Thanks, everyone. ;o)
I can only say from making lime marmalade that they seem to help- couldn't begin to quantify it.
Thank you, Smaug. (Love your name by the way, though it does strike a bit of terror.) I'm wondering if I should be saving them, the way I save lemon pips, when squeezing for juice, etc., to use in low pectin preserves. Sounds like it can't hurt. I go through dozens of lemons and limes throughout the year and make at least five or six different kinds of preserves in the summer - many but not all involving plums, and I cannot always get them underripe.
I asked my 88 yo Mom who knows all things fruit and veg. Her answer was "oh, yes, but unripe plums are so much better..does she have a plum tree?" Lol. Thanks Mom.
Thanks, Susan W. I actually have an "ornamental" (huge, though) Japanese plum tree, which does get small plums on it. I'm thinking I should grab a few and test this out, when the time comes.
When I've picked up the fallen / rotten ones while doing yard work, I've noticed that they are about 80% stone, but if I'm just using the flesh to supplement other fruits, that won't matter. I can easily get them when under-ripe, as my husband built an spectacular "tree house" (redwood deck accessed by climbing the tree with the assistance of a sturdy, well-knotted rope), so I think I'll give it a go.
Thank you so much. I'll report back in July. ;o)