Why Do Purple Beans Turn Green After Cooking?

Royal Burgundy bush bean

There’s something so majestic about purple beans. The pods are richly colored and easy to spot among the leaves, and they look beautiful tossed into a green salad. I call them magic beans, and the magic happens when you cook them…

Purple "green" beans

The purple in purple beans like the Royal Burgundy (Phaseolus vulgaris) bush variety comes from plant pigments called anthocyanins. Anthocyanins are water-soluble pigments that also give red cabbage, purple cauliflower, and purple asparagus their vibrant color. But if you’ve ever grown these plants yourself, you may have noticed that their color tends to change slightly from season to season, or don’t appear the same color as other plants you’ve seen.

This 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. Anthocyanins tend to turn red in acidic soil, blue in neutral soil, and yellow in alkaline soil. This is why “red” cabbage may appear more purple, and “purple” cauliflower may appear more pink. It’s also the reason hydrangeas are famous for their color-shifting ways, as the petals respond well to varying soil conditions (and gardeners can even change the color of the flowers quite easily by amending their soil to be more or less acidic).

Anthocyanin pH indicator

Anthocyanins are also highly susceptible to heat and light. You may have seen this anomaly in apples, which sometimes appear more red on one side than the other. The red side was exposed to more sunshine, which spurred a chemical reaction in the plant cells that produces more pigments.

When it comes to purple beans, however, heat plays a role when you cook them. Boiling, baking or sauteing at high temperatures causes the anthocyanins to deteriorate. The heat breaks down the plant cells, diluting the acidity of the cell sap as the pigments are dispersed in a more neutral solution (water). What’s left behind is green chlorophyll, which was always present in the beans but masked by the plant’s anthocyanins. So, your purple beans end up as green beans.

Purple beans turn green after cooking

There’s not much you can do to preserve the color if you’re cooking these beans (and why you don’t see any purple bean casseroles!). But if you’re simply adding them to a salad or stir-fry, a blanch (followed by a plunge into an icy cold bath) or a quick toss for just a few minutes will keep some of the color, albeit a bit faded. Or, just pick the purple beans when they’re young and tender and enjoy them raw!

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July 17 2013      27 comments     Linda Ly
Jardín   Leguminosas   Verduras

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  • Madame X

    Are there any health benefits to eating them in their more colorful form, or is it better to get them to a less acidic state?

    • http://www.gardenbetty.com/ Linda Ly

      The health benefit to eating green beans raw (or cooked very minimally) is that you retain more of their nutrients. Plant acidity is not an issue.

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  • Julia

    Is this the same pigment that’s in purple basil? I’ve always wondered why some of my basils are more purple than others, and why they turn partially green as the season progresses.

    • http://www.gardenbetty.com/ Linda Ly

      Yes, anthocyanins are also the purple pigments found in purple basil (throughout the season, my basil shifts between different shades of purple and green).

  • Sarah

    Thank you for this post! I’ve been planning on growing these beauties for next summer, so now I think my info is complete!

    • http://www.gardenbetty.com/ Linda Ly

      Royal Burgundy beans are one of my favorites; I grow them every summer!

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