The Brainliest Answer!
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..................

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Just like human when we are alive we don't got erode where when we die we decompose... Here's you answer> Fermentation occur in anaerobic environment but in environment there is oxygen ;)... other one is  yeast in required for fermentation   but hanging one don't have that.. 
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