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.
Preserves are fruits in which the tissues of the fruit have absorbed a heavy sugar sirup until they are filled with sirup instead of with water. A good preserved fruit is plump and tender in texture and filled with sweetness. It is bright in color, clear and sparkling.
Cook Hard Fruits Before Placing in Sirup - Stew hard fruits, such as hard pears, underripe peaches, pineapples, sweet apples, quinces, watermelon-rind or citron, until tender before placing them in a heavy sirup. This makes the fruit soft, so that the sirup can enter the cells of the fruit. If these fruits are not treated in this way, the preserves will be hard and tough instead of plump and tender. Tender fruits such as berries, ripe peaches or cherries may be placed at once in a heavy sirup.
Cook Rapidly in Sirup - Cook the fruit rapidly in the sirup, and only long enough for the sirup to fill the fruit. Too long cooking gives a dark, stiff product.
When foods have been given a preliminary cooking to make them tender, drain them before adding them to the sirup. The water in which they were cooked should be used for making the sirup. Place the fruit in the sirup and bring it quickly to the boiling-point; continue the cooking rapidly until the product has a bright, clear, shiny look, showing that the fruit is filled with the clear sirup.
Plumping - If an extra fine quality of preserve is desired, add the fruit to the sirup and heat it only until it bubbles; then set it away in a covered enamel preserving-kettle for several hours, or over night. Then continue the cooking. In this way, more sirup is absorbed by the fruit. If the amount of extra work entailed is not too exacting, the heating and cooling process may be repeated several times. Pears, peaches, green tomatoes, whole tomatoes, crabapples, citron and melon-rind are especially adapted to plumping. Fruit to be candied should be plumped.
Sealing - The best method of keeping preserves is to seal them in hot clean jars. If trouble has been experienced with molds, it may be desirable to hold the jars of preserves in steam or boiling water for ten minutes as an extra precaution against molds. All jars, rubbers, spoons and utensils that are to be used in placing preserves in the jars should be sterilized in boiling water.