The art of canning was discovered more than a [two] hundred years ago by a Frenchman named Appert. About 70 years later another Frenchman, Pasteur, proved that foods spoil because of the growth of very small plants, or microorganisms, of various kinds, such as yeasts, molds, and bacteria. These small organisms are looked upon as plants rather than animals, although they are not like any of the green plants with which we are so familiar. They not only consume our food, but they also defile it by producing various sub- stances which spoil its flavor and appearance. In some instances they produce substances which cause illness or even death.
The appearance of molds is familiar to us all, because they usually grow on the surface, at least when they form their fruits or spores. These black, or white, or variously colored spores are formed in such abundance that their fuzzy mass is very plainly visible to the naked eye. They grow readily in, and on the surface of, such materials as bread, fruits, vegetables, jelly, cheese, sour milk, and even leather and cloth, in the form of “mildew”. Their spores or seeds are blown about in the air, and are always ready to grow on the surface of jelly, preserves, and many other foods which have been left exposed. Molds, however, do not give much trouble in canning by modern methods.
Household yeasts are also more or less familiar to most of us, because one of the wild yeasts has been “tamed” or domesticated and is used, either in the form of yeast cakes or in some liquid form, for the making of bread. This same yeast is also useful in start- ing the process of vinegar making. There are many kinds of yeasts. When any kind of yeast grows in jelly, preserves, or canned products, its presence is highly objectionable because the fermentation which it brings about destroys the attractiveness and wholesomeness of such foods, so far as ordinary table uses are concerned. However, yeasts (like molds) are usually killed by heating for a short time, and therefore give comparatively little trouble in canning.
The destruction of bacteria is an altogether different matter. These organisms are very much smaller than are yeasts and molds and are much more likely to cause trouble in canning. Bacteria are so small that it would take more than 10,000 of them of average length, placed end to end, to measure 1 inch.
There are always microorganisms of one kind or another present on the skin or rind of fruits and vegetables. Bacteria, yeasts, and molds are found to a greater or less extent in the air, in natural waters, on our own skin as on all animal bodies, in the soil, and in dust. If we are canning food which we expect to keep perfectly for a period of weeks, months, or even years, we must make every effort to see that we do not allow any bacteria, yeasts, or molds to remain in the jar or can to grow on the food in the sealed container and spoil it. All utensils and materials must be kept as clean as possi- ble and must be carefully handled to prevent the entrance of dirt and bacteria or other organisms, so that the chances of spoilage will be greatly lessened. Bacteria multiply with astonishing rapidity, especially in a warm, moist place. Many bacteria, yeasts, and molds can not grow except in the presence of air or oxygen, but-^there are many varieties of bacteria which grow very well indeed in the sealed can or jar, after all the air or oxygen it is possible to take out has been removed.
Source: U.S.D.A., 1922