Why does lettuce turn to mush when frozen?
Freezing and thawing lettuce tends to damage it because the water in the leaves forms large ice crystals that perforate cell walls. When the lettuce is thawed, the ice melts, causing the cells to collapse and water to leak out everywhere. Since lettuce lacks the starch of other vegetables to serve as a replacement structure, this renders a once-crisp leaf of lettuce a dark green, watery mush.
The key to preserving any vegetable through the freezing and thawing process is speed. Quick freezing means that ice crystals don’t have time to grow very large, reducing the amount of damage they do to the cell walls. Also, reducing excess moisture prevents surface freezing, further reducing the amount of damage done to the leaves.
Certain varieties of lettuce hold up better than others. Locally raised heirloom varieties may be sturdier than the iceberg or romaine heads at the supermarket, and less time between field and freezer means sturdier cell walls. Separating the leaves increases the surface area available for heat to transfer out of the lettuce, and chilling the lettuce in the coldest part of the refrigerator before moving it to the freezer minimizes the amount of transition time to a frozen state. Even with these precautions, however, frozen lettuce rarely resembles its fresh state.