This section is from the book "The American Woman's Cook Book", by Ruth Berolzheimer. Also available from Amazon: The Domestic Arts Edition of the American Woman's Cook Book.
Jam is made from whole small fruits which are either mashed or cooked to a pulp with sugar. Good jam is soft, tender and jelly-like in texture, bright and sparkling in color and of the same consistency throughout the mixture.
Some Underripe Fruit Desirable - Portions of fruit left from canning, or broken fruit, may be used for jam, but at least a portion of the fruit should be underripe. Overripe fruit lacks pectin and some pectin, a jellying substance, is necessary for good jam.
Cook the Fruit Before Adding Sugar - In order to develop the pectin substance, the fruit should be cooked for a few minutes before the sugar is added. If the fruit does not have sufficient juice, add just enough water to keep it from burning and cook it in a covered kettle.
Not too Much Sugar - The best jam is made by using not more than three-fourths pound of sugar to each pound of fruit.
Cook Quickly and Not too Long - After the sugar is added to the fruit, continue the cooking quickly until the jam gives a jelly-like appearance. It should hang in sheets from the spoon or set quickly if a portion is dropped on a cool plate. It should be tender and jelly-like, not thick and tough. Jam thickens on cooling, and an allowance must be made for this or the jam will be overcooked. Overcooking also darkens the product. It is better to make a small amount of jam at a time. Use enamel or porcelain cooking utensils, if possible.
Stir to Prevent Burning - Jam is a highly concentrated mass and will burn quickly unless it is stirred from the bottom. Use a wooden spoon and lift the mass from the bottom. It is better to cook jam briskly and watch it carefully for twenty or thirty minutes than to let it simmer for hours.
Seal in Hot, Clean Jars - Jams, like preserves, are safer from molds if they are sealed in hot, clean jars.