Successful preservation of food by canning depends on two things: first, the sterilization of the food and the can, that is, the complete destruction by heat of all life in or on the food, and on all parts of the can that are to come in contact with the food; second, subsequent care to prevent further entrance of micro-organisms.
The presence of air in a can will not cause food to spoil, provided the air is sterile, that is, freed from all living organisms. A half-filled can of fruit will keep perfectly if fruit, can, rubber, and cover are sterile, if the air space above the fruit is sterile, and if micro-organisms cannot enter the can. The precaution sometimes taken to run a knife or a spoon down the sides of a can in order to remove the few bubbles that may be there, is consequently unnecessary. Unless the spoon or knife has been boiled, its use in removing air may even endanger the keeping qualities of a can of food, for it may hold organisms that thus find their way into the can.
It is now known that some micro-organisms that cause foods to spoil may assume two forms, the spore and the vegetative form. When conditions are unfavorable to their growth, they go into the spore form, cease growing and reproducing, and become inactive and very resistant to the influence of heat. It is their method of tiding over a hard time.
In the spore form micro-organisms are much more difficult to destroy than in the vegetative form, and some of them are able to resist for many hours a temperature even as high as the boiling point of water. During a dry season spores occur much more frequently than usual on fruits and vegetables, and the difficulties of successful canning may, therefore, be greatly increased. As soon as growth conditions become favorable- when warmth, moisture, and food are supplied-spores begin changing over to the active, growing, vegetative form, and in their greater liveliness they lose much of their power to resist heat, cold, and other unfavorable influences. These facts explain why canned foods sometimes spoil even after long boiling and careful and complete sealing; the boiling temperature may not have been sufficient to destroy the spores, which soon change over to the active growing state. The problem of successful canning is, therefore, how even the most resistant microorganisms may be killed.
The commercial canner has solved the problem of sterilization by the use of steam under pressure. In this way a tempera-ture higher than the boiling point of water, and hence more destructive, is obtained. With a sufficiently high temperature, a relatively short time of cooking is required to sterilize food.