The gases produced in, or incorporated into, the dough and entrapped in it in the form of gas bubbles require the development of pores. The formation of extensible pore walls, both in wheat and rye doughs, depends on certain proteins present in the flour that exhibit specific film-forming properties. Such properties are particularly pronounced in the gluten proteins (gliadin and glutenin) found in wheat flour. When mixed with water under the action of mechanical energy, wheat gluten forms a viscoelastic mass that encloses all other constituents of doughs, primarily starch, in a network. The latter can be two-dimensionally stretched to films largely impermeable to gas. These physical properties of gluten enable it to entrap gases as bubbles, forming a porous, three-dimensionally extending structure. The structure sets to form the crumb of the bakery products when the starch gelatinizes and the gluten coagulates under the action of heat during baking.
Figure 2 shows gluten protein particles isolated from wheat flour that have been converted into a viscoelastic mass by moistening and kneading. When the residue was freeze-dried and examined using a scanning electron microscope, it was possible to identify a membranelike network that corresponds to the structure that is formed by the gluten proteins and is required for the development of the membrane and network in actual doughs. The formation of the structure depends on the amino acid sequences of the gluten proteins and their chemical reactivity. In this connection, it was possible to prove experimentally that the disulfide bonds of wheat gluten contribute substantially to the characteristics of the dough and thus to its baking performance (16,17). The molecular composition of wheat gluten is now also known to a large extent (18). In this context, it is interesting to note that the relative composition of gluten in respect to its molecular subfractions gliadin and glutenin has a crucial influence on its stretching properties, which are responsible for providing the volume in bakery products (19). As regards the baking volume, flours from different wheat varieties thus exhibit differences in volume-forming capacity. The latter depends on an optimal development of the viscoelasticity of the dough by kneading. The mechanical energy input by kneaders necessary for an optimal viscoelas-ticity is in the range of 10-12 Wh/kg dough (20).
The proteins present in rye flours do not possess the same viscous properties as wheat gluten even though rye flour protein contains fractions that, owing to their foam-forming properties, are suited to incorporate gases in doughs in the form of bubbles (14). However, such doughs are considerably less firm than wheat flour doughs due to the lack of an elastic protein fraction in their foam structure. This difference between wheat and rye flour doughs is evident in the surface structure of each type of dough, as shown in Fig. 3. The wheat flour dough has a smooth structure, whereas the structure of the rye flour dough is loose and fractured. Unlike wheat flour doughs, rye flour doughs therefore tend to flow so that they generally result in bakery products that are flatter than those made of wheat flour dough. The difference in gas retention between wheat and rye flour doughs results in the shown large differences between the volumes of the breads baked in both cases from the same dough weight.
Figure 3 Doughs and breads made from wheat and rye flour.
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