Grapes hanging on the plant do not ferment because they are still receiving nutrients and moisture, which helps preserve them. Once the grapes are plucked, they begin to break down, and the conditions for fermentation become favorable.
Fermentation occurs when the grapes are exposed to yeast (which can be naturally present on their skin) and a suitable environment (like warmth and the right level of sugar). In this environment, the sugars in the grapes are converted into alcohol and carbon dioxide by the yeast.
This process is a chemical change because it results in the formation of new substances (alcohol and carbon dioxide) and cannot be easily reversed.
