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.
The thing which has done more to simplify the keeping of vegetables and fruits by canning-with the possible exception of the cold-pack method-is the use of a little acid (usually lemon juice) in the liquid or syrup in which the products are put up. Experiments at the California Experiment Station showed that peas heated to the boiling-point (212 degrees F.) kept perfectly when five ounces of lemon juice to the gallon was added; while without the lemon juice, under the same conditions, they quickly spoiled. Corresponding results were obtained with beans, beets, asparagus, pumpkin and other Vegetables which are considered Very hard to keep. The amount of lemon juice is so small that in most cases, if noticeable at all, it improves the flavor.
Experiments with fruits established the fact that many varieties could be sterilized at a temperature considerably under that usually used (212 degrees F.). The object of this low temperature in sterilization is to keep the fruit as near the fresh form as possible, as cooking changes the flavor, texture and looks of the fruit. By carefully controlling the temperature, peaches, apricots, pears, cherries and berries were found to keep perfectly after being sterilized at 165 to 175 degrees F. and were only slightly altered in flavor and texture from the fresh fruit. This seemed to be particularly true of the peaches.