Purple beans are really quite special. The pods are richly colored and easy to spot among the leaves (which is why they’re my favorite kind of “green” bean—so easy to harvest!), and they’re a beautiful contrast to all the emeralds and chartreuses and hunter greens happening in the garden.
I call them magic beans, and the magic happens when you cook them…
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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.
Recommended reading: Ruby Kraut (and Why It’s So Good For You)
(But note that anthocyanins are only responsible for certain reds. Some of the red pigments you see, like those in beets and chard, come from betalains, a class of plant pigments that are only found in Caryophyllales. Other red pigments, like those in red peppers and red tomatoes, come from carotenoids.)
Anthocyanins exist in all tissues of a plant and may appear in the roots, stems, leaves, flowers, and fruits (though not all at once).
With purple beans, the pigments primarily show up in the flowers, which gradually give way to purple-tinged pods.
How and why anthocyanins change color in the garden
If you’ve ever grown purple beans yourself, you may have noticed that their color tends to change slightly from season to season, or they don’t appear the same color as other plants you’ve seen. Purple striations may feature more or less, and one year may yield deep purple pods while the next year’s crop appears lighter.
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.
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 magenta.
It’s also the reason hydrangeas are famous for their color-shifting ways, as the petals respond highly 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).
Think of these anthocyanin-rich plants as a litmus test of sorts. (In fact, red cabbage makes a good indicator in pH experiments at home.)
How heat affects anthocyanins
Anthocyanins are 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. This happens when the red side was exposed to more sunshine, spurring a chemical reaction in the plant cells that produces more pigments.
When it comes to purple beans, however, heat plays the principal role. 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 and the boiled water looks blue from all the pigments that escaped.
How do you keep purple beans from turning green?
There’s not much you can do to preserve the color if you’re cooking purple beans (and why you don’t see any purple bean casseroles).
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.
You can also try to increase the acidity (thus retaining some of the anthocyanins) by briefly soaking the beans in lemon juice or vinegar before cooking them. This trick works best for recipes that already have a bit of tanginess to them, like lemony pastas, Thai-style noodles, or rice bowls with citrus shrimp.
Or, just pick the purple beans when they’re young and tender and enjoy them raw off the vine.
Recipe to try: Three Bean Summer Salad
Why don’t other purple vegetables change color after cooking?
As you may have noticed in the kitchen at one point or another, not all purple vegetables turn green when they’re exposed to heat.
Purple sweet potatoes and purple potatoes retain their vibrant color, in part, because there’s no chlorophyll in the flesh—the tubers grow underground. When they’re cooked (typically by steaming, roasting, or boiling), they may veer more violet since water has a neutral pH, but more or less keep their color.
(Conversely, blue potatoes may take on reddish tones—appearing more purple—if cooked with acidic ingredients.)
But the main reason these tubers—along with certain varieties of purple Brussels sprouts, black carrots, red cabbage, red corn, and red radishes—don’t lose their color is because they contain higher concentrations of acylated anthocyanins.
You see, not all anthocyanins are the same.
In contrast with non-acylated anthocyanins (like those found in purple beans and purple peas), acylated anthocyanins have higher heat and light stability, and lower sensitivity to pH changes.
When these vegetables are cooked past the stage of being crisp-tender, they’ll only fade from purple to lavender because the acylated anthocyanins don’t undergo as drastic of a change.
Acylated anthocyanins are so stable, in fact, that their pigments have been studied as a potential (and more natural and healthful) replacement for synthetic dyes such as FD&C Red No. 40.
The drawback of cooking purple beans
So is there a benefit to preserving the purple color in your beans?
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. (And while you’re at it, pick some of the leaves too—green bean leaves, on their own, are edible and delicious.)
This post updated from an article that originally appeared on July 17, 2013.
View the Web Story on why purple beans turn green after cooking.