176 AGBICULTUEAL AND INDUSTRIAL BACTEEIOLOGY 



water, convection does not occur, and heat passes to the 

 center much more slowly and only by conduction. Cans of 

 corn heat more slowly at the center than cherries. Sub- 

 stances like pumpkin paste and spinach heat most slowly 

 of all. With the latter the rate of heat conduction is 

 practically the same as the rate of heat conduction through 

 water. 



Fourth, the hydrogen ion concentration, that is, the 

 actual acidity of the food material, is also important. It is 

 common experience that foods such as peaches or apples are 

 much more easily sterilized by heat than vegetables such as 

 peas, beans and corn. The presence of acid greatly increases 

 the death rate of microorganisms at high temperatures. 



Fifth, in some commercial canneries devices (agitators) 

 are employed to keep the canned foods continually in 

 motion during sterilization. This induces constant mixing 

 of the food material and increases markedly the rate at 

 which the heating will occur. Agitation acts in very much 

 the same fashion as convection currents in water. The 

 method of cooling and the time required will also determine 

 to some extent the efficiency of sterilization. 



Sixth, the temperature which must be employed is of 

 importance. Home canning is ordinarily accomplished by 

 the use of boiling water, and 100° c. is the highest tem- 

 perature available. By the use of pressure cookers or in 

 commercial establishments by the use of large autoclaves or 

 pressure cookers, higher temperatures may be employed and 

 the length of time required materially decreased. 



Several methods having their counterpart in commercial 

 processes have been utilized in the home canning of foods, 

 important among them being the intermitten process of 

 sterilization and the cold pack process. In intermittent 

 sterilization it is cvistomary to heat the food to be preserved 

 in its container for a certain length of time, frequently an 

 hour, on each of two, three or more successive days. The 



