Reduced to a single sentence, then, the whole theory of preserving food by canning is as follows, we (1) take the food or vegetable product to be preserved, in or upon which there are already present the germs which, if left to themselves, would soon destroy it; (2) place the product in a container and apply heat enough for a sufficiently long time to destroy completely the germs and germ spores which may be present; (3) seal it absolutely air-tight so that no more germs can gain access to the product. In some instances a preservative is used in addition to sterilization.

How can the point of actual sterilization be determined?

Coming to the practical side of the matter, that is naturally the first question which must be answered. There is no way of telling by watching the progress of the work while it is being done and no uniform rule which will apply to everything. As a general rule, the less cooking or heating required to make the product safe, the better; for then the more nearly it will resemble in color and flavor the fresh product. There are, however, a few general principles which may be kept in mind and will serve to some extent as a basis on which to estimate the treatment which may be required by different vegetables or fruit. (For more specific information see the "Time Table" at the end of this chapter. It is based upon the result of actual practice.)

In the first place, the molds and yeasts- which, as we have seen, are more likely to attack fruit than vegetables-succumb at a comparatively low temperature - 150 to 165 degrees F., which is considerably below the boiling-point of water (212 degrees F.). Many of the bacteria, however, which usually attack vegetable products, will withstand the boiling temperature for a long period; some of them over an hour. By the usual process, employing heat alone, therefore, the thorough sterilizing of most vegetables is a long process. The acid vegetables, tomatoes and rhubarb, can be made safe for keeping in a comparatively short time, because they do not furnish a favorable environment for the development of bacteria. A temperature of 212 degrees can, of course, be obtained by keeping the containers in which the product is placed immersed in boiling water. To shorten the time required, however, a higher temperature may be obtained by using vessels in which the free escape of steam is prevented, or in which steam itself under pressure is used for supplying the heat. (See Chapter 8, on equipment.)