Although in some respects a simpler process than those already described, the discussion of canning has been left until the last because it is a later discovery.
When fruits and vegetables are freed from bac teria and packed in air-tight cans, little or no preservative material need be combined with them. Hence, canned fruits, being in a more natural form and more dilute than jams and preserves, are con sidered to be more digestible than such prepara tions dense with sugar.
Acid materials, as rhubarb or cranberries, may be canned without cooking. The cut pieces are put in glass jars, the spaces filled with fresh cold water, and the jars sealed. Thus the sour juices act some thing like vinegar as a preservative.
Usually, however, sterilization by heat is essen tial. The fresher and cleaner the article to be canned, the more certain we are of securing com plete sterilization. Overripe fruit, or that exposed in dusty markets, may harbor bacteria not easily destroyed at the boiling - point. Here the home canner cannot compete with the factory, as there it is possible through steam under pressure to secure a higher temperature.
Firm fruits may be stewed or steamed and then packed in jars. The softer fruits may be steamed in thin syrup or, better still to preserve their form and flavor, put in jars and set in a pan of water in the oven or in a steamer to cook and then be filled with thin syrup. Before sealing, a spoon should be put down between jar and fruit to let out all air-bubbles.
The pressure of the atmosphere on the surface of the preserving kettle causes some vari ation in the dens ity of syrup, how ever the sugar and water were pro portioned at first. When canning acid fruits, the syrup used to fill the jars may be made of equal measures of sugar and water, while, for sweet fruits, the sugar may be reduced.
The canning of vegetables is usually considered a more difficult process under ordinary conditions than that of canning fruits. With due precautions as to cleanliness and a long period of cooking in the jars placed in a steam cooker or wash-boiler (Fig. 240), many housekeepers are as successful with vegetables as with fruits.
Some vegetables are more subject to fermenta tion than others. Where the skin is cut, as in
sweet corn, there is greater opportunity for bac terial action. String beans may well be parboiled in salted water before putting into the jars, where the cooking process must be continued two or three hours. Tomatoes are less liable to spoil if thoroughly skimmed while cooking. When they have proved most trouble some to housekeepers, it ap pears that they have not been cooked long enough for the center of the tomato to be raised to the boiling point.
The country housekeeper who can bring perfect fruits and vegetables from her gar den directly into the preserving kettle and air-tight can will have little trouble with "germs"; but the city woman who must secure raw materials through many middlemen would better depend on reliable canneries for her main supply.
Utensils.
While excellent results have been accomplished by many housekeepers with very poor appliances, any one who is to make preserves as a busi ness needs the best utensils, not the most expensive, but those best adapted to the purpose. Everything should be of shape and substance easy to handle, not readily affected by acids, and affording little hiding-place for molds and ferments.
Scales g i v e greater accuracy than measures. A silver-plated fruit knife with sharp edge is best for paring and cor ing, or steel knives, if used, should be kept bright. Wooden, enamel, or silver spoons should be used, never tin or iron.
The old porcelain lined iron kettles trans mitted moderate heat with little danger of burning the con tents. There are brown earthenware kettles, raised from the stove by short legs or a metal rim, that are useful when slow evaporation is essential, as for marmalades or ketchups. Agate ware kettles are light, easy to lift, and clean, and with asbestos or a metal trivet underneath do not burn readily. There should be several of different sizes, and new ones are desirable since fruit acids often remove stains which cannot be scoured off,— and that does not improve the hue of a jelly. Broad rather than deep kettles should be chosen, since evaporation is thus hast ened, and whole fruits should be cooked in shallow layers.