[vhfpmr (OP)]

 When cooked, frozen broad beans are smooth, green and hard, and the cooking water turns black, but fresh ones are wrinkly, purple and soft, and the cooking water turns red. Why the difference?

And why are the last bag of frozen beans I bought purple, soft and wrinkly unlike all the previous ones?

(The cooking water's clear when it's drained off, it turns coloured if it's left a few hours.)

[Zer0]

Pesticides?

Or some chemical reaction that takes time, hence the water color changes?

Is it a possibility that Beans harvested during Different time periods vary alike seasonal changes?

[alancalverd]

Don't know about beans, but most frozen vegetables are "fresher than fresh" because they are harvested at peak flavor, then cleaned, blanched and frozen within hours, whereas fresh vegetables will have been harvested by demand rather than optimally,  then stored unprocessed for several days at ambient temperature.

From the given account it looks as though there is some hydrolytic or thermal reaction that produces a red color  (is the water really black or very deep red?) either slowly at room temperature (in the fresh or late-harvested veg) or quickly when boiled. Redness is usually associated with ripening so it may be that optimum harvest for beans is at maximum water content (certainly the case for peas, preferentially harvested at dawn) so the frozen ones aren't wrinkly.

But Mendel's famous peas were definitely two related genetic strains, one inherently wrinkly - maybe the frozen bean grower's contract is principally for the smooth variety. 

[Bored chemist]

Pesticides?

That's probably the only thing we can rule out.

The problem is that "broad beans" covers a large range of slightly different plants.
So the differences might not arise from how they were treated but simply be because they are different strains.

[vhfpmr (OP)]

Re the freshness issue, my only experience of unfrozen broad beans was the ones grown in the garden when I was a child, I've never seen them in the supermarkets, so I think they would have been even fresher than frozen. On reflection, the recent bag I referred to was from Sainsburys, and all the others from Tesco, so different variety may be the explanation. I assumed that the water colour was probably a time delay effect, although the other possibility I'd considered was a reaction with any washing up liquid already in the bowl.

(I didn't make it clear above that both types look smooth and green whilst frozen, although fresh ones look a paler green IIRC, it's the skins that go 'slack', and grey/purple on cooking.)