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Figure 8-4. Industrial production of bakers' yeast. From Lallemand Baking Update, Volume 1/Number 9 (with permission).

fate, and lesser amounts of the trace minerals zinc and iron. Since the goal is to produce cell mass as the end product (rather than metabolic products), growth occurs under highly aerobic conditions by providing continuous agitation and oxygen (or air). In addition, the yeast propagation step is performed at optimum temperature and under pH control (usually around 30°C and at a pH of 4.0 to 5.0), with nutrients provided on a continuous basis. At the end of the growth fermentation, the yeast is collected by centrifugation, resulting in a yeast "cream" that contains about 20% total yeast solids and a yeast concentration of about 1010 cells/g.

Bakers' yeast preparations are available in several forms.The yeast cream can be used directly, although this form is highly perishable. Most commercial bakers use compressed yeast cultures. These are produced by pumping the yeast cream through a filtration press or vacuum filter to remove more of the water. The yeast is collected in the form of moist cakes, separated by wax paper. Compressed yeast cakes (about 30 cm x 30 cm x 2 cm) still have a high moisture content (70% to 75%), require refrigeration, and last only a few weeks. However, because the cells are metabolically active, once they are introduced into the dough, fermentation can occur very quickly.

Compressed yeast can be further dried to about 90% solids to provide dry active yeast. This is the form that is familiar to consumers who make homemade bread, but small manufacturing operations or those located where compressed yeast is not available also use dry yeast preparations. Dry active yeast preparations last six months or longer, even at room temperature.They do require a hydration step, and in general, are not as active as compressed yeast, although improved drying technologies have greatly enhanced the activity of dried yeast. In addition, dry active yeasts can be "in-stantized" such that they rehydrate quickly.

Bread Manufacturing Principles

In general, bread manufacture involves just a few steps. First the ingredients are assembled, weighed, and mixed to make a dough and the "bulk" dough is allowed to ferment. The fermented dough is then portioned and shaped, given a second opportunity to ferment, and then baked, cooled, sliced, and packaged. Of course, the actual procedures used by modern bakeries are often more involved, as will be described later. Described below is what is essentially the "straight dough" system.

Ingredients

Most ingredient statements on a loaf of commercial bread are a paragraph long and list thirty or more ingredients. However, only four ingredients—flour, water, salt, and bakers's yeast—are actually required to make a perfectly acceptable, if not exceptional, bread. Although the other ingredients do indeed provide important functional properties, not only in the finished product, but also during the dough development steps, they are strictly optional. Of course, one could argue that consumer preferences for breads with a soft crumb texture; long, mold-free shelf-life; and resistance to staling have led to such widespread use of some of these ingredients that they can hardly be considered "optional." Moreover, large scale manufacturing operations that demand high throughput could not function nearly as well without these extra ingredients and process aids.

Wheat flour is the main ingredient of bread, representing about 60% to 70% of a typical formulation. The flour contains the proteins that are essential for dough formation and the starch that absorbs water and serves as an energy source for the yeasts. Water, added at about 30% to 40%, acts as the solvent necessary to hydrate the flour and other ingredients. Salt, at 1% to 2%, toughens the gluten, controls the fermentation, and gives a desirable flavor. Finally, the yeast, also added at about 1% to 2%, provides the means by which leavening and flavor formation occur.

Many optional ingredients are often included in bread formulations, depending on the needs and wants of the manufacturer.

• Sugars. In the United States, many large-scale bakeries add either sucrose or glucose (about 2% to 3%) as an additional source of readily fermentable sugars.They also supply flavor and, when the dough is baked, color. Wheat flour contains only modest amounts of maltose and glucose, and although yeasts do express amylases that can release fermentable sugars from starch, sugar availability may still be growth-limiting.

• Enzymes. Another way to increase the amount of free sugars in the dough is to add a- and p-amylases, enzymes that specifically hydrolyze the a-1,4 glucosidic bonds of amy-lose and amylopectin (Figure 8-5). Flour naturally contains both of these enzymes, but a-amylase, in particular, is present at very low levels. These enzymes are also available in the form of microbial preparations or in the form of malt. Not only do these enzymes collectively increase the free sugar concentration by hydrolyzing amylose and amy-lopectin to maltose, but they also can enhance bread quality. Controlled hydrolysis of starch—especially the amylopectin portion—by a-amylase tends to increase loaf volume and,by virtue of softening the crumb texture, staling is delayed. Of course, if too much enzyme is added, the dough will be sticky and unmanageable and the excess free sugars that are formed could contribute to over-browning during baking.

One other important factor that must be considered when enzymes are used as process aids is their temperature inactivation profile. Heat-resistant amylases, such as those produced by bacteria, may survive baking and continue to hydrolyze starch, which could result in a soft and sticky bread. Fungal enzymes are generally heat labile and are inactivated during baking.

Increasing the amylase activity in dough, and thereby increasing the free sugar concentration, can also be accomplished by adding malt powder. Malt, an essential ingredient in beer manufacture (see Chapter 9), is prepared from germinated grains, usually barley and wheat. Malt contains both a- and p-amylases that are both somewhat more heat stable than fungal enzymes.

Finally, the addition of proteolytic enzymes has been advocated as a means of achieving partial hydrolysis of the gluten. In theory, this would reduce mixing time and would make a softer dough.

• Fat. The addition of 0.1% to 0.2% fat,as either a shortening or oil, is now commonplace in most commercial breads. Whereas a generation or two ago, animal fats with excellent "shortening" properties were used in breads, the use of such fats has about disappeared in the United States. Now most breads are made with partially hydrogenated vegetable oils. The composition of these oils is not that different from animal fats, in that they contain mostly long chain, saturated fatty acids, with a high melting point. Not surprisingly, they impart similar properties as the animal fats, giving a soft, cake-like texture.The precise mechanism, however, for how these functional effects occur is not clearly known. Fats with high-melting points (i.e., containing long, saturated fatty acids, such as palmitic and stearic acids) clearly provide discernable improvements, especially for short-proof breads. There is currently a movement to use non-hydrogenated oils to eliminate trans fatty acids.This does change the dough properties compared to the use of hydrogenated vegetable shortenings.

• Yeast nutrients. Various nutrients can be added to the dough mixture to enhance growth of the yeast, including ammonium sulfate, ammonium chloride, and ammonium phosphate, all added as sources of nitrogen. Phosphate and carbonate salts may also be added to adjust the acidity or alkalinity, and calcium salts can be added to increase mineral content and water hardness.

• Vitamins. There has been a flour and bread enrichment program in the United States for more than sixty years. Flour currently is fortified with four B vitamins—thiamine, ribo-flavin, niacin, and folic acid—and one mineral, iron. However, non-enriched flour is also available and bread manufacturers can enrich

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