Preserving fruit pulp. Jam is mashed fruit pulp combined with water and sugar and cooked until thick. It should have a bright color end a soft consistency that spreads easily, but has no runny juice.
The thickening occurs when the correct proportions of pectin and acid (which are present in all fruits in varying degrees) and sugar are combined and cooked to a certain temperature. If sufficient pectin is not present for a good jell, you’ll need to add commercial pectin, available as a liquid or a powder. (The powder must be added to fruit before cooking.) Some people prefer to use pectin in all their jam and jelly making; the cooking time is less and a jell is guaranteed. However, pectin does affect flavor and generally necessitates the use of more sugar.
Acid contributes not only to the jelling but to the good flavor of jams and jellies. Mix 1 teaspoon lemon juice with 3 tablespoons water and 1/2 teaspoon sugar. If your fruit or juice does not taste as tart as this mixture, you will need to add lemon juice-about 1 tablespoon per cup of fruit or juice.
Fruits rich in pectin and acid are cranberries, Concord grapes, tart apples, and damson plums. Strawberries and greengage plums have average pectin and acid; peaches and raspberries have low amounts. By combining a low-pectin fruit, such as pears, with a high-pectin fruit, such as cranberries, you can improve the jelling quality of the low-pectin fruit.
Underripe fruits are higher in pectin and acid than ripe ones. By combining one part underripe fruit with three parts ripe, you can enhance the jelling property of the ripe fruit.
Unless your recipe calls for commercial pectin, you must measure the pectin content of your fruit. Put 1 tablespoon unsweetened, cooked fruit juice and 1 tablespoon 190-proof grain alcohol or rubbing alcohol in a saucer and blend them. If a thick clot forms, the fruit is rich in pectin; if there are stringy clots, pectin content is average; numerous small clumps indicate that pectin must be added.
Caution: Don’t taste the mixture containing alcohol; it’s poisonous.
Preparing the jam
Unlike fruits for freezing or canning, those for j am can be less than perfect. Hull and crush berries in a bowl. Peel and remove blemishes, stems, and seeds or cores of other fruits; cut them into small pieces and measure them. For best results, cook no more than 6 to 8 cups of prepared fruit at a time.
Some recipes call for mixing the fruit with sugar and letting it stand for a time before boiling. Otherwise, put the fruit into a large, heavy pot of enamel or stainless steel (at least four times the volume of the fruit). To fruits other than berries, add a little water to prevent burning, about 1/2 cup for every 4 cups of fruit.
Cook the fruit until tender, stirring frequently to prevent sticking. Add the sugar-usually about 3/4 cup per cup of fruit-and stir until dissolved. Boil the mixture rapidly until it starts to thicken; then insert a candy thermometer and read it at eye level. Most fruits reach the jam stage-220°F or 8°F above the boiling point of water at high elevations -within 5 to 15 minutes (after 1 minute of cooking with added pectin). Don’t overcookjam; itwillbecome too thick.
The thermometer is the most reliable test for doneness. If you don’t have one, you can test by removing the pot from the heat, spreading a teaspoon of the mixture on a cold saucer, then chilling it quickly in the freezer. If the skin wrinkles when you push it with a finger, the jam is ready.
Another test for doneness is to dip out a metal spoonful of syrup, let it cool slightly, then tip it over above the pot. If the syrup droplets merge as they slide off the spoon, the jam is done. Remove the pot from the heat. Skim the foam with a slotted spoon.
If you live in a hot climate, it’s best to sterilize the jam and the jars in a hot-water bath. Otherwise, sterilize the jars, then seal them with sterilized canning lids or paraffin. (Sterilize jars and lids in a 225°F oven for 10 minutes; or boil them for 15 minutes, inverted in 2 inches of water. Keep them hot until you are ready to fill them.)
Fill the sterilized jars, leaving 1/2 inch of space at the top for paraffin, 1/4 inch for a sealed lid. Apply the sealing lid with sterilized tongs; or pour 1/8 inch of melted paraffin over the jam, using a circular motion so that the wax adheres to the sides of the jar. Clean the jar rim with a damp cloth.
Caution: Paraffin is highly flammable. Melt it in a double boiler or in a saucepan set in hot water-never over direct heat.
When cool, recheck the seal of each jar, put on outer lids, label, and store the jars in a cool, dry, dark place.