This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
Quivering, tempting, fine-flavored jelly is not in the least difficult to make if one knows just how to go about it. A jelly with a delicate natural flavor is much to be preferred to the strong jelly in which the natural fruit flavor has been destroyed in the making.
In the first place great care should be taken in selecting the fruit. Some people have an idea that any sort of inferior fruit will do for jelly, but this is a mistake. Never use fruit for jelly that is overripe, for it not only requires more boiling to make it jelly, but it will also be much darker. Peotin, the substance in fruit that causes it to jelly, is at its best in fruits partly ripe or in their prime. Gooseberries, grapes, currants, elderberries, strawberries, blackberries, raspberries, and such like fruits will jelly best when only partially ripe; while peaches, pears, quinces, apricots and such like fruits will be best when perfectly ripe, but not overripe.