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The pKa of propionic acid is 4.87, and, according to the Henderson-Hasselbalch equation, calcium propionate would be in the acid form below this pH and in the salt form above this pH. It should be noted that the ordinary pH range of bread is between 5.0 and 6.0 (i.e.,above the pKa of propionic acid). However, enough of the acid form will still be present to diffuse into the cell and cause inhibitory effects (Figure 2, middle and lower panels).

In the home, a good strong pair of hands can do the mixing and kneading, but commercial bread manufacturers rely on large mixers that knead and tumble the dough. There are actually many different types of mixers, ranging from low- or slow-speed mixers to high-speed devices that require as little as five minutes to complete the job. The latter are used for processes, such as no-time bread manufacturing methods (discussed later) that rely on mechanical development of the dough and shorter fermentations. This is in contrast to traditional low-speed mixing methods in which dough development (defined loosely as the cumulative changes that give the dough its necessary physical and chemical properties) depends on an extended period of yeast activity and fermentation.

Several events occur during mixing. Upon hydration, gliadin and glutenin form a visco-elastic gluten complex. The sulfur-containing amino acid cysteine is present in both gliadin and glutenin, where it forms disulfide bonds or crosslinks between poypeptide chains. However, in gliadin the disulfide crosslinks occur within a single protein molecule, whereas it is generally believed that the disulfide crosslinks in glutenin form between different protein strands (according to most cereal chemists). The more crosslinking, the more elastic the dough. Dough mixing and elasticity are affected by oxidizing and reducing agents, as described above.Eventually, as the dough is mixed and the gluten complex is formed, a continuous network or film will surround the starch granules. Naturally occurring or exogenous amylases then begin to hydrolyze the starch, generating maltose, maltodextrins, and other sugars.

Fermentation

Yeast growth is initiated as soon as the flour, water, yeasts, and other ingredients are combined and the dough is adequately mixed.A lag phase usually occurs, the duration of which depends on the form of the yeast and the availability of fermentable sugars. Bakers' yeast (S. cerevisiae) has a facultative metabolism, meaning that it can use glucose by either aerobic (i.e., via the tricar-boxylic acid or TCA cycle) or anaerobic pathways (Figure 8-6, upper panel). The former pathway yields much more cell mass and more ATP per glucose than the anaerobic pathway. However, despite the incorporation of oxygen into the dough during the mixing step, oxida-tive metabolism of carbohydrates occurs only briefly, if at all. Instead, carbohydrates are metabolized by the glycolytic fermentative pathway. This is due, in part, to the presence of glucose, which inhibits synthesis of TCA cycle enzymes via catabolite repression, but also because the dough quickly becomes anaerobic due to the evolved CO2. Also, as shown in Figure 8-6 (lower panel), the ethanol-forming reduction reaction generates oxidized NAD, which is necessary to maintain glycolysis.

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