Heating vegetables stops this enzyme activity. If a vegetable is not heated prior to freezing, enzyme activity will continue even at freezer temperatures. In addition, heating wilts or softens vegetables and makes them easier to pack. It also kills microorganisms, brightens the color, and helps retain nutritional value during freezing. The heating or blanching time varies with the kind of vegetable and size of pieces.
Timing is important.
Underblanching can stimulate the activity of some enzymes and is worse than not blanching at all. Overblanching results in loss of vitamins, minerals, flavor, and color. See the directions in Table 1 for heating times of specific vegetables. There are several methods you can use to heat vegetables. The most satisfactory way to heat-treat most vegetables is in boiling water.