This section is from the book "Mrs. Rorer's Vegetable Cookery And Meat Substitutes", by Sarah Tyson Rorer. Also available from Amazon: Mrs. Rorer's Vegetable Cookery And Meat Substitutes.......
Jelly-making is divided into two classes. In the first class, fruits containing pectose, a jelly-making material, we have guavas, crab-apples, quinces, Japanese quinces, lady blush apples, pippins, black currants and wild grapes (fox and chicken grapes). These are all easily made into jelly. In the other class, a large group containing very little pectose, some pectin and a fair quantity of acid, we have the dark and white ripe grapes, raspberries, strawberries, currants and cranberries. Some of these fruits contain pectose, but they also contain a fair quantity of fruit sugar and acid. To make jelly easily from acid fruits they must be selected just a little under-ripe and should be perfectly fresh from the vines, the juice boiled without the sugar, and the sugar added at the last. The heat and the acid of the fruit will frequently split the sugar into levulose and dextrose, which prevent the jelly from forming. They make a syrup, not a jelly, and continued boiling does not improve it.
If jelly fails to form at first, it cannot be reheated. If sugar crystals form in the cold jelly, too much sugar has been used.
If dust falls in jelly while it is cooling and paraffine is put on top, the jelly will sour and spoil.
Cool under glass or in a clean room.