From The Food Encyclopedia by Jacques Rolland and Carol Sherman
Many species of single-cell fungi that grow quickly in warm temperatures and react differently in different conditions, including altitude. Most yeast used in baking and fermenting is derived from Saccharomyces strains. In breadmaking, when it is open to the air, the growing yeast creates carbon dioxide which causes the dough to rise, producing a small amount of alcohol in the process; inside brewing vats, where little air is available, yeast converts sugar into alcohol, creating a small amount of carbon dioxide in the process. Yeast is also used in the production of various other foods and drinks, including chocolate, cheese, kefir, pasta and soy sauce. It is available powdered (in several active varieties used for baking and beer, cider and wine making, and in a non-active form - a by-product of beer making, called "brewer's yeast" - used as a dietary supplement), in fresh yeast cakes (used for baking), in yeast starters (called sponges and used for sourdough breads), as a paste (used as a nutritional spread) and, in Asian markets, formed into balls (used for making rice wine). See also active dry yeast, nutritional yeast, vegemite.
