Obvious changes in color, texture, and taste take place in a vegetable that is left out on a table or in the refrigerator. The vegetable's own enzymes are largely responsible for that transformation. The extremely cold temperatures of freezing slow down the changes but do not completely stop them. In fact, some enzymatic activity has been observed at minus 100 degrees F, and the temperature in most home and commercial freezers is not below 0 degrees F.
Extreme heat, on the other hand, inactivates enzymes and sets color and flavor. For this reason, most vegetables should be blanched before they are frozen. The process involves partially cooking the food in boiling water or steam for up to several minutes. Some cooks choose to boil or steam their vegetables as they would be serving. However, blanching is preferable to full cooking because it does less damage to the vegetable's texture, color, and taste.