Here's a set of questions from readers.
- Q: Why does bread need to rise twice?
A: You let bread rise over several hours to develop its flavor. The longer the yeast cells have to work (up to a point), the more maltose and alcohol they can produce.
- Q: Why does bread need to rise in a warm place?
A: Yeast cells are like most single-cell organisms -- they are more active when it is warm. The whole idea behind a refrigerator is to make food cold so that the bacteria, which all foods contain, have a low level of activity and therefore reproduce less. Warm yeast cells do their work faster up to a point -- beyond that point, the temperature gets too high and the yeast cells die. See How Cells Work and How Food Preservation Works for details.
- Q: Does the yeast reproduce in the bread?
A: Yeast reproduces by cell division. Over the course of two hours, yeast does not really have time to reproduce. The yeast cells in the envelope of yeast are the cells that do the work in your loaf of bread. That is why, if you use old yeast, your bread will not rise. Most of the yeast cells in an old envelope of yeast have died, so there are not enough cells to power the expansion.
- Q: What is sourdough bread?
A: Read How Sourdough Bread Works -- it's a great article. But a quick answer is that sourdough bread uses different strains of yeast or other bacteria (like Lactobacillus). Centuries ago, before you could go to the grocery store and buy yeast, people needed a way to have yeast around the house. The way they did it was by keeping a pot of live culture (living in perhaps a quart of water/flour medium) and "feeding" it daily or weekly so that the yeast remained alive and active. To start the culture, you take 2 cups of flour and 2 cups of water, mix the two together, and then add in some sourdough culture that you obtain from a friend (sourdough, in times past, passed from friend to friend like this). When it came time to bake bread, a cup of this live culture would be added to the dough to provide the yeast needed to leaven the bread. The pot would be replenished by adding back an equal amount of flour and water. If you take some of the culture out and feed the pot more flour and water each week, the culture will stay alive. Most people let the culture live at the back of their refrigerator (otherwise, it requires daily feeding). The strains of yeast and bacteria used in sourdough are acid-producing, hence the unique flavor of sourdough bread.