388 Bacteria in Relation to Country Life 



some instances, while other samples from the same lot, 

 kept under ordinary conditions, turned sour in a few 

 days. While carbon dioxid, under pressure, retards' the 

 development of the bacteria, and particularly of the lactic- 

 acid bacteria, it does not destroy them. In one series of 

 experiments, milk was kept for some hours under a 

 pressure of fifty atmospheres of carbon dioxid, at the 

 end of which time the organisms were found capable of 

 growth when the pressure was removed. 



Centrifugal force. — When subjected to centrifugal 

 force, milk may be deprived of the heavier particles of 

 dirt and, to some extent, also, of the smaller particles 

 including the bacteria. It has been suggested, therefore, 

 that cream separators, run at a low speed might prove 

 useful for removing bacteria from milk, and for improv- 

 ing its keeping quality. 



The investigations made in this connection have 

 demonstrated that a portion of the solid impurities in 

 milk can be thus removed. They have shown, at the 

 same time, that the number of bacteria in milk is either 

 slightly or not at all diminished by this treatment, and 

 that the keeping quality is scarcely improved. The 

 expense and labor involved, as well as the partial sepa- 

 ration of the cream when excessive speed is used, together 

 with the slight benefits derived, may, therefore, be urged 

 as serious objections against the use of the centrifuge 

 for this purpose. 



Sterilization by heat. — Theoretically, the most effec- 

 tive means for the destruction of bacteria in milk would 

 be its boiling for some time. It is well known that all 

 bacteria that do not produce spores are readily killed 



