When grapes are hanging from the plants, they are alive and
their immune system is also working properly. This immune system or the
internal defense mechanism of the plant body does not allow the
microorganism to ferment the sugar present inside grape fruit. When the
fruits are picked from the plant their immune system stop working and
fermentation can be possible. Under anaerobic condition microorganisms like yeast ferment grapes and wine is formed. This is a chemical change..................