By LEE REICH
For The Associated Press
It sure would be hard to explain to any young child why cooking would turn
purple beans, such as the variety Royal Burgundy, from purple to green.
Which cooking does.
AP Photo/Lee Reich
This photo taken Aug. 25, 2009 shows purple beans. A natural group of
chemical, called anthocyanins, are what put the purple in purple green
beans, as well as the purple in grapes, plums, and, less familiar,
purple broccoli.
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, are what put 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.
A 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 what anthocyanin is still left
becomes bathed in liquid insufficiently acidic to keep it purple.
A similar thing happens when you cook red cabbage. It turns colorless after
awhile. You can also expect purple broccoli, purple asparagus, purple
tomatillos, even purple peppers to lose their purple color after cooking.
Red peppers stay red, though, because carotenoids give them their red color.
PURPLE HAS ITS PURPOSES
It may seem foolish to grow a purple vegetable if it is going to turn green
anyway after you cook it. It's not as if that purple color does anything for
flavor.
In nature, anthocyanins do have a purpose though, attracting insects to
flowers and protecting plants from ultraviolet radiation, which is why you
find purple in many alpine plants. Carrot shoulders exposed to sunlight
sometimes even turn purple.
Actually, that purple in a vegetable does do us gardeners some good. The color
can be very pretty in the vegetable garden. Also, because Royal Burgundy's
foliage stays green, it is easier to pick out the purple pods from among the
leaves. And perhaps birds can more easily spot and devour green cabbage
worms on purple broccoli and red cabbage than on green varieties of either
vegetable.
Furthermore, those purple beans do not really have to turn green before you
eat them. You can eat green — or purple — beans raw, as many children,
especially, often do. The way to prevent or lessen the color change of any
cooked purple vegetable is to soak it before cooking in vinegar or lemon
juice, increasing the acidity. Then minimize cooking. Because anthocyanins
are tasteless, preserving the purple color will have no effect on flavor.
Copyright 2009 The Associated Press.
Wowza!
String bean bounty at farmstands everywhere! Summer is clearly upon us,
if only all those heirloom tomatoes would get all yummy and juicy and
amazing sooner. Julia started a “tomato fund”, I’m so not joking.
Literally though, the Fair Food Farmstand is getting in bushel boxes
full of string beans in all colors: green, yellow, burgundy. It’s hard
to keep up with them all.
So, I whipped this awesome salad up for our 4th of July Grillout
Chillout back yard bbq last weekend and not only was it super easy, but
it was mighty tasty. Loren couldn’t get enough of the stuff and on her
way out demanded the recipe. Unfortunately, in all the whirlwind of
preparing picnic salads that afternoon, I didn’t write down the recipe
as I was tossing things into the marinade. Typical.
Not to worry though, just an excuse to buy more beans from the
Passyunk Fountain Farmer’s Market today and try and recreate it.
Luckily, there wasn’t too much out of the summer salad norm (red wine
vinegar, olive oil, s+p, some fresh herbs) so remembering wasn’t too
tough, and I wrote it down this time!
Sweet and Spicy String Bean Salad
1 qt string beans (the more color, the better!)
1/2 c sweet onion, thinly sliced (such as Vidalia or Walla Walla)
1 med red bell pepper, thinly sliced
1/2 c olive oil
1/4 – 1/3 red wine vinegar (really though, as much as you’d like)
1 tbsp packed light brown sugar
1 tbsp fresh thyme, roughly chopped
1 tbsp fresh chervil, roughly chopped
1/4 tsp red pepper flakes
1/4 tsp mace
1/4 tsp garam masala
s&p to taste
First things first, sort through
your beans, pull out the funky ones, and blanch* them for about 3
minutes in some boiling water. Just enough to take away the raw starchy
taste yet keep them crisp and crunchy. Place them into a large bowl of
ice water to ensure they stop cooking when you want them too.
Cut the beans into approximately 1
inch long pieces, or whatever suits your fancy. Add the sliced onion
and bell pepper to the mix.
Whisk the rest of the ingredients
together in a small bowl and mix into the veggies. Go read a book in
the sunshine for an hour while your beans get happy, and then try to
not eat the whole darn bowl in one sitting. Wish you good luck on that
last part.
*Warning! If you get some
beautiful burgundy beans, like I did, they’re gonna turn green when you
blanch them! Either suck it up and wave their purple hue goodbye as
they simmer, or toss em in raw. The plus side is that their color change is like a built in timer alerting you when they’re done blanching.
(Keep the color from article above & enjoy. SCF 
: The way to prevent or lessen the color change of any
cooked purple vegetable is to soak it before cooking in vinegar or lemon
juice, increasing the acidity. Then minimize cooking. Because anthocyanins
are tasteless, preserving the purple color will have no effect on flavor.)
Posted by Nate