46 PRINCIPLES OF BACTERIOLOGY 



heating to 100 C. an exposure on three consecutive days, and to 

 115 C. one or two suffices. 



PASTEURIZATION. It is sometimes undesirable to expose food, such 

 as milk, to such a temperature as will destroy spores, because of the 

 deleterious effects upon food values of such high temperatures, and yet 

 where a partial destruction is necessary. In these cases we heat 

 the foodstuffs for from five to thirty minutes to such a temperature 

 (70 C.) as will kill most of the bacteria in the vegetative form, but 

 allow the spores to remain alive. Even this amount of sterilization kills 

 about 90 to 98 per cent, of the bacteria. The exposure to this degree 

 of heat alters the chemical composition of the milk but little. 



