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.
By "canning" we mean the saving of fruits or vegetables for future use by placing the product in a container of tin, glass, or other material, which is afterward hermetically sealed. The product "keeps," because the bacteria, which would otherwise cause it to spoil, are actually prevented from getting at it. If we merely put it in cans or jars and filled it up, however, millions of the bacteria would be sealed up with it and continue their nefarious work. Therefore, before sealing it up, it is necessary to destroy all the bacteria already in the product. This is done by heating to a certain temperature for a required length of time. The bacteria already in the food, when sufficiently cooked to be destroyed, have become harmless. This may not sound particularly appetizing, but it is what actually happens. The food has been "sterilized," and if it can be sealed up be fore any other bacteria can get to it, it will be safe to keep for an indefinite period until wanted for use.
These facts have been known for a long time. Recently, however, a further fact has been discovered which has very greatly simplified the different methods of canning. For generations housewives have known that corn was very hard to keep even if carefully canned. Corn and tomatoes put up together, however, keep very well. The reason for this, as we have now come to know, is that the acid in the mixture, supplied by the tomatoes, makes living conditions which the bacteria refuse to tolerate. Experiments proved that they expired much more quickly in, and were easily kept out of, a slightly acid solution. As a result, the modern practice of putting a little acid, such as lemon juice, with vegetables which are hard to keep, has resulted in the saving of a tremendous amount of work in canning these things and a great saving in the amounts lost by "spoilage" after canning.