SAPROPHYTES 417 



tendency. Food would doubtless be much more expensive 

 today than it is if we did not have these modern methods of 

 preserving it. 



481. Domestic Canning. As stated above, the canning of 

 certain things, such as fruits and some of the vegetables, has 

 been practised in the homes for a long time. This practice 

 began long before the development of the science of bacteri- 

 ology and, therefore, the reasons for the success of the methods 

 used were not understood. The methods ordinarily used in 

 the home are successful only with fruits and with certain vege- 

 tables like tomatoes which contain a considerable quantity 

 of acids. You will recall the fact that acids are generally 

 detrimental to the growth of bacteria. It is doubtless on 

 account of the influence of these acids that food, which con- 

 tains them in sufficient quantities, may be canned successfully 

 with the moderate degree of heat available in the ordinary 

 household. 



It is difficult to heat food in the ordinary household to a 

 temperature higher than that of boiling water and this is not 

 sufficient to kill, in a reasonable length of time, the resistant 

 spores of certain bacteria w r hich are always found on such foods 

 as green corn, beans, and peas. Consequently these foods ire 

 seldom successfully canned in the home. There is a method, 

 however, by which such foods may be successfully canned in 

 the home without special apparatus. This consists in heating 

 the food in the cans to the boiling temperature for about a half 

 hour on each of three successive days and then sealing up 

 the cans with lids and rubbers which have also been heated 

 along with the vegetables. The heating the first day kills all 

 active growing bacteria and probably serves to stimulate the 

 spores to begin to grow. These then are killed with the second 

 heating, and the third heating serves to kill any that may have 

 escaped the first two heatings. If the fuel used in this process 

 is expensive, it is likely that the food canned in this way will, 

 in the end, cost about as much as it would cost to buy the same 

 amount of food which has been canned in a factory. 



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