In bread making, yeast has different roles. Most of us are familiar with yeast's leavening ability. But you may not be aware that its fermentation helps to develop gluten in dough and also contributes to flavor from the wheat flour in the bread.

In a process called fermentation, yeast converts the complex carbohydrates in the bread recipe's flour into simple sugars that it feeds on. With an almost instant action it starts to release carbon dioxide and alcohol, all very important by-products in bread-making. Fermentation can be quickened by warm rising temperatures, 75 to 85 degrees F or slowed by cool ones, such as in a refrigerator. It is important to realize that yeast, although needing warmth, can be killed if it becomes too hot, above 140 degrees F.

When yeast ferments, the carbon dioxide gas released by it is trapped in the tiny air cells in the bread's strong and elastic gluten strands. Gluten is created when wheat flour and moisture, usually water, are mixed and two proteins contained in the flour, gliadin and glutenin form gluten; when the dough is mixed the gluten fibers become parallel and cross-bond to form the elastic but strong structure, much like rubber-bands. Once flour and water are mixed together, any further working of the dough, such as kneading or handling, allows more proteins and water to find each other and link together, further creating and developing the gluten into a web.

The gluten gives the bread dough structure and the elasticity it needs to stretch as carbon dioxide is released from the yeast. The gluten structure must also be strong enough to trap and hold the leavening gases released slowly by yeast over a period of several hours, that's why bread flour with a high gluten-forming potential is typically used in bread recipes. As more and more tiny air cells fill with carbon dioxide, they begin to inflate the cells, just like someone blowing up bubblegum, causing the bread dough to rise. It was been hypothesized that hydrated gluten strands contract around air bubbles and, with proper mixing, supply the dough with well-distributed air bubbles. Good bread texture depends to some extent upon the uniform distribution of the gluten done through the mixing and kneading, as well as rising steps. A dough that is mixed properly and develops the gluten in the flour, will yield a lighter loaf of bread; too much flour or gluten, results in bread that it is dry and dense. 

The alcohol from the fermentation process burns off during baking leaving behind irresistible flavors and aroma. It also tenderizes the gluten structure, and furnishes the vapor for the initial oven spring just after placing the bread loaf in the oven. When protein, starch, and fat molecules break down into their building blocks during fermentation, it breaks down large molecules into smaller, flavorful ones.-- proteins into amino acids, starches into sugars, or fats into free fatty acids -- they all have marvelous flavors. 

Each bread making step affects the yeast cells in some way: the first rising is to allow time for the yeast cells to grow. As this takes places, yeast generates carbon dioxide which makes the dough rise, and alcohol for flavor. Punching down the dough redistributes the cells more evenly throughout the dough mass. After shaping, the dough rises again, but this rising takes place more quickly as the yeast cells are by now highly active. The same process takes place whether you're making foccaccia, pizza dough or just plain bread.

At this stage, most bakers stretch and tuck the dough underneath itself, into a round to give it a smooth, tight top that will trap the gases produced by fermentation. Then they let this very springy dough stand for 10 to 15 minutes. This lets the gluten bonds relax a little and makes the final shaping of the dough easier. This rounding and resting step isn't included in many home baking recipes, but it's a good thing to do.

Some bread recipes require less fermentation time than others because of the type of flour used; recipes with rye flour requires less than those containing only wheat flour. Rye flour is more fermentative than wheat flour because it contains a larger percentage of natural sugars, diastase and protease enzymes. It is also slightly higher in natural acidity than wheat flour. All of these factors have an acceleration effect on gas production and gas retention.