HERMETICAL SEALING 



195 



2. Preservation. After the food has once been deprived 

 of bacteria {sterilized), it must be protected from the 

 subsequent access of all kinds of microorgan- 

 isms. Since bacteria are always present in the 

 air, any of these sterilized products will surely 

 be reinoculated if exposed, and the new bac- 

 teria would soon spoil the food. Therefore 

 the practical method of keeping bacteria out is 

 that of seahng 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. Bacteriologists 

 have found that the air which passes through 

 cotton is deprived of all bacteria. If, there- 

 fore, any sterilized material is placed in bottles, 

 tubes, or vials which are tightly plugged with 

 cotton, as shown in Fig. 64, it will be per- 

 fectly protected from the invasion of bacteria. 

 A knowledge of this fact may be of some 

 practical importance, even in the household, 

 in case it is desired to preserve something for Fig. 64. Pre- 

 a short time only and one does not want to 

 go to the trouble of hermetical sealing. But 

 such a method is quite impracticable for the 

 ordinary canning of food. At best it is of 

 only temporary utility, for though cotton keeps 

 all bacteria away from the sterilized material, 

 it will not wholly exclude molds, and, moreover, allows the 

 material to become dry. Hermetical sealing keeps food 

 moist as well as sterile. 



served cher- 

 ries, showing 

 that the ex- 

 clusion of air 

 is not neces- 

 sary for pres- 

 ervation. 



