The many methods used, and the many things to be handled in different ways, may make the art of canning seem at first a rather complicated one. Nevertheless, the general principles involved are always the same, no matter what is being canned, nor how, nor on what scale it is done. Once the general principles are understood, therefore, one has a key to the whole matter, and it becomes accordingly simplified and easy to understand.

The spoiling or decay of various fruits and vegetables is caused by the attack of one or more of three destructive agencies: bacteria, yeast, molds. Hot weather, or dry or moist air, which are often considered the causes of spoiling, are not direct causes. It is only as they make for conditions which are favorable or unfavorable to the growth of the or ganisms which do attack food products, that they may be considered a cause of spoiling. when vegetables or fruits decay or turn sour or ferment, it is not a simple physical change, but the result of the growth of certain living vegetable organisms, or to use the common phrase, "germs." The individual germ, of course, is too small to be seen without a microscope; but growing in the tissues of the vegetable or fruit they multiply with such rapidity that under favorable conditions they may cause it to change entirely, or decay or sour in a few hours.

As an illustration of how such changes are possible, take for instance the action of yeast; a cake of yeast is, in reality, nothing more nor less than a compact mass of millions and millions of germs which, under favorable conditions, soon permeate the whole mass of dough with which they are mixed. The action of "mother of vinegar" is another familiar example.

Fig. 2 - An aluminum pressure cooker, and a gas range, make an ideal outfit for canning purposes.

Fig. 2 - An aluminum pressure cooker, and a gas range, make an ideal outfit for canning purposes. The former save, time and, the latter work and heat.

The germs which attack fruits and vegetables belong to three distinct classes, viz., bacteria, molds, and yeasts. Their development will cause vegetables and fruits to decay or mold, canned fruits or vegetables to sour or spoil, and jellies, preserves and pickles to mold. They are not, however, all alike in their tastes. The bacteria group thrive where there is little or no acid; the yeast and molds, on the other hand, thrive only where there is acid. Most fruits are acid and most vegetables are not; therefore, it is usually yeast or mold that causes the spoiling of fruits, and bacteria which causes the spoiling of vegetables. It is important to know this, because the yeast or molds can be killed at a much lower temperature than the bacteria.