Silver Grapes (銀色のブドウ, Giniro no Budou) are poisonous grapes that can be found in Atagoal. They also have a sour taste
These grapes can also sometimes temporarily turn someone into their favorite food
One cat turned into a giant tuna fish after eating these grapes, and Hideyoshi turned into an giant octopus after eating these grapes.
Atagoal Monogatari Chapter 8 - Grape Forest (ブドウの森)