Brewing

Fermentation happens when yeast attacks a solution of sugar dissolved in water. Yeast eats sugar and gives off alcohol, heat, and carbon dioxide; it also creates chemicals called esters, which are volatile flavor compounds. Being volatile means that esters evaporate easily and, in doing so, make themselves available to the sense of smell. Esters derive from the combination of organic acids and the alcohols formed during fermentation. Most of the fruity aromas of beer (and, indeed, many of the fruity aromas of fruit) come from esters. In bread making, the alcohol evaporates during baking; in wine making, the carbon dioxide is released into the air; and in beer making, both products are preserved.