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bread by adding these nutrients directly to the dough.

• Dough improvers. Mixing the ingredients and making an elastic dough can often be a slow process, due to the sticky gluten protein that forms when the flour is hydrated. Reducing agents, such as cysteine, are used to speed up dough mixing, by decreasing the number of disulfide cross-links that make gluten so highly elastic (as described in the next section). In other words,these agents weaken the dough structure and decrease mixing times. Alternatively, other bread dough improvers include oxidizing agents, such as ascorbic acid and potassium bromate, that can improve dough structure by increasing the number of disulfide bonds (and cross-linkages) in the gluten network.Their effect is to increase elasticity and gas retention. Bromates have long been considered as the most effective oxidizing agent; however, recent questions regarding their safety has led some government authorities to discourage or prohibit the use of this additive in bread manufacture.

• Biological Preservatives. Biological spoilage of bread (in contrast to staling or other chemical-physical causes) is invariably caused by fungi. Thus, mold inhibitors are routinely added to most breads. The use of these inhibitors is somewhat less common today than twenty years ago, however, due to the increased consumer demand for natural foods that are free of chemical preservatives. By far, the most common anti-fungal agents are the salts of weak organic acids; they include potassium acetate, sodium diacetate, sodium propionate, and calcium propionate. The latter is the more effective and is the most widely used bread preservative in the United States. As illustrated in Box 8-3, the mode of action of these agents against target cells is similar and depends on the form of the acid and the pH of the food. They are typically added at 0.15% to 0.2% of the flour weight; the maximum amount allowed for acetates and propionates in the United States is 0.32% and 0.40%, respectively (less in Europe). It is important to note that, despite their effective ness against fungi, these agents are only inhibitory and are not cidal.They also are not inhibitory to bakers' yeast, and, therefore, have no effect on the fermentation.

• Emulsifiers. Although bread dough does not ordinarily require emulsification, emulsifying agents may improve the functional properties of the dough by increasing water absorption and gas retention, decreasing proofing times, and reducing the staling rate. As such, bread emulsifiers are perhaps best considered as dough conditioners. By far, the most commonly added emulsifiers are mono- and di-glycerides, derived from hydrogenated vegetable oils. In the United States, they are added at a concentration of 0.5% (flour weight).

• Gluten. Gluten, as discussed below, is formed naturally during dough hydration and mixing steps. However, it is also a common practice to add dried gluten, in the form of vital wheat gluten, as an ingredient to the dough mixture. Not only does the added gluten increase the protein content of the bread, but it also increases loaf volume and extends shelf-life. In addition, high gluten breads may have improved texture properties by virtue of strengthening the dough and bread structure.In general, vital gluten is most commonly added to flour in crop years when protein quality or quantity are low; it is also added to whole grain and specialty breads to increase loaf volume.

Hydration and Mixing

Depending on the nature of the ingredients (solid or liquid), they are either weighed or metered into mixing vats. Once all of the ingredients are combined, they are vigorously mixed. This is a very important step because the flour particles and starch granules are hard and dense and water penetrates slowly. Mixing, therefore, is the primary driving force for the water molecules to diffuse into the wheat particles. Mixing also acts to distribute the yeast cells, yeast nutrients, salt, air, and other ingredients throughout the dough. Mixing, in other words, is necessary to develop the dough.

Box 8-3. How Propionates Preserve Bread

Calcium propionate (Figure 1) is one of the most widely used antimicrobial preservatives in the fermented foods industry, due to its widespread application as an anti-fungal agent in bread. It also inhibits rope-producing species of Bacillus.Although it is the propionic acid moiety that is the active agent, the calcium salt of this weak organic acid is used commercially since it is readily soluble and is easier to handle. Calcium propionate is a GRAS substance (generally recognized as safe), and occurs naturally in various cheeses (e.g., Swiss cheese).

The inhibitory activity of calcium propionate against fungi is due to its ability to penetrate the cytoplasmic membrane and gain entry inside the fungal cell. However, propionate can only traverse the membrane when it is in the acid form.This is because the lipid-rich, non-polar, hy-drophobic cytoplasmic membranes of cells are largely impermeable to charged, highly polar molecules, such as the anion or salt form of organic acids. Rather, only the uncharged, undisso-ciated, lipophilic acid form can diffuse across the membrane.

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