174 BACTERIA, YEASTS, AND MOLDS 



The remedy in all such cases is greater heat, since no 

 satisfactory means of destroying bacteria is known except 

 the application of heat. Even spores may be perfectly 

 destroyed if the proper method is adopted. 



Higher Heat. Common liquids, when boiled in open 

 vessels, cannot be heated above 212", no matter how brisk 

 the boiling ; but if boiled in closed vessels under pressure 

 the temperature may be raised much higher. A tempera- 

 ture of 212° does not destroy spores, but a few degrees 

 higher will do so. If the material is boiled under pressure 

 of a few pounds only, such a temperature is easily obtained, 

 and if it is maintained for a short time the spores will be 

 destroyed. In the household it is rarely possible to use 

 apparatus for this purpose ; but in canning factories there 

 is no difficulty in constructing and using devices for heat- 

 ing under pressure. This method of heating is commonly 

 adopted for the sterilization of food products which are 

 difficult to can because of the presence of spores. 



Longer Heating. Higher temperatures are not easily 

 obtained in the household, but the spores may be killed 

 by simply prolonging the boiling. If spore-bearing mate- 

 rial is boiled for a sufficient time, the spores are eventually 

 totally destroyed. The length of time necessary for the 

 purpose cannot be stated exactly, for it will depend very 

 largely upon the vigor of the boiling and the nature of the 

 food. For the thorough sterilization of peas an hour, or 

 even two hours, may be needed, and an equal time is 

 required for corn or beans. Tomatoes do not require 

 quite so much time. Any material will be more surely 

 sterilized if placed at first in cold water and then brought 

 to a boil, than if placed immediately in boiling water. 



