Why does lettuce when left whole turn limp, while lettuce torn up into small pieces stay crisp and fresh?
Limpness has to do with how moist the lettuce is in storage. Tearing up the leaves and separating them allows the lettuce to dry. A whole head of lettuce retains the moisture it came with and picks up more moisture in the refrigerator, unless it is stored in the crisper drawer. For appetizingly crisp leaf lettuce, wash and dry it, layer the leaves between clean paper towels, and place them in a plastic bag. Store in the crisper drawer until you’re ready to serve it. If you purchase a cellophane-wrapped head of iceberg lettuce, leave it in the wrapper until you are ready to use it. Lettuce sold with its roots intact (hydroponic lettuce) keeps best if you wrap the roots in damp paper towels, then place the whole head in a plastic bag. Hydroponic lettuce also can be placed upright in a glass of water (like a bouquet of flowers), covered with a plastic bag, and refrigerated.