Leavening whose fermentation causes yeast bread to rise. The combination of warmth, food (sugar) and liquid causes yeast to release carbon dioxide bubbles that cause dough to rise. Yeast is very sensitive; too much heat will kill it, and cold will stunt its growth. Always use yeast before its expiration date. Varieties include Regular Active Dry Yeast, Quick Active Dry Yeast and Bread Machine Yeast.