194 BACTERIA, YEASTS, AND MOLDS 



Higher Heat. Common liquids, when boiled in open ves- 

 sels, 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. If the material is 

 boiled under pressure of a few pounds only, a sufficient tem- 

 perature is obtained to destroy the spores in a comparatively 

 short time. Household devices for cooking under pressure 

 are now on the market, but as they are rather expensive, 

 this method of sterilizing is in common use only in canning 

 factories. This method of heating is called "processing" 

 and is very efficient. 



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 material 

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

 totally destroyed. The length of the time necessary is often 

 as much as two 01 three hours and depends very largely 

 upon the vigor of the boiling and the nature of the food 

 (see page 201). 



Intermittent Heating. Sometimes a better product is 

 obtained if, instead of boiling once for a long time, the 

 food be boiled for shorter periods on three successive days. 

 The principle upon which this method depends is that the 

 spores not killed by the first day's boiling will germinate 

 and be in a condition to be readily destroyed the second 

 day, while the third day's heating takes care of the few that 

 escape both previous heatings. This is known as "frac- 

 tional " sterilization, because only a part of the bacteria are 

 destroyed at each heating. The length of boiling necessary 

 on each of the three days varies with the kind of food it 

 is desired to preserve. 



