Wednesday, June 30, 2010

My Magic Beans.

Remember my purple Royal Burgundy beans that turned green when cooked? If not, then I'd worry since I wrote about them in my very last post.
In that post, I queried why do purple beans turn green.
In my comments I got two wonderful answers.
From Kathy, of Reinventing a Boomer blogdom:
Kathy said...


From Zzzadig:
zzzadig said...

The heat causes the cyanoresinoids to lose a hydrogen atom thereby rendering the spectral reflectivity only reactive in the blue-green spectrum. If you believe that, I have a bridge for sale.

Well, Kathy and Zzzadig, thank you for responding. As it turns out, you're both spot on. What happens is this:

Atchally, the purple is from a pigment known as anthocyanin. The heat from cooking breaks apart the molecules on the surface of the bean and exposes the underlying chlorophyll, which is green. Another example: cooking destroys a lobster's greenish coat and uncovers its carotene underlayer. Back to anthocyanins - this natural group of chemicals is what puts the purple in purple beans, as well as in grapes. They're what also make roses and geraniums red and cornlowers (bachelor buttons)

and delphiniums blue. Yellows and oranges come from carotenoids. The red pigment called betacyanin is responsible for the reds in beets and bougainvilleas. Acidity in anthocyanin is the key. The anthocyanin that is red in a rose petal is highly acidic. The anthocyanin that is blue in a cornflower petal is less acidic. Anthocyanins change color as the acidity of the cell sap changes.

In the case of my Royal Burgundy beans turning green, a direct effect of heat causes the decomposition of anthocyanin. Less anthocyanin means less purple.

The indirect effect of heat is to burst the cells apart, diluting the acidity of the cell sap. The green color from chlorophyll, which was present but masked by the anthocyanin, becomes prominent once the anthocyanin concentration drops.

And there you have it. And you are entirely welcome.

1 comment:

Kathy said...

Like I said - magic.