The amount of water and flour are the most significant measurements in a bread recipe, as they affect texture and crumb the most. Professional bakers use a system of percentages known as Bakers' Percentage in their recipe formulations, and measure ingredients by weight instead of by volume. Measurement by weight is much more accurate and consistent than measurement by volume, especially for the dry ingredients.
Flour is always 100%, and the rest of the ingredients are a percent of that amount by weight. Common table bread in the U.S. uses approximately 50% water, resulting in a finely-textured, light, bread. Most artisan bread formulas contain anywhere from 60 to 75% water. In yeast breads, the higher water percentages result in more CO2 bubbles, and a coarser bread crumb.
The quantity and quality of the proteins contained in the flour serve as the best indicator of the quality of the bread dough and the finished bread. While bread can be made from all-purpose wheat flour, for quality bread a specialty bread flour, containing more protein, is recommended.
Wheat flour in addition to its starch contains three water-soluble protein groups, albumin, globulin, proteoses, and two non-water soluble protein groups, glutenin and gliadin. When flour is mixed with water the water-soluble proteins dissolve, leaving the glutenin and gliadin to form the structure of the resulting dough. When worked by kneading, the glutenin forms strands of long thin chainlike molecules while the shorter gliadin forms bridges between the strands of glutenin. The resulting networks of strands produced by these two proteins is known as gluten. Gluten development improves if the dough is allowed to autolyse.
During proofing, yeast converts glucose and other carbohydrates to carbon dioxide gas which gives the bread rise and alcohol which gives it flavor. Bacteria which coexists with the yeast consume this alcohol, producing lactic and acetic acids.
Proofing is divided into a number of different categories including fermentation, proofing, retarding, autolyse. Fermenting is any stage of proofing which is completed prior to the shaping of the bread. Often a third of a bread's rise will occur during this stage. Proofing is the general term for allowing a bread to rise while at room temperature after it has been shaped. Retarding is the stage in which bread is placed into a dough retarder, refrigerator, or other cold environment to slow the activity of the yeast. The retarding stage is rarely found in recipes with commercial yeast but often used in sourdough bread recipes to allow the bread to develop it characteristic flavor. Autolyse is a period of rest allowed for dough to relax. After the initial mixing of flour and water, the dough is allowed to sit. This rest period allows for better absorption of water and allows the gluten and starches to align. Breads made with autolysed dough are easier to form into shapes and have more volume and improved structure.
The sour taste of sourdoughs actually comes not from the yeast, but from a lactobacillus, with which the yeast lives in symbiosis. The lactobacillus feeds on the byproducts of the yeast fermentation, and in turn makes the culture go sour by excreting lactic acid, which protects it from spoiling (since most microbes are unable to survive in an acid environment).
All yeast-leavened breads used to be sourdoughs, and the leavening process was not understood until the 19th century, when with the advance of microscopes, scientists were able to discover the microbes that make the dough rise. Since then, strains of yeast have been selected and cultured mainly for reliability and quickness of fermentation. Billions of cells of these strains are then packaged and marketed as "Baker's Yeast". Bread made with baker's yeast is not sour because of the absence of the lactobacillus.
-from wiki
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