Blanching is the scalding of vegetables in boiling water or steam. Blanching slows or stops the action of enzymes.
Up until harvest time, enzymes cause vegetables to grow and mature. If vegetables are not blanched, or blanching is not long enough, the enzymes continue to be active during frozen storage causing off-colors, off-flavors and toughening.
Blanching time is crucial and varies with the vegetable and size of the pieces to be frozen. Under-blanching speeds up the activity of enzymes and is worse than no blanching. Over-blanching causes loss of flavor, color, vitamins and minerals.
Blanching can be done ahead of time if you are going to use the vegetables for cooking. Or crisp blanched vegetables can be either chilled or served immediately.