Baking the Bread What Heat does
Baking seems simple to us: put the well-risen, proofed dough in the hot oven and take it out when it is fully baked. If all went well (and there is no reason why it shouldn't), we place a still-steaming, irresistibly-perfumed, brown-crusted, mouth-wateringly beautiful loaf of bread on a wire rack, and we are ready to cut into it after a short cooling period. But the baking process is anything but simple. There is a series of very complex chemical reactions and physical processes that happen during bread baking, so complex that even food scientists who have studied the baking process for decades are far from fully understanding it. For our purposes as home chefs we don't need to know more about these complex reactions than the very basics which are simple.
In a nutshell, here is what happens in the oven. There are three stages of baking.
1. The first stage covers the first quarter of baking time, until the temperature of the dough reaches 140° F (60° C). That is the temperature when the yeast cells die. Up to that point the rising heat keeps the yeast more and more active to produce a great amount of carbon dioxide gas. All the gas trapped in the dough now expands rapidly as we still remember from our physics class—heat expands gases. Another thing happens, too. The by-product alcohol the yeast produce after gobbling up the sugar evaporates and turns into gas in the hot oven. The result? Even more gases in the dough.
As a consequence, the dough expands rapidly. Bread bakers call this process oven spring— the bread dough springs up. Anticipating oven spring is the reason why you don't let the dough fully double in the last rise. If you allowed the dough to rise too much, the expanding gases during oven spring may rupture the barely solidified gluten structure, and the loaf may partially deflate. Also, if you let the dough rise too much, its structure becomes too unstable, and even such last-minute action as slashing and glazing may partially deflate it. Should that happen to you, don't trash the bread—it is still edible but a little dense and too firm. It may still be fine for toast.
At the end of this first stage the gluten begins to coagulate and the starch to gelatinize. Both processes are changes from soft, flaccid phase to firm and solid, and both occur at close to the same temperature, about 145° F (63° C). Once both gluten and starch are solid, the oven spring ends, the structure cannot expand any more—but by then the yeast cells are dead and they cannot produce more gas anyway.
2. During the second phase of baking, that makes up about one-half of the total baking time, the structure becomes more solid, progressing from the solidified crust toward the center. This phase is over when the center finally also turns solid. In the same time, near the end of the phase, the top crust begins to brown.
3. In the third phase, the final quarter of baking, the top surface dries out and turns brown. These two processes form that splendid crisp crust of a fresh-baked bread. Even though browning only takes place on the thin outer surface, it affects the flavor of the entire loaf because the flavors (produced by the browning reaction) disseminate inward. To prove the importance of this stage, try baking one light-colored and one dark-colored loaf from the same dough. The darker one will have noticeably more flavor. When knowledgeable housewives bought their breads in European village bakeries, they always asked for the darker loaves.
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