Blanching is an important preliminary step in the cold-pack method of canning. It is used for some fruits and may be used for all vegetables. Blanching may be done with either boiling water or steam.

When boiling water is used, the fruit or the vegetable is placed in a piece of cheese-cloth or a crate, lowered entirely under the water, and heated for the required number of minutes (pages 612 to 614). Only a small quantity of the product should be blanched at a time in order that the water may be kept as near the boiling point as possible.

Delicately flavored greens are generally blanched by steam in order to avoid the loss of iron and other nutrients that occurs when they are immersed in boiling water. For blanching in steam, the food is placed in some perforated utensil or a piece of cheese-cloth and suspended in a tightly closed steamer. The food should be blanched until no further shrinkage will occur. Blanching in steam generally requires a longer time than blanching in boiling water. The steam must penetrate to all parts of the mass, and for this reason the fruits or vegetables should not be crowded together. Blanching in steam is not recommended for strong-flavored greens.

Blanching may accomplish one or more results: (1) it helps to insure a close pack either by contracting the tissue and making the product flexible, as in the case of string beans and asparagus, or by causing a decided shrinkage, as with greens; (2) it may partly eliminate strong acids or bitter flavors; (3) it may set the color; (4) it begins the sterilization of the food; (5) it loosens the skins of certain fruits and vegetables from the pulp so that they may be slipped or scraped off easily, as in the case of peaches, tomatoes, or carrots. When blanching serves this last purpose, it is frequently called scalding. Blanching may not be necessary but it is believed to give a superior product in many cases.

The cold dip is the rapid chilling of the outside of the blanched fruit or vegetable by plunging it into cold water. While the food may need to be cold dipped in order to be cooled sufficiently to make it easily handled and to insure the cooling of the center of the mass, it should not be allowed to remain long in the cold water.