Cooking

Why Does Salad Dressing Make Lettuce Wilt?

Eating healthy isn't easy. Sometimes, you're inspired to pack a salad for lunch, only to see that your virtuous mason-jar combo of lettuce, veggies, and light vinaigrette has become a soggy, wilted mess by noon. (Next time, the dressing goes in its own container.) What is it about salad dressing that makes lettuce wilt so fast?

Dressing On Trial

A classic vinaigrette salad dressing is a mix of oil and vinegar — sometimes people swap the vinegar for lemon juice, and sometimes they add an emulsifier. But if you want to uncover the culprit behind mushy lettuce, those first two ingredients should be your main suspects.

First up is vinegar. Vinegar is acidic, and chefs have long claimed that this caustic chemical is to blame for eating away at delicate lettuce leaves. According to Indiana Public Media's A Moment Of Science, some cookbooks even recommend tossing greens in oil first to protect them from the vinegar. Vinegar, consider yourself a suspect.

Next up is oil. Whereas vinegar is water-based, so it has surface tension that makes it easily bead up and roll off of lettuce leaves, oil is, well, oil. Its lower surface tension means it takes less energy for it to stick to the lettuce and leave a shiny coating on your entire salad. That's a lot of space to do damage! Oil, you have the right to remain silent.

If we were to test this scientifically, we'd need identical salads, some with only oil, some with only vinegar, and some with water, as a control. Luckily, both Cook's Illustrated and Serious Eats have performed experiments similar to this one. Their verdict? The greens dressed in oil wilted much faster than the others. In the case of oil vs. salad, we find the defendant guilty as charged.

Keepin' It Crispy

So oil is to blame for soggy salads. But why? And what's a salad lover (or, you know, a salad tolerator) to do about it?

It comes down to the way the lettuce defends itself. Greens are armed with a waxy coating that keeps water in and invaders out. But that waxy coating is closer to oil, molecularly speaking, than it is to water, which makes it easy for the oil to break through and seep into the spaces between the leaf's cells, causing damage.

While pretty much any oil-based dressing will make your salad wilt eventually, there's a way to stave off the inevitable. Adding an emulsifier, like mustard, honey, or egg yolk — along with giving the whole thing a good mix in a jar or a blender — keeps the dressing ingredients from repelling one another and letting the oil do its damage solo. Instead, the oil breaks down into tiny droplets that are surrounded by a protective layer of vinegar. Not only does that keep the lettuce crisper, longer, but it helps the dressing cling to your salad instead of pooling at the bottom of the bowl. That makes for a tastier salad all around!

For more cooking tips from the science lab, check out "Cook's Science: How to Unlock Flavor in 50 of our Favorite Ingredients" from Cook's Illustrated. If you make a purchase through that link, Curiosity will get a share of the sale, which helps support the work that we do.

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Written by Ashley Hamer December 20, 2017

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