416 MICROORGANISMS 



479. Oxygen and Bacteria. We have mentioned in Art. 

 465, the fact that some bacteria require free oxygen in order 

 to be able to carry on their processes, while others can, not 

 only get along without free oxygen, but can not thrive in its 

 presence. The only importance that this fact has in con- 

 nection with the preservation of food is that it shows that we 

 can not hope to preserve the food by shutting it away from free 

 oxygen. As far as the oxygen relation is concerned, the 

 anaerobic bacteria find ideal conditions within a sealed can 

 of fruit or vegetables. 



PRESERVATION OF FOOD BY CANNING 



480. Why We Can Food. We know that one of the most 

 extensively used methods of preserving food at the present time 

 is that of canning. This consists of putting the food into cans, 

 heating it to a high temperature for a time sufficiently long to 

 kill both the active growing bacteria and the more resistant 

 spores, and then sealing it air-tight. 



Canning has been practised to some extent as a household 

 industry for about a century, but the large canning factories 

 which now preserve annually immense quantities of food 

 have come into existence during the last 20 or 30 years. 

 The importance of canning and other modern methods of pre- 

 serving food as means of cheapening the cost of living is very 

 great. Before these methods came into use, many kinds 

 of food could be used only while it was fresh and in season, and 

 a large percentage of each crop was allowed to go to waste for 

 the lack of suitable methods for preserving it. Now, the 

 whole crop may be saved and be sold at reasonable prices 

 throughout the year. It is a remarkable fact, however, that, 

 during the same period of time in which the process of canning 

 and other methods of preserving food have been brought to 

 their present high state of development, the cost of food has 

 greatly increased instead of decreasing. The increase in the 

 cost of food has come about, however, in spite of the perfec- 

 tion of these processes which naturally have the opposite 



