174 AGRICULTTJEAL AND INDUSTEIAL BACTEEIOLOGT 



that are slightly acid as well as upon those which are neu- 

 tral or slightly alkaline. 



Methods of Preventing Deterioration. — In order to pre- 

 vent deterioration food must be kept so that there can be 

 no appreciable activity of autolytie enzymes and no growth 

 of bacteria, yeasts, or molds. This may be accomplished by 

 heating to a temperature and for a time sufficient to destroy 

 all microorganisms and enzymes present, or by the process 

 termed pasteurization, whereby those organisms capable of 

 bringing about detrimental changes are destroyed without 

 necessarily making the material sterile; by holding the 

 foods at temperatures so low that undesirable changes can- 

 not be brought about; by the presence of chemicals which 

 will act as preservatives or antiseptics; and finally by the 

 elimination of water to such a degree that microorganisms 

 cannot grow and enzymes cannot act. 



Sterilization by Heat. — If food materials are heated to 

 such a temperature and for such a time as will destroy aU 

 of the living microorganisms present, and so sealed that 

 microorganisms from the environment cannot come in con- 

 tact, they may be preserved indefinitely. The process most 

 used for this purpose is that commonly termed canning, in 

 which the food material either before or after sterilization 

 is hermetically sealed in containers. Meat, fruits, vege- 

 tables, and in general those foods not seriously damaged by 

 heat and which cannot readily be preserved by drying, are 

 utilized. 



In any method of canning it is necessary that the food be 

 properly prepared, placed in suitable containers, the air 

 exhausted and the food hermetically sealed, and sterilized. 

 We are here concerned primarily with the factors which 

 determine the time and the temperature necessary for 

 sterilization. 



The first of the factors determining the time necessary for 

 sterilization is the initial infection, its extent, and the kind. 



