This Is Why Purple Beans Turn Green When You Cook Them

Why do purple beans (and other purple vegetables) turn green after cooking? How do you stop them from changing color?

And conversely, why do other vegetables stay purple no matter how much you cook them?

Here's the science behind the vibrant pigments called anthocyanins, and why these plants are like their own litmus test.

Where do purple beans get their color?

The intense purple color in string beans and snap beans (like Royal Burgundy and Dragon Tongue), yardlong beans (like Chinese Red Noodle), and hyacinth beans (like Ruby Moon) comes from plant pigments called anthocyanins.

How and why anthocyanins change color in the garden

This natural fluctuation in color occurs because anthocyanins are sensitive to the pH level of the “juice” inside the plant cells (the cell sap). The acidity of the cell sap is dependent on both genetic and environmental factors.

Magic in the kitchen: how heat affects anthocyanins

When it comes to purple beans, heat plays the principal role. Boiling, baking, or sauteing at high temperatures causes the anthocyanins to deteriorate. What’s left behind is green chlorophyll, which was always present in the beans but masked by the plant’s anthocyanins.

How do you keep purple beans from turning green?

If you want to minimize color loss, you can blanch them, singe them on the grill, or toss them into a stir-fry at the end for a couple minutes. The color will fade a bit, but remain purplish.

All anthocyanins are tasteless, so they have no effect on flavor. But cooking your purple beans does destroy some of the anthocyanins available. You’ll reap the most nutritional benefits by eating them raw or cooking them very lightly. Just pick your purple beans when they’re small and tender for the best flavor and texture.

Swipe up to learn more.