Hypothesis for how linear glutenin holds gas in dough

Abstract

A hypothesis is put forward to explain how dough resists uncontrolled expansion of gas cells. Hand kneading, mixer action or gas pressure shears swollen protein in dough. Shearing orients linear glutenin molecules parallel to the shearing forces. Molecules with little overlap are pulled apart in the shear gradient, giving them further chances of getting better overlap with other molecules. Mixing thus forms a well overlapped and coherent gluten. When starch granules are pressed together the protein phase between them gets squeezed out sideways and so undergoes shear at angles of up to 90° to the main direction of flow. A similar result is found if the swollen protein suspension is regarded as being pressed through the tortuous pores between granules: since there will be flow in every pore, the gluten is sheared locally parallel to the pore direction. The pores between starch granules form a continuous three-dimensional network; therefore the protein also forms such a network, which appears isotropic because the orientation is local. Unless a gluten is weak, bubbles can expand without bursting because this network of protein resists forces in all directions until gelling of starch sets the crumb structure; further expansion then tears the starch/protein matrix enough to release the pressure in the bubbles. Elasticity does not arise from stretching the bonds of polypeptide chains but because folded conformations of chains are incomparably more probable than unfolded ones. Brownian motion helps unfolded chains to refold rapidly. Relaxation during normal proof times, while letting unfolded chains refold, does not destroy all orientation and overlap. Part of the action of fungal α-amylase may be to widen the pores between starch granules, reduce the size of small granules that could block such pores and delay starch gelation by slowing the entry of water. Fat could ease the relative movement of granules and protein, and the work of expanding gas cells. The fibrils seen when flour particles are wetted may illustrate the effects on glutenin of shear, produced in this case by sudden release of osmotic pressure. Moulding by sheeting gives an overall direction of orientation (DO) lacking in newly mixed dough. For the surfaces of a rolled sheet this DO lies in the machine direction, but inside the sheet it is sideways. In one-piece bread the asymmetry of gas cells results from easier expansion in the main DO, i.e. along the loaf. In four-piece bread the lack of room to expand horizontally forces cells to elongate vertically.