Why do most vegetables that freeze well require a quick blanching treatment before freezing?

Blanching is necessary to manage enzyme activity which drives deterioration in low-acid vegetables.

Enzymes naturally present in vegetables are chemical agents that continue to drive processes like ripening and deterioration even when the food is frozen, although their activity is slowed. In low-acid vegetables, this residual enzyme activity can lead to unwanted deterioration over time if not managed. Blanching, which involves briefly scalding the vegetables in hot water, serves as a crucial pre-treatment step because it effectively deactivates these destructive enzymes. By halting this enzymatic process before freezing occurs, blanching helps to effectively lock in the vegetable's color, texture, and vital nutrient content, ensuring better quality retention over the extended frozen storage period.