biological

• yeast is a biological raising agent used in bread dough
• yeast is a micro organism that causes fermentation - a process that releases alcohol and carbon dioxide
• doughs containing yeast are often proved to allow fermentation to take place
• proving is important because its when the carbon dioxide is released and trapped in the dough, causing it to rise - fermentation stops during the baking as the yeast is killed by the heat
• when the dough is baked, the carbon dioxide expands, causing the bread to rise even more - any alcohol produced by the yeast evaporates