Guests are coming for dinner, and you're trying to prepare ahead. You drizzle dressing on the salad and refrigerate it, with plenty of time before the guests arrive. A couple hours later when you go to serve the sprightly greens, you find that they've transformed into a soggy, discolored mess. What happened? Why does dressing make lettuce wilt so quickly?
First, let's line up the suspects known to hang out in salad dressing, namely oil and vinegar. Traditionally, chefs have maintained that vinegar is the culprit. It makes sense that acidic vinegar might eat away at the delicate cells of fresh greens. Some older cookbooks advise tossing greens in oil first, to protect them from acidic vinegar or lemon juice.
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. Here's why. Lettuce has a waxy coating that protects the leaves from rainwater and water loss. Water tends to bead up and roll off a lettuce leaf. However, 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. That's why the best dressed lettuce is simply undressed, at least until the last minute!