This section is from the book "Save It For Winter.", by Frederick Fry Rockwell. Also available from Amazon: Save It For Winter; Modern Methods Of Canning, Dehydrating, Preserving And Storing Vegetables And Fruit For Winter Use, With Comments On The Best ... For Saving, And When And How To Grow Them.
As already explained, Vegetables and fruits "spoil" as the result of the presence of cer tain vegetable bacteria or molds or yeasts, which begin to attack them soon, sometimes within a few hours, after they have been harvested or are fully matured. Among the several conditions which make it impossible for the bacteria or molds to exist, is a surplus amount of acid or sugar. In pickles, the acid or saline condition of the product keeps them safe from attack. In preserves, the surplus of sugar accomplishes the same purpose. Most of the jellies are too acid to afford favorable conditions to bacteria. The molds which might develop upon them have to have for their existence a continuous and abundant supply of oxygen, so that if they are sealed tightly or covered with paraffin they are safe from attack because the supply of oxygen is shut off.