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Figure 2. The increase in volume of bread dough in a resistance-heating oven. Doughs contained no shortening or 3% (flour basis) shortening. Initial rate of expansion is the same for both doughs. In the absence of shortening some (as yet unknown) event takes place at 55°C that results in a decreased rate of expansion. This slowdown occurs at about 75°C in the presence of shortening. Both loaves cease all expansion at about 90°C. Source: Ref. 19.

continues to expand up to 80°C; the longer time at the initial expansion rate results in a larger final volume. Similar results were found in the presence of 0.6% monoglyceride or 0.5% sodium stearoyl lactylate (17).

Much of the C02 formed during fermentation is lost to the atmosphere (20). If the internal gases in a bread were at equilibrium with the atmosphere, there would be no excess C02 pressure inside the gas cells. The fact that there is such a pressure (as shown by increased dough volume) is due to inhibition of diffusion of C02 from the gas cells to the surrounding air. The causes of this inhibition are not known, but increased viscosity due to water-soluble pentosans and/or the developed gluten phase has been suggested (21). At the end of proof, about 35% of the C02 formed during fermentation is present in gas cells, about 5% is dissolved in the aqueous phase, and 60% has been lost to the atmosphere (22). When the solubility of C02 in bread dough was measured experimentally, a value of 0.81

mL (equivalent) dissolved per gram of dough was found (21). Others calculated 0.32 mL (equivalent) dissolved per gram of dough, based on the known solubility of C02 in water (20).

A mass balance calculation for a pup loaf based on 100 g of flour (173.5 g of dough) found that C02 and air occluded during mixing accounted for 370 mL of loaf volume at the end of proofing, and in the oven this gas phase expanded by 62 mL (20). Dissolved C02, driven out of the aqueous phase owing to rising temperature, contributed 43 mL to oven expansion, and C02 generated by increased yeast activity during early stages of baking amounted to 4 mL. The total volume expansion in the oven from C02 and air was 109 mL, but the actual increase in loaf volume was 360 mL. (If the solubility data of Ref. 22 are used, the volume of C02 expelled from the aqueous phase would be 109 mL, and the total volume expansion due to C02 and air would be 175 mL.)

The difference, 251 mL (or 185 mL, if Ref. 23 data are used), must be supplied by the evaporation of ethanol and water. The amount of dough expansion that could be expected owing to evaporation of water, based on the vapor pressure of water as internal dough temperature rises was calculated (6). This is a slight overestimate, as the aqueous phase of the dough is roughly 0.5 M in salt and 0.3 M in sugar, as well as containing water-soluble proteins, flour pentosans, and fermentation acids. These solutes lower the vapor pressure of the water by roughly 3%.

The amount of ethanol formed during fermentation and proofing usually amounts to 1% of total dough weight (24), or about 2.5% by weight of the liquid phase. About half this ethanol is vaporized during baking. The boiling point of the ethanol azeotrope (95% ethanol, 5% water) is 78°C, so all of it would be expected to vaporize by the end of the bake (although half is apparently trapped and does not escape to the surrounding atmosphere). At 1% of the weight of the pup loaf, 1.7 g of ethanol vaporizes to 830 mL. Vaporization of only half the ethanol is more than sufficient to supply the volume of oven spring over and above that from C02 and air.

To summarize, C02 contributes about 25% of the gas pressure responsible for oven spring in the standard white loaf considered; expansion of air accounts for about 15%; and ethanol vaporization makes up the rest. The actual magnitude of oven spring depends on other dough components that govern the temperature at which rapid expansion ceases (the foam structure of leavened dough converts to a porous sponge, allowing leavening gases to escape).

Proofing Temperature

Another factor that influences the amount of oven spring is the temperature of proofing. In a study using a sponge and dough variety bread (containing wheat bran) it was found that lower proofing temperatures produced larger final loaf volumes (25). The 1-lb loaves were proofed to a height of 0.5 in. above the pan rim in 50 to 55 min. Additional yeast was added on the dough side to maintain this proof time at each experimental temperature (75-125°F, in 10-degree steps). The bread proofed at 75° had a loaf volume of 3058 cc, while that proofed at 125° had a final vol ume of2365 cc. Further, the crumb was progressively more open and overall bread quality was poorer as proofing temperatures increased.

The probable mechanism for this enhanced oven spring was analyzed in some detail (26). Expansion of C02 and air accounted for only a small part of the difference. The main contributor to better bread quality is probably twofold: (1) improved gluten strength (less weakening effect from yeast cells dying at the higher temperature), and (2) a longer loaf expansion time until the foam (dough) converts to a sponge (bread) structure.

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