This section is from the book "A Book Of Recipes For The Cooking School", by Carrie Alberta Lyford. Also available from Amazon: A book of recipes for the cooking school.
In the can-cooked, or cold-pack, method the food is scalded or blanched in boiling water or steam, dipped into cold water, packed directly into the jars, and covered with boiling water, or with boiling syrup of the desired density, or with boiling fruit juice, usually sweetened. The covers are adjusted and the jars are placed on a rack in a large kettle and entirely surrounded by water, which should extend an inch over the top. The water should be quickly brought to the boiling point and boiled as long as is necessary to sterilize and cook the food which is contained in the jars. The can-cooked, or cold-pack, method can be used for fruits, vegetables, and meats. It offers little opportunity for spoilage because the jar and its contents are both thoroughly sterilized and the product canned retains its shape, color, and flavor well. The can-cooked, or cold-pack, method involves more labor and time on the part of the housewife and requires more fuel, but the excellent results that can be obtained when canning a considerable quantity of food are worth the additional labor and expense involved.
For those vegetables which contain spores that are especially resistant to the heat, an intermittent process of sterilization is considered necessary in some parts of the country. When this method is used the vegetable is cooked in the can one hour or longer in the water bath on three or four successive days. At the end of each period of sterilization the jars are removed from the water bath.
The three main processes necessary to success in the cold-pack method are blanching, dipping, and the sterilization, or processing, of the food which is being canned.
Blanching - Blanching consists in dipping the food into boiling water or live steam and leaving it immersed for from two to eight minutes according to the nature and the maturity of the food. (See time-table). Live steam is best for greens. Boiling water is desirable for other products. Berries and some soft fruits need not be blanched. The purpose of blanching is (1) to prepare for the removal of the skin; (2) to remove the strong flavor; (3) to eliminate objectionable acids; (4) to bring out the color; (5) to shrink the product, making possible a fuller pack; and (6) to make the product more flexible.
Dipping - After blanching, the food should be cold-dipped: that is, it should be dipped quickly into water which is as cold as possible and it should be removed immediately. Fresh water should be used for each dipping. If possible, it is desirable to accomplish the dipping process by allowing the water to run through the food. The purpose of dipping is (1) to set or coagulate the coloring matter so that it does not dissolve so easily during the sterilization period; (2) to harden the pulp beneath the skin; and (3) to make the food easier to handle in packing.
Sterilization - Sterilization, or processing, consists in boiling the packed jars in a boiler or canner full of water, for a designated length of time. (See time-table.) The jars should be placed on a wooden rack in a covered boiler, which is used as the canner. The canner should be filled with sufficient water to come over the top of the jars for at least one inch, thus forming a hot-water bath. The water should be boiling when the jars are put into the boiler. The jars should be in the boiler and the water about them should be kept at the boiling point long enough to make tender and to thoroughly sterilize the contents. Sterilization, or processing, kills or renders ineffective the organisms within the jars which would cause the food to spoil.