I have successfully “rehydrated” store grapes to full plump by submerging them in ice cold acidulated water (little lemon juice) in the fridge for 2-5 days.

The lemon juice is for light crisp taste. Gapes can double in size! Too long of a soak and grapes will crack.

Remove. Dry. Place back into the product vented ziploc bag, or a bowl… and keep fresh in fridge.