 in milk are governed by the specific bacterial flora which it contains 

 and the temperature at which it is kept. Normal clean milk, if 

 kept at a temperature of between 10 and 21 C., passes through a 

 sequence of changes which can be divided into four stages. 



First Stage. The first of these is known as the germicidal stage, 

 and lasts a few hours after the milk has been drawn from the udder. 

 During this stage there is a decrease in the number of organisms, 

 as shown by the plate method. The extent of this decrease varies 

 with the milk of different cows and the temperature at which the 

 milk is kept. The higher the temperature, the more marked the 

 decrease, the sooner the end of the germicidal period is reached. 

 There is a great difference in opinion among bacteriologists con- 

 cerning the nature of the phenomenon. Some would account for 

 it on the grounds that milk is a favorable cultural media for many 

 bacteria, but not all. The ones for which it is unsuited rapidly 

 die off. Others consider that the milk, like the blood and many 

 other body fluids, possesses bactericidal power which is very weak 

 and soon lost. Rosenau and McCoy, however, consider that the 

 bacteria are agglutinated and not killed. On plating, the clump 

 gives rise to the colony in place of each individual organism, as is nor- 

 mally the case. 



This germicidal power is lost on boiling the milk or heating to a 

 temperature of 80 C., and some have urged this as an objection 

 against pasteurization, but in the "holder" process this is not a 

 warranted objection. 



