246 THE PRESERVATION OF FOOD PRODUCTS. 
organisms are so abundant everywhere that every bit of food is 
certain to be infested with them. The first step taken must be 
the killing of all the organisms that may be adhering to the food to 
be preserved. This is done by heat, the material being commonly 
placed first in the receptacles in which it is to be finally sealed. The 
amount of heat necessary varies much with the nature of the 
material. If it is of a proteid nature, like meat, beans, peas, or 
corn, it is likely to contain spore-forming bacteria, and will require 
high and prolonged heating. This is especially true of corn, 
peas, and beans, since these materials contain such resisting spores 
that it was once thought that they could not be successfully canned. 
But modern methods of applying heat above the boiling tempera- 
ture have made it possible to sterilize thoroughly even these resisting 
substances, and their canning is now perfectly feasible. To-day 
temperatures of 230 to 250 are commonly used for such materials. 
In canning fruit, as a rule, such high heat is not needed, since 
fruits are more often spoiled by yeasts and molds than by spore- 
bearing bacteria, and these organisms are easily killed by simple 
boiling, or by even less heat. 
The second step is the sealing. This consists simply in closing 
the vessel containing the sterilized mass in such a way as absolutely 
to exclude the air and with it all microorganisms. It is sometimes 
done in tin cans, when a small hole is left in each can so that the 
air and steam may escape during the sterilizing, after which process 
the hole is sealed by a drop of solder. Sometimes glass jars are 
used, and these are sealed by covers pressed forcibly down upon 
a soft rubber ring. The principle is the same in either case. The 
sterilized food thus removed from any possible means of contamina- 
tion will keep indefinitely. 
The development of the canning industry does not belong to 
our immediate subject, but certain facts connected with the matter 
have produced great changes in the possibilities of agriculture. 
It has made possible the utilization of a great quantity of food 
products which otherwise could not be used. Certain of our fruits 
are extremely palatable but very perishable, and if it were necessary 
to use them fresh or in a dry condition, only comparatively small 
