This section is from the book "Everywoman's Canning Book", by Mary B. Hughes. Also available from Amazon: Everywomans canning book; the A B C of safe home canning and preserving.
Correct Processing is absolutely essential to successful canning, and to understand the importance of it, let us consider why we process. Every piece of fruit or vegetable, no matter how fresh, will have on its surface tiny, invisible microorganisms. The air contains many microorganisms, an important thing to remember in canning. If living organisms remain in the jars, they increase rapidly, causing food to decay and spoil. We "process" to kill the microorganisms, and thus to render food in the jars sterile; and we "seal" to keep out other organisms which are in the air.
Microorganisms are divided into three main groups: yeasts, bacteria, and molds. In canning, we may find yeast plants on fruits and vegetables, bacteria on vegetables and meats, and molds from the air may form on jams and jellies. Canning immediately after the vegetables are gathered, and cleanliness in all handling, lessens the danger from these organisms.
Different organisms require different degrees of heat and different lengths of time to render them sterile or inactive. Yeasts are killed by lower degrees of heat than bacteria. A short period of processing, from fifteen to twenty-five minutes, with the water actively boiling around the jars, is all that is necessary for the heat to penetrate sufficiently to kill the yeasts on fruit; while bacteria in the spore state may resist two and three hours' active boiling. Spores have been known to resist twelve hours' processing in the hot water bath, at a temperature of 212° F., and to become active in the jars after ten or twelve hours. This is the reason that an occasional jar of beans or corn may spoil, while others done in the same way at the same time keep in perfect condition.
There is only one short process by which the spores in vegetables may be killed, and that is by the use of the Steam Pressure Canner, with which a temperature of 250° F. can be obtained. These pressure canners are expensive, but they can be used for a great deal of cooking in addition to the canning, hence are desirable when they can be afforded.