cranberry shell beans

Maybe I will run a bean lab.
That tablespoon of red wine vinegar didn't make them tough at all. Perhaps being just picked helped there. They were lovely (went into a colorful succotash).

I mispoke when I said the heat broke down the pigment--cooking breaks the sort-of sacs that hold the red pigments, and they dissolve into the water.

Acid does tend to keep red pigments red and it works well with cabbage. The problem I see with using it for cranberry beans is that acid (the vinegar) strengthens cell walls, and adding it during cooking could make the beans permanently tough. A lot of people do the opposite of adding acid (they add baking soda) to help beans cook. mainecook, did you add your shot of vinegar at the end? I can imagine that would help restore a little color but not a heck of a lot, because most of the pigment is completely gone. Come to think of it, I make lots of cranberry beans with oil and vinegar, and I've never noticed any return to rosiness. Fun topic. If I had yours big crop, I'd set up a few experiments.

Actually, I discovered a "kind of" answer when I recalled that Julia Child advised adding a shot of red wine vinegar to red cabbage for the same reason, to help set the color. So I did that and the beans came out much pinker, kind of a dusty rose. I have an enormous crop of these beans, so am still curious if anyone else has an opinion.

Nope - it's what they do! But they sure taste wonderful.

Alas, no. Heat breaks down the pigment. Maybe it'll help if you think of the cooked beans as Borlottis.