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Yeast is a living organism and is an essential ingredient in bread making as it causes the bread to rise. Some yeast products are dehydrated, so the yeast will remain dormant until it becomes activated when warm water and a source of carbohydrate (e.g. flour, sugar) is added. Fermentation occurs from yeast cells creating carbon dioxide and ethanol. The carbon dioxide becomes trapped in the bread dough by a network of gluten strands, causing the dough to rise. During baking the ethanol is evaporated and the yeast cells are destroyed.