Factors affecting growth

Temperature and relative humidity have an important effect on yeast growth and fermentative capacity. Most strains of S. cerevisiae used for baking have an optimum temperature of about 36°C to 39°C, although seldom are such high temperatures used during the dough fermentation. Rather, doughs are ordinarily held at temperatures of about 25°C to 28°C (and sponges slightly lower). Although higher temperatures can accelerate fermentation and gassing rates (defined as the amount of CO2 produced per unit time), elevated temperatures also can enhance growth of microbial contaminants, including wild yeasts and mold.Also, the temperature at which yeast activity declines is only a few degrees higher than its optima. Relative humidity must also be controlled, usually at about 70% to 80%. Lower levels of moisture in the air may cause the dough to dry at the surface, leading to formation of a crust-like material, as well as inhibiting the fermentation within the areas near the surface.

Dividing, Rounding, and Panning

The fermentation described above for a straight dough or bulk fermentation (see below) usually occurs in troughs. Once the fermentation is considered complete, the dough is then ready to be divided, on a volumetric basis, into loaf-sized portions. Dividing is usually a simple process that involves cutting extruded dough at set time intervals, such that each piece has very near the same weight. Other devices, driven by suction, are also common. Despite the differences in design, the main requirement of the dividing step is that it be done quickly, since the fermentation is still ongoing and the weight-to-volume ratio (i.e., density) of the dough pieces may change.

The divided dough is then conveyed to a rounding station, where ball-shaped pieces are formed.At this point the dough is given a short (less than twenty minutes) opportunity to recover from the physical strains and stresses caused by being cut, compressed, and bounced about. A portion of the gas is also lost during the dividing and rounding steps.Thus, not only does the dough have a chance to rest (structurally, that is), but the fermentation also continues, adding a bit more gas into the dough. Following this so-called intermediate proofing step, the dough pieces are delivered into a molding system that first sheets the dough between rollers, then rolls the dough into a cylinder shape via a curling device, and finally, shapes the dough into the desired final form. The shaped loafs are then transferred to pans for proofing and baking.

Proofing

Most of the CO2 is expelled during the sheeting step, and it is during the final proofing step where the dough is re-gassed and the fermen tation is completed. For some bread production systems (e.g., no-time processes described below), the entire fermentation takes place during the proofing step. Proofing is usually done in cabinets or rooms between 35°C and 42°C, giving dough temperatures near the optimum for S. cerevisiae (35°C to 38°C). Proofing rooms are also maintained at high relative humidity (>85%). Total proofing times vary, depending on temperature, but are usually less than one hour.The increase in dough volume, measured as the height of the loaf, is often used to determine when the dough is sufficiently proofed.

Baking

A remarkable confluence of physical, chemical, and biological events occurs when the dough loaves are placed into a hot oven. Into the oven goes a glutenous, sticky, spongy mass, with a pronounced yeasty aroma and inedible character, and out comes an airy, open-textured material, with a unique aroma and complex flavor. It is a transformation like none other in food science.

Most modern commercial bakers use continuous conveyer-type ovens, rather than batch-type, constant temperature ovens. Most of these tunnel-like ovens are configured as single or double-lap types, in which the dough loaves are loaded and unloaded (after baking) at the same end. Alternatively, in other ovens, the dough is fed at one end and the baked breads are discharged from the other end. It usually takes about twenty-five to twenty-eight minutes for a loaf to traverse through the oven.

The temperature with these ovens is not constant, but rather increases in several stages along the route, starting at about 200°C for six to eight minutes, and then increasing to about 240°C for the next twelve to fourteen minutes. Finally, the temperature is reduced slightly to about 220°C to 235°C for the remaining four to eight minutes. It should be evident that the temperature of the dough itself will always be less than the oven temperature.The actual tem perature of the loaf during baking depends on those general factors affecting heat transfer kinetics, including dough composition, moisture content, loaf size and shape, and the type or form of heating applied. Although ovens may transfer heat by convection currents (i.e., via the air and water vapor), heat transfer within the dough is via conduction.Thus, there is a decided temperature gradient within the loaf, with the interior being much cooler than the surface. Moreover, many of the readily observed physical changes that occur in the bread during heating are localized, the formation of crust color being the most obvious example.

Several events rapidly occur when the dough is placed into the first stage or zone of the oven. First, as the interior dough temperature increases (about 5°C per minute), there is an immediate increase in loaf volume ("oven spring") due to expansion of the heated CO2.When the dough temperature reaches 60°C, all of the dissolved CO2 is evolved, causing further expansion of the dough. Ethanol also is volatilized and lost to the atmosphere. At very near the same time, there is a transitory increase in yeast and amylase activity, at least up to dough temperatures of 55°C to 60°C, at which point the yeasts are killed. Starch swelling and gelatinization increases, and the gluten begins to dehydrate and denature, causing the bread to become more rigid and more structured.All of these events occur within about six to seven minutes.

In the meantime, the surface temperature will have reached close to 100°C, and the early stages of crust formation will have begun. During the next heating stage, evaporation of water, gelatinization of starch, and denaturation and coagulation of gluten continue. Remaining enzymes and microorganisms are inactivated as the internal temperature approaches 80°C. Finally, when the temperature reaches 95°C, the dough takes on a crumb-like texture and the crust become firmer, due to dehydration at the surface.A brown crust color is formed as a result of caramelization and Maillard non-enzymatic browning reactions.The latter reaction also generates volatile flavor and aroma compounds.

Cooling and Packaging

Once out of the oven, the baked bread is susceptible to microbial spoilage. Therefore, cooling must be performed under conditions in which exposure to airborne microorganisms, particularly mold spores, is minimized. The bread also must be cool enough so that condensate will not form inside the package, a situation that could also lead to microbial problems. Various cooling systems are used, including tunnel-type conveyers in which slightly cool air passes counter-current to the direction of the bread, as well as forced air, rack-type coolers. For sandwich breads, the cooled loaves are sliced by continuous slicing machines. Ultimately, commercial breads are packaged in moisture impermeable polyethylene bags.

Modern Bread Technology

Although hundreds of variations exist, most leavened breads are manufactured by one of four general processes (Figure 8-9). These processes vary in several respects, including how the ingredients are mixed, where and for how long the fermentation occurs, and the overall time involved in the manufacturing process. Of course, how the bread is made also has a profound effect on the quality characteristics of the finished product.

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