So I ventured out and grabbed a few handfuls of grapes to experiment with.
This is what we wound up with when it was all separated.
Since I had washed the grapes before separating, the next step was to mash them up.
I used a flat potato masher, but you can also use a foley mill. Just be careful not to squash the seeds or you will release a lot of bitterness.
This juice was left in the refrigerator for 2 days so the tartaric acid (cream of tartar!) can come out. About 1/4 of the juice is tartaric acid. It tastes dry and unpleasant, so it's best to let it settle out. I felt like it was closer to 1/3 tartaric acid.
Then pour the juice off and either drink it, save it, or make jelly.
I would not make jam with riverbank grapes because the skins and seeds contain even more tartaric acid. It's best to take them all out. You can eat the grapes raw and they taste fine - a bit tart. The seeds are crunchy and bland - kind of annoying but totally edible (and loaded with reservatol and fatty acids - grapeseed oil.)
I drank some of the grape juice and made the rest into a syrup for pancakes. It tastes a million times better without all the tartaric acid, but it's still quite sour. The syrup did not require any pectin and neither should the jelly. They are quite naturally high in pectin.
I picked these grapes on August 23rd. We have had a very cool end to summer, but these grapes are also known as "frost grapes" because their sugar content increases once the frost hits.
I left a big patch of grapes untouched, and I will gather them up after the frost. Of course, waiting also means birds will probably eat most of the grapes. It's something to pay attention to and do what's best based on your situation. I think I will make the frosted grapes into fruit leather and raisins, freeze the juice for smoothies, and can as juice.
In an emergency, you'll have to weigh the taste versus your current food supplies.
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