Some older cookbooks advise tossing greens in oil first, to protect them from acidic vinegar or lemon juice. But a simple kitchen experiment challenges this timeworn advice. Put one small piece of lettuce in a saucer of vinegar, and one in a saucer of oil. Check them every five minutes. The one in the oil wilts first.
Water tends to bead up and roll off a lettuce leaf.
But oil clings to its waxy surface, quickly seeping into cracks, entering the air pockets between cells, and making the leaf darken and wilt. Vinegar will also damage lettuce leaves eventually, but because it is water-soluble it takes much longer than oil to get past the waxy coating.