TIME-RESOLVED SHEAR VISCOSITY OF WHEAT-FLOUR DOUGHS - EFFECT OF MIXING, SHEAR RATE, AND RESTING ON THE VISCOSITY OF DOUGHS OF DIFFERENT FLOURS

The shear viscosity of three doughs of different wheat cultivars mixed to a farinograph level of 500 BU was measured at low shear rates as a function of the shear deformation using a cone-and-plate viscometer. Cyanoacrylate adhesive was used to attach the dough samples to the ins trument surfaces to eliminate wall slip. Flours used were Dragon, Kosa ck, and a fodder wheat. A distinct difference was observed between the viscosities of the different flour cultivars. The strongest dough (Dr agon), with the highest protein content and a good resistance in the f arinograph, had the highest maximum viscosity. The doughs showed disti nct strain hardening, more pronounced for the strong doughs. Maximum v iscosity was obtained at a strain of approximate to 4, almost independ ent of the shear rate, but at higher values for stronger doughs (5 for Dragon, 4 for Kosack, and 3.5 for fodder wheat). The maximum was most pronounced for well-mixed doughs after resting. The viscosity and its variation with strain may be used as a measure of quality; a higher v iscosity and a maximum occurring at high strains indicating good quali ty (related to the farinogram). The viscosity gradually decreased at h igher strains. Apparent viscosity increases with strain and reaches a maximum value at a common strain, which suggests the presence of entan gled molecules. The increase of maximum viscosity with increase in mix ing also supports this theory. Resting the dough increases the maximum viscosity, which suggests the formation of new cross-links in the non equilibrium entangled network during resting.