418 MICROORGANISMS 



482. Factory Canning. As has been stated, the canning 

 process as ordinarily carried on in the household, finds its limi- 

 tations in the fact that the temperature of boiling water, 

 which is the highest temperature conveniently obtainable 

 without special apparatus, is not sufficiently high to kill the 

 more resistant spores of molds and bacteria in a reasonable 

 length of time. Canning factories, however, are equipped 

 with special apparatus by means of which the food that is 

 canned may be raised to any desired temperature in the process. 

 Such apparatus generally consists of what might be called a 

 large steamer of boiler iron construction. A large number of 

 cans containing the food, are placed in this steamer and then 

 steam is turned, into this apparatus until a certain pressure is 

 reached. You are familiar with the fact that steam arising 

 from boiling water is practically of the temperature of the boil- 

 ing water; that when the steam is under 1 atmosphere of pres- 

 sure its temperature is 100C., or 212F.; but when steam is 

 held under pressure, the temperature increases with the pres- 

 sure (see Arts. 155 and 160). Consequently, by surrounding 

 cans containing food with steam under pressure, the food 

 may be raised to a temperature sufficiently high to kill in a 

 fe% minutes the most resistant spores of microorganisms. 



When the cans are placed in the heating apparatus, they 

 are generally sealed up except for a very small hole in the 

 top of the can. As soon as the cans come from the heating 

 apparatus, a drop of solder is dropped on this hole and it is 

 sealed. This hole is left in the can while it is being heated 

 for the purpose of permitting the air which is contained in the 

 can to escape as it is expanded by the heat. Since the hole 

 is sealed up while the contents of the can are still very hot 

 and therefore more' or less expanded, the contents of the can 

 will shrink slightly on cooling and the ends of the can will 

 become slightly concave as a result of the atmospheric pres- 

 sure on the outside. This fact is important as it affords a 

 means of detecting cans which have not been perfectly pre- 

 served, for some kinds of bacteria are gas producers and when 



