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Bread baking transforms an ordinary kitchen into a laboratory of earthy sights, smells, and tastes. A recipe, after all, reads like a scientific experiment. The baker combines flour, yeast, liquid, and salt in a bowl, shapes it all into a dough, lets the dough rise, and bakes it in a hot oven. Flour, which gives bread its structure, is made by milling cereal grains such as wheat, barley, or rye. In this process, the grain seeds are crushed, releasing starch and proteins. Starch molecules are long, gangly polymers of simple sugars linked head to tail by chemical bonds. Proteins are more complex--a single protein may contain hundreds of amino acids strung together like beads on a necklace. What gives bread its light, fluffy texture? The answer is gliadin and glutenin, two proteins found in flour. When flour is added to water and kneaded, these proteins swell up like sponges and form a tough elastic substance called gluten. Gluten can stretch and trap the bubbles of gas that make dough rise. That gas comes from the leavening action of tiny one-celled fungi called yeast. When you combine yeast with flour and water, you'll end up with a sticky white dough. Inside the dough, fermentation is occurring and molecules are on the move. Enzymes from the yeast cells attack starch, breaking it down into glucose. Other enzymes transform glucose molecules into carbon dioxide and ethanol. The carbon dioxide (CO2) gas then bubbles up through the mixture, causing the dough to rise. Breads which are leavened by baking powder instead of yeast lack the tasty molecules of fermented bread. That's because when baking powder gets wet, a chemical reaction occurs that releases only carbon dioxide, salt, and water. In breads leavened with yeast, however, the yeast cells grow under anaerobic conditions and cannot convert glucose molecules completely to gas. Some sugar molecules get sidetracked and are converted into alcohols, acids, and esters--substances which add to bread's flavor. Salt strengthens gluten by slowing down the enzymes which catalyze the breakdown of proteins. If you add too little salt, the dough is tough and sticky. If you add too much, water flows out of yeast cells by osmosis. Then nutrients are lost and production of carbon dioxide slows down. After dough rises a couple of hours in a warm place, it's ready to go into the oven. There, heat causes pockets of gas in the dough to expand. Eventually the crust becomes toasty brown--and soon you're enjoying a slice of warm, home-baked bread.