Bread and brewing

The metabolic products of yeast metabolism are exploited by humans for bread making and alcohol brewing. Yeast metabolism of flour starch by respiration generates CO₂, which is trapped within the gluten-rich dough and forces bread to rise. In the brewing of alcohol, yeasts are forced into fermentative metabolism in the sugar-rich, low- oxygen environments of beer wort or crushed grape juice. The fermentative metabolism is inefficient, and only partially metabolizes the available substrates, yielding CO₂ and ethanol. Both processes considerably enhance the value of the original substrate while contributing little to its nutritional status.