296 FILTRATION AND HEATING OF MILK 



The great reduction in numbers by pasteurization is 

 seen in the following table compiled from Professor 

 Stewart's experiments : 



It is important to bear in mind that pasteurization is 

 a double process, namely, the application of a tempera- 

 ture sufficient to destroy many living bacteria, followed 

 by rapid cooling. To disregard the latter part of the 

 process is to render it useless ; for if this is neglected, 

 and the heated milk allowed to cool slowly, the remain- 

 ing uninjured organisms multiply rapidly when the 

 optimum temperature is reached, and the milk may 

 contain more bacteria after treatment than before. 



Ex. 142. Take 250 c.c. of fresh evening's milk and keep it 

 in the pantry until next morning. 



Dilute a small amount of it to i in 1000 with sterile water, 

 and prepare two litmus lactose agar plates, using .5 c.c. for 

 inoculation. 



Heat (i) 50 c.c. of the milk to 50 C. for 10 minutes. 



(2) 60 C. 



(3) 7 C- ,, 

 Cool quickly. 



Dilute a small amount to i in 100, and prepare two litmus 

 lactose agar plates from each sample, using .5 c.c. for 

 inoculation 



