The process of fermentation actually involves breaking down of large molecules into simple ones in the presence of enzymes. The source of these enzymes is yeast. The various reactions taking place during fermentation of carbohydrates are

                  ${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{Invertase}/\mathrm{sucrose}}{\to } {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$
$\mathrm{Sucrose} \mathrm{Glucose} \mathrm{fructose}$
$$

                        ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6 \stackrel{\mathrm{Zymase}}{\to }2{\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH} +2{\mathrm{CO}}_2$
$\mathrm{Glucose}/\mathrm{Fructose} \mathrm{Ethanol}$

In wine making, grapes are the source of sugars and yeast. As grapes ripen, the quantity of sugar increases and yeast grows on the outer skin. When grapes are crushed, sugar and the enzyme come in contact and fermentation starts. Fermentation takes place in anaerobic conditions ie., in absence of air. ${\mathrm{CO}}_2$ gas is released during fermentation. The action of zymase is inhibited once the percentage of alcohol formed exceeds 14 percent. If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks.