End products

The most obvious and most important end product of the bread fermentation is CO2. It is, after all, the CO2 that is mainly responsible for transforming the stiff, heavy, dough into a spongy, elastic, and airy material. Some of the CO2 dissolves in the water phase (even though CO2 solubility is generally low), causing a slight decrease in pH. Eventually, the CO2 saturates the aqueous environment surrounding the yeast microcolonies.At that point, the CO2 evolves into the dough. The CO2 causes the gluten proteins to stretch, and some escape, but most of the gas is retained and is trapped within the matrix. This process is known as leavening.

The retained CO2 ordinarily collects in the dough to form large irregular gas cells that are dispersed in a non-uniform heterogenous manner. This is one reason why doughs are "punched down" during the course of the fer-mentation.This step re-disperses the gas, forming smaller, more regular, and evenly dispersed gas cells.About 45% of the gas is lost. However, re-mixing causes the yeast cells and sugars to be re-distributed in the dough such that more substrate is made available to the yeast and additional CO2 can be formed.

Other metabolic products are also produced during the yeast fermentation, including various acids, that give the dough a slightly acidic, but pleasant flavor and aroma.The yeast itself can produce acids, as well as other organic compounds. Lactic acid bacteria, which are inevitably present, either in the yeast cake or in the flour, also begin to grow, ferment sugars, and produce acids. Ultimately, the pH of the dough will drop from about 6.0 to 5.0.This decrease in pH, however, has little effect on yeast growth and metabolism; in fact, pH 5 is very near the optimum for S. cerevisiae. Only when there is enough acid produced in the dough to lower the pH to 4.0 or less (as is the case for sourdough breads, discussed below) will inhibition of bakers' yeast strains occur. Despite their modest accumulation in dough, the acids produced by yeasts and lactic acid bacteria make important contributions to the flavor, as well as rheology of the dough. For example, low pH improves the water-binding capacity and swelling of gluten, making it more elastic and pliable.

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