Blanching is the method of briefly boiling vegetables and then plunging them into ice water to stop the cooking.
The hot-water stage partially cooks the food, while the cold-water “shock” rapidly removes heat and limits further softening. This helps vegetables retain a brighter color and a firmer texture. Timing depends on the vegetable’s size, shape, and density; over-blanching can make food limp and cause water-soluble nutrients and flavor compounds to leach into the cooking water.
Blanching is especially important before freezing vegetables because heat deactivates enzymes that would otherwise cause color, flavor, and texture deterioration during storage. It is also used to loosen the skins of tomatoes, peaches, almonds, and pistachios. Although boiling water is traditional, steam can be used for the heating stage, and cold running water can replace an ice bath.