This is very important, don't fail to read it thoroughly.
If everyone understood thoroughly why foods spoil, they would also understand the reason for the different methods of canning, which in itself is simply a method of preparing food so it will keep. The causes for so-called spoiling in foods of all kinds are microscopic forms of plant life, called bacteria, yeasts and molds, which cannot be seen with the naked eye. These cells, in growing, bring about putrid conditions in meats and vegetables, cause the souring of fruits, etc., and produce mold on bread and fruits with which we are all more or less familiar. The bacteria are the hardest to kill and they grow on vegetables and meats almost exclusively, rather than on the fruits which contain too much acid for their growth. The yeasts and molds, however, can live and grow on the fruits, even the very acid fruits, such as lemons. The yeasts and molds are easily killed and the question of the canning of fruits is really a simple one. In vegetables and meats, however, the bacteria, under favorable conditions, such as drouth and heat, stop working and go into what is called the spore or resting stage, and boiling for the length of time ordinarily given to the fruits will not kill them; these spores afterward become active and their growth brings about a spoiled condition in the contents of the can. Many of the bacteria can withstand the boiling temperature for 30-60 minutes, the time generally given to fruits, or for even a greater length of time; and after the jar cools then they start to work and cause "spoiling" of the contents. If, however, the boiling is repeated on the second and third days for one hour each, any spores which have become active in the meantime, are finally killed and the contents left sterile, which means that there are no more living organisms present.