When (a microscopic organism) is mixed with flour and warm water or milk, kneaded and left for a while, carbon dioxide bubbles are released. This causes products that are baked with yeast – like bread - to rise, thus acting like a . The yeast, along with any alcohol that is generated in the process, is burnt off when bread is baked.
Yeast – for baking - comes in many forms with many differing names: Like active, deactivated, compressed, cream, instant and rapid-rise yeast. Active (which is moist and shaped in blocks, must be refrigerated), compressed (which is dry and grainy and can be stored on a shelf) and instant (which is also dry, but more refined than compressed yeast) are the most commonly used types in cooking today.