Yeast during bread baking

Like many fungi types, yeasts for bread dough responds to warm water, which begins to bring the little cells to life. Then when exposed to sugars in bread and in flour, it begins to eat, digesting portions of these sugars. This eating process goes on for a short period of time only. Eventually the yeast will die within a few hours, especially if the dough is allowed to grow cold or exposed to too much air. Don’t feel too sorry for it when you have a slice of bread, as its life would be short under any circumstances. At least by using it for bread or wine, you get to give this fungus a happy and full life.

It might be embarrassing to the yeast to have to admit that this rapid eating/digestion cycle makes it just a trifle gassy. As Saccharomyces cerivisiae is feasting, it begins to release gas bubbles of carbon dioxide, and small amounts of ethanol alcohol. These bubbles, trapped in the bread dough, cause the rising action with which we’re familiar. This is why bread making can be time consuming; you’ve got to let this fungus work for a couple of hours in order to sufficiently rise dough.

Once dough has been acted upon by fungi, not all of the cells are quite dead. Putting the bread in the oven is relatively macabre, from the standpoint of the yeast (if it had a point of view). The heat from the oven makes remaining cells go into overdrive, madly munching away at the sugars and expelling carbon dioxide prior to expiring from the oven heat. This is why bread continues to rise during its early cooking stages, and then may deflate slightly as cooking continues.

http://www.wisegeek.com/why-does-yeast-make-bread-rise.htm

 -Li Hua