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 (SECTION E1). 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!