When grapes are hanging on the plant, they are alive and so the cells do not allow bacterial growth. Also the cells have active metabolism. But, when they are plucked from the plant, all the cells of the grape either become dormant i.e seeds or die i.e parenchyma in the fleshy part of the fruit.
Hence, bacteria can easily attack and decompose the grape.
Fermentation is a chemical change in which sugar is converted to alcohol.
