Question

Grapes hanging on the plant do not ferment but after being
plucked from the plant can be fermented. Under what conditions
do these grapes ferment? Is it a chemical or a physical change?

Answer 1

When grapes are attached to the plant, aerobic respiration takes place in the cells as the oxygen reaches there. But when plucked, no oxygen reaches the cell, therefore fermentation starts. It is obvious that fermentation takes place only in the absence of oxygen (anaerobic condition).
Fermentation is a chemical change in which sugar is converted to alcohol.

Answer 2

When grapes are still on plants, their self-defense mechanism is at work. This prevents fermentation of grapes. When grapes are plucked form plants, then yeast (a type of fungi) carry out fermentation. During fermentation, sugar changes into alcohol. So, it is a chemical change.