Water, enough to cover the mashed grapes by an inch or two.
Instructions
Harvest ripe wild grapes. I like to use a scissors to remove the whole clusters to make sure they don't get bruised or smashed during transport.
Inspect the grapes and eat one of two to see if they have any grit, mine are often so clean I don't bother to wash them, but this can vary depending on where you harvest. Grapes on the edge of a working field might be very dirty.
Put the grapes into a pot, stems and all, then mash them up until juice nearly comes to the top with a potato masher or another blunt object.
When the grapes are well mashed, add some water to cover them by about an inch.
Cover the pot and put it on a burner, then warm it until the pot is steaming. Do not allow it to boil.
Working in a place that's easy to clean, like a garage or outside, put a large colander into an even larger bowl then carefully pour the grape juice and stems into the colander. Allow the juice to drain out, then remove the wild grape mash and allow to cool until you can handle it.
Wring out the excess grape juice from the warm mash. Reserve the mashed grapes to make wild grape vinegar (optional). Strain the juice through a fine mesh strainer, then portion into containers and freeze, or use to make jams, jellies, wine, grape molasses, etc.
Racking off the tartaric acid
Before using wild grape juice, allow the tartaric acid to settle overnight in a fridge in a clear container. Pour off the juice and discard the tartaric acid sludge at the bottom.
Making vinegar with the leftover skins and seeds
Save the leftover skins and stems to make wild grape vinegar (refer to my recipe for fruit scrap vinegar)
Video
Notes
Wild grapes can be stored in a cool dry place after picking for 24-48 hours, but will deteriorate after picking and should be processed quickly for the best result.