Just add heat, and purple beans will turn green

It is difficult to explain to any young child why cooking turns purple beans, such as the variety Royal Burgundy, from purple to green.

You can’t respond by just saying, "Anthocyanins, my dear.” You may have to wait a few years before offering a thorough answer.

A natural group of chemicals called anthocyanins is what puts the purple in purple green beans, as well as in grapes, plums and, less familiar, purple broccoli. Anthocyanins are also what make roses and geraniums red, and cornflowers and delphiniums blue.

Not yellows and oranges, though; those colors come from carotenoids, which also are responsible for certain reds in plants. In the case of beets and bougainvilleas, the red comes from yet another natural pigment called betacyanin.

Color-changing chemical

But back to anthocyanins. How can the very same anthocyanin — and there are a few different kinds — that makes one vegetable or flower red make another vegetable or flower blue?

Acidity of the cell sap is the key. The particular anthocyanin that is red in the very acidic sap of a rose petal is blue in the less acidic sap of a cornflower petal. Anthocyanins change color with changes in acidity, and eventually turn colorless as acid levels drop.

Two things happen during cooking to make Royal Burgundy beans turn from purple to green. A direct effect of the heat is to cause decomposition of anthocyanin. Less anthocyanin means less purple.

The indirect effect of heat is to burst apart cells, diluting the acidity of the cell sap. The green color, which was present but masked by the anthocyanin, becomes prominent once the anthocyanin concentration drops, and any anthocyanin remaining becomes bathed in liquid insufficiently acidic to keep it purple.

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