This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
There should be no trouble in having canned fruit keep well if perfect or "chemical cleanliness" is observed in regard to jars, lids, etc., and if the fruit or vegetables are in good order, not overripe or beginning to ferment where bruised or crushed. Fruit will never come out of jars better than it goes in. It is better to put up a little fruit at a time when it is just ripe than to wait for a large amount to ripen, when the first may be overripe and fermenting and likely to spoil the whole lot. Use only the finest flavored fruit.
Have everything ready before beginning canning. Put water in each jar, fit on rubbers and tops, and invert the jar on the table. If any water oozes out try another top and rubber until sure the jar is air-tight. Wash jars and tops, put them in cold water and bring to a boil. When the fruit is cooked ready take a jar from the boiling water, set it on a damp cloth laid in a soup plate, dip a rubber in boiling water, and fit it on firmly. Fill the jar to overflowing, wipe the brim, screw on the top, and turn it upside down on a table. If any syrup oozes out empty the jar back into the kettle and fit on a tighter rubber. Let it stand upside down till cold, wipe clean, wrap in thick paper, and keep in a cool, dry place.
These general directions are for all fruits and vegetables that are cooked before putting in the jars. Fruit keeps its shape better if cooked in the jars, which should be prepared as above, the fruit carefully looked over and filled into the jars. If a juicy fruit, like blackberries or raspberries, put the sugar in with it in alternate layers. For cherries the amount of sugar depends on the acidity of the fruit and is best made into a syrup with a little water and poured down through them. Peaches and pears after paring, are packed into the jars and a syrup of about a quarter of a pound of sugar to a pound of fruit poured over them. Most fruits need to be cooked from 10 to 15 minutes after the water around them begins to boil.
Red raspberries ought not to be boiled. Put them into jars as gently as possible; they are the tenderest of all fruits and will bear the slightest handling. Drop them in loosely, fold a saucer into a clean cloth, and lay over the top, set on a perforated board in a boiler, pour water to two-thirds, cover and set over a slow fire. As the fruit settles add more until full. When it is cooked soft lift the jar out and fill to the top with boiling syrup of equal parts of sugar and water, and seal.
Do not can all the fruit, for jams and jellies are a welcome change and also easier to keep. Raspberries and currants mixed make delicious jam. Use the juice of a third as many currants and 3/4 of a pound of sugar to a pound of fruit.
The flavor of all kinds of fruit is injured by cooking it long with the sugar, so heat the latter in the oven and add when the fruit is nearly done.
Jelly is best made on a clear day, for small fruits absorb moisture, and if picked after a rain require longer boiling, and every minute of unnecessary boiling gives jelly a less delicate color and flavor. When jelly is syrupy, it has been boiled too long; if it drops from the spoon with a spring, or wrinkles as you push it with the spoon in a saucer while cooling, it is done enough. Try it after 5 minutes' boil. Cook the fruit only until the skin is broken and pulp softened. Strain without squeezing for jelly, and use the last juice you squeeze out for jam-Measure the juice and boil uncovered, skimming off. For sweet fruits J of a pound of sugar is enough to a pint of juice. Heat the sugar in the oven, add to the boiling juice; stir till dissolved. When it boils up, draw to the back of the stove. Scald the jelly glasses, fill and let stand in a clean, cool place till next day; then cover. Blackberries make jelly of a delicious flavor and jelly easily when a little underripe. Currants should be barely ripe; the ends of the bunches may be rather green.
A highly prized way of canning cherries: Stone and let them stand overnight. In the morning pour off the juice, add sugar to taste, and some water if there is not much juice, and boil and skim till it is a rich syrup. If the cherries are sweet a pint of juice and 3/4 of a pint of sugar will be right. Heat the jars, put in the uncooked cherries till they are nearly full; then pour over them the boiling syrup and fasten on the covers. Set the jars in a washboiler, fill it with very hot water and let it stand all night. The heat of the syrup and of the water will cook the fruit, but the flavor and color will be that of fresh and uncooked cherries.