Short definition
The process where yeast converts grape sugar into alcohol and carbon dioxide, turning juice into wine.
Yeast eats sugar, produces alcohol and CO2. That is fermentation in one sentence. It can take days or weeks, depending on temperature and yeast strain. Cool fermentation preserves delicate fruit aromas; warm fermentation extracts more color and tannin. The winemaker controls the speed and temperature to shape the final wine's character.