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We often get snap beans from the CSA (Community Sponsored Agriculture) that are either entirely purple or mottled purple. When cooked (at least when steamed or blanched), however, the purple color disappears and they turn to a bright green color. As far as I can tell, this happens with any purple podded snap bean.

Why does this happen? Are there any methods of cooking purple beans that retains their original color? Are there other types of vegetables that have the same color changing reaction when cooked?

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This is another case for anthocyanins, the same compound that causes garlic and ginger to turn blue. Anthocyanins are pH sensitive, only appearing blue or purple when in the presence of acidity. As you cook the beans, the cells begin to rupture and lose water, which causes the acid around the anthocyanins to be diluted. As this happens, they lose their color allowing the green chlorophyll (which was always present) to show through.

The Science Behind Purple Beans

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I can't answer WHY but I can say that there is no trick to keep them purple after cooking. They always turn green when cooked.
So why Purple ? They have a different taste, they cook fast and are easier to harvest because of the color difference with the leafs.

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