Blanching vegetables is submitting them briefly to boiling water or steam. This destroys enzymes, changes the texture and sets the color. There are a number of reasons one might want to do this.
For extended storage, particularly freezing, destroying the enzymes improves the keeping quality of the vegetables. The enzyme that causes fruits and vegetables to ripen also cause them to rot. Though blanching prior to freezing is not necessary, vegetables prepared this way will be fresher looking and tasting for a longer period of time.
Blanching is sometimes used to prepare vegetable salads. It will brighten the color of some vegetables (green and yellow ones especially), but primarily changes the texture. Again, this step is not necessary, but the slightly softer texture may make the vegetables more palatable.
Blanching for a salad also allows the vegetables to take up the flavors of the dressing or marinade better. This is due to the breakdown of the cells, allowing flavor components and water to flow into the vegetables.
Sometimes vegetables such as green beans may be blanched early in the meal preparation, to be finished off with a saut or other method just before serving.
Blanching can be done in boiling water or steam. The amount of time needed depends on the vegetable and the use. Steaming usually takes one to two minutes longer, but preserves more of the vitamins. Blanching times can range from thirty seconds for asparagus to six minutes or more for something like lima beans (for freezing), but usually is done for just one to two minutes. If using boiling water, blanching time starts from the time that the water returns to a boil after adding the vegetables. The larger the volume of water, the faster it will return to a boil.
As soon as the vegetables are ready, they should be removed from the steam or water and plunged immediately into an ice bath. This is simply a bowl or other container of water and ice. They should sit in this for the same length of time as blanching. Drain well, and dry on a kitchen towel if need be.
Another reason for blanching is to facilitate peeling. This is usually done for tomatoes and peaches, and the boiling water method is used. Immerse the tomato in the water until the skin splits. Transfer to the ice bath. When cool the peel will slip right off, and the tomato can be squeezed to remove the seeds if desired.
Transferring the vegetables from pot to water bath can be facilitated by the use of a wire basket or colander that can be inserted into your pot. A slotted spoon or skimmer can also be used for small quantities of vegetables.