FIRST METHOD OF FOOD-PEESERVATION 165 



and the actual canning, or it may be the result of defective sealing. 

 In whichever way it has resulted, it is always due to the multiplica- 

 tion of microorganisms which have not been eliminated from the food. 

 From what has just been stated we must, in considering the adapt- 

 ability of certain foods for canning, pay attention to the chances of 

 the presence of these spores in the food. Tomatoes are more difficult 

 to can than other fruits, owing to the greater frequency of the presence 

 of species that form spores ; the difficulty has, however, been largely 

 overcome by slightly raising the temperature, and by somewhat pro- 

 longing the time of exposure to the heat. This results in the weaken- 

 ing or the death of the spores, thus greatly lessening the danger of 

 putrefaction. By experience alone we can tell the degree of tempera- 

 ture, or the length of exposure which the fruit can stand without 

 being appreciably deteriorated in value. 



In America maize is now extensively canned, though for a long time 

 the canning of this substance was thought to be impossible. This 

 difficulty was overcome in the same way : it was found possible to 

 make a slight increase in temperature, and give a longer heat exposure, 

 without injuring the food. 



Some time ago a sensation was caused in this country by the intro- 

 duction of poisoned canned meat from America. This was probably 

 not the result of defective canning, but rather the result of the use of 

 poisoned meat for canning. The bacteria had done their work, leaving 

 their poisons behind them, they themselves being either dead before 

 reaching the factory or killed during the heating process. The 

 meat was just as poisoned before being canned, as it was when the 

 cans were opened in this country, for, as has already been explained, it 

 does not directly matter whether the bacteria themselves are present 

 or not, what does matter is the poison secretion of these organisms. 

 In spite of these mishaps, however, there can be no doubt that the 

 development of the canning industry has been a boon to mankind, and 

 has vastly increased the agricultural resources of several countries. 



3. SECOND METHOD OF FOOD-PRESERVATION- 

 DRYING. 



This method consists in the removal of water, sufficiently to prevent 

 the multiplication of the organisms, no attempt being made to kill 

 them. No organism can grow if there is not a sufficient amount of 

 water present. Living bacteria contain from 83 to 86 per cent, of 



