When the water in food freezes, it stops much of the cellular activity that normally causes spoiling. But there are certain enzymes in vegetables that can continue their nefarious deeds in the quiet and dark of the freezer. Blanching - either steaming or boiling the food briefly - destroys the enzymes that cause the loss of nutritional value and flavor. Onions, peppers, and herbs do not need to be blanched. Squash, sweet potatoes, and pumpkin should be fully cooked before freezing. All other vegetables should be blanched.
To blanch by boiling, use at least a gallon of water for a pound of vegetables. Put the vegetables in a wire basket, submerge them completely in the boiling water, cover with a lid, and begin timing. To blanch by steaming, put the vegetables in a steamer basket and suspend it above an inch or two of boiling water. Cover the pot, and begin timing as soon as steam starts to escape from under the lid. With either method, shake the basket a couple of times to ensure that all vegetable surfaces are exposed to the heat. After the allotted time, remove the basket, and plunge the vegetables into a bowl of ice water to stop the cooking. Once cool, remove them, drain thoroughly, and package for freezing.
The old Joy of Cooking provides the following blanching times for various vegetables. The Mmes. Rombauer and Becker say that to preserve the natural color of artichoke, eggplant, mushroom, and sweet potato, soak the vegetables for 5 minutes in a solution of 1 teaspoon of ascorbic acid (vitamin C) to 1 quart of water.Vegetable Boiling Time Steaming Time Artichoke, medium 8 to 10 Asparagus, medium spears 3 4 Bamboo shoots 7 to 13 Broad beans, in pod 4 Bean sprouts 4 to 6 Beet greens 2-1/2 Beets, small until tender Black-eyed peas 2 2-1/2 Broccoli, split 3 to 4-1/2 3 to 5 Brussels sprouts 3 to 4-1/2 3 to 5 Cabbage, leaf or shredded 1-1/2 2 Carrots, sliced 3 minutes 3-1/2 Cauliflower, florets 3 3-1/2 Celery, diced 3 3-1/2 Chard 2-1/2 Chayote, diced 2 2-1/2 Chinese cabbage, shredded 1-1/2 2 Collard greens 2-1/2 Corn, to be cut from the cob 3 to 7 4 to 8 Corn, frozen on cob 6 to 10 7 to 11 Dasheen 2-1/2 3 Eggplant, 1-1/2-inch slices 4 4-1/2 Green beans 2-1/2 3 Kale 2-1/2 Kohlrabi, diced 1 1-3/4 Lima beans 1-1/2 2 Mushrooms, medium, whole 5 Mustard greens 2-1/2 Okra 3 to 4 4 Parsnips 2 3 Peas 1-1/2 to 2-1/2 2 to 3 Peppers 2 3 Pumpkin until tender Rutabaga, diced 2 2-1/2 Shell beans 1-3/4 Soy beans, in pod 4 Spinach 2-1/2 Sweet potatoes until tender Turnip greens 2-1/2 Turnips, diced 2 2-1/2 Wax beans 2-1/2 3