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.
In the chapters on canning, drying and storing, the necessity for getting the products while young and tender was emphasized. All that was said there applies equally to vegetables or fruits meant for pickling. For preserves and jellies, however, another fact must be taken into consideration. That is, the sugar contents of vegetables, and to a greater extent of fruits, increases rapidly as they approach the stage of the becoming fully ripe. If fruits are gathered too green the sugar content is low and both the keeping qualities and the table qualities are affected.
In the making of jellies, of course, the test that is always looked for anxiously is to see whether the product is going to "jell." The reason why it will do so sometimes, and will not others, under apparently similar conditions, is that the "jellying," or setting or hardening of the syrup, is due to the presence of pectin in the fruit. This material, like sugar, increases as the plant approaches ripeness-but it again diminishes rapidly as the fruit gets overripe.
Because of the necessity for getting the sugar and the pectin in as great abundance as possible, fruits for preserves and jellies should be taken as near the stage of just becoming fully ripe as possible. But to be on the safe side, it is best to aim at getting them slightly before that period is reached. This is a very important point, and one that too often is not taken into consideration. Fruits for pickling, on the other hand, may be obtained in a much greener condition; in fact for some purposes they are preferred green, as they are much more firm and will hold their shape better.