This section is from the book "Mrs. Allen's Cook Book", by Mrs. Ida C. Bailey Allen. See also: The Conscious Cook: Delicious Meatless Recipes That Will Change the Way You Eat.
Success in jelly-making largely depends, not upon "good luck or bad luck" but upon whether or not pectin (a vegetable starch that stiffens the jelly) is present in sufficient quantities to make it harden. Most of the pectin is present in the skins and cores of the fruits and is found in greater abundance in under-ripe fruit, and in lesser amount in fruit that is ripe. So, if possible, choose fruits that are a little green, or at least not over-ripe.
Wash the fruit, and, if it is of a juicy type like currants or grapes, crush it in a preserving kettle, setting this in turn in a larger utensil containing hot water to form a hot-water bath. Cook gently until the fruit is tender and the juice is running freely. Then drain through a flannelette jelly bag, but do not squeeze it if a clear effect is desired. In using hard fruits, as apples, add enough water to keep them from burning and boil them until they are soft.
To make sure that the jelly will stiffen, put 2 teaspoon-fuls of the unboiled and unsweetened juice in a tumbler and add 2 teaspoonfuls of grain alcohol. Stir until well-mixed, and let stand for half an hour. If a jelly-like substance collects in the bottom of the tumbler, it is evident that pectin or the jelly-making principle of fruits is present. If pectin is lacking, boil a few apples (as these are rich in pectin), some green citron melon, or the white inner skin of a few oranges, and add it to the fruit juices. This inner skin of the oranges may be saved the year through and dried to be ready for such an emergency.
Measure the juice. Then bring to boiling point and boil rapidly for twenty minutes. Add to this three-fourths the quantity of granulated sugar, warmed in the oven, or one-half the quantity of sugar, and one-fourth the quantity of white corn syrup. Boil briskly until two rows of drops form on the end of a spoon held sidewise. The temperature is usually about 220o F. Pour into sterilized glasses. Let stiffen and, when cool, seal with melted paraffine.
Most common fruits may be made into jellies if they are sound, not too ripe, although pear jelly is difficult to make. If, however, the pears are combined with apples this difficulty may be overcome. Apples and cranberries in the proportion of one quart of cranberries to a peck of apples; apples and quinces, in the proportion of 2 quinces to 2 quarts of apples; apples, with any other canned fruit juices in the proportion of a quart of juice to a peck of apples, will make delicious jelly. Green-skinned apples alone make a clear amber jelly and red-skinned apples make jelly of a deep pink color. Currants and raspberries, or loganberries, are delicious in combination, while blackberries may be reinforced by apples to keep down expense. Barberries and apples in the proportion of a quart of barberries and a peck of apples make a delicious jelly. They are especially suitable to serve with game. Elderberries may be used either alone or in combination with equal parts of ripe grapes or currants.
It is usually customary to make jellies of the fruit juices and sugar only, but if desired a little whole spice may be cooked with the fruit juice, a little orange or lemon rind may be added, or, in case the juice seems insipid, a little lemon juice. Old-fashioned cooks still use a rose geranium leaf or a spray of lemon verbena in their apple jelly.