HERMETICAL SEALING 175 



Nearly all failures in canning are due to an insufficient 

 amount of heating at the outset. In the canning of 

 fruits, which is the kind of preserving most commonly 

 performed at home, there is seldom any special difficulty, 

 since fruits do not as a rule contain resisting spores. 

 In the majority of cases, therefore, a vigorous boiling of 

 fruits for a few moments is sufficient to destroy bacterial 

 life, after which the materials can be canned with perfect 

 success. It must be remembered, however, that absolute 

 certainty cannot be reached by simple boiling, and that 

 the employing of this method will result in occasional 

 failures. Once in a while a can will become decayed, 

 though the rest of the same lot will be preserved in the 

 proper fashion. The difference in these cases is doubt- 

 less due to the accidental presence of some spore-producing 

 bacterium which happened to get into one of the cans and 

 not into the others. 



2. Preservation. , After the food has once been de- 

 prived of bacteria (sterilized), it must be protected from 

 the subsequent access of all kinds of microorganisms. 

 Since bacteria are always present in the air, any of 

 these sterilized products will surely be reinoculated if 

 exposed, and the new bacteria would soon spoil the food. 

 The practical method of keeping bacteria out is, there- 

 fore, that of sealing the contents hermetically. In the 

 laboratory it is possible to preserve foods without sealing 

 by simply filtering all the air that reaches them through 

 something fine enough to 'exclude bacteria. Bacteriolo- 

 gists have found that the air which passes through cotton 

 is deprived of all bacteria. If, therefore, any sterilized 

 material is placed in bottles, tubes, or vials which are 



