Rolling of bread dough: Experiments and simulations

Bread dough (a flour-water system) has been rheologically characterized using shear rheometry at room temperature. its flow curve shows a viscoplastic shear-thinning material, which obeys the Herschel-Bulkley model. In addition, substantial wall slip is exhibited by the material, and an appropriate slip law has been formulated. The material has been rolled using the Sentmanat Extensional Rheometer (SER) (Xpansion Instruments) with an aspect ratio of radius to minimum gap of R/H-0 = 4.3. Rolling experiments have been performed for different feed thickness ratios and roll speeds. The exit thickness ratios and the torques to roll the samples have been measured. They have been found to increase substantially with Toll speed and initial sample thickness. Two-dimensional finite-element simulations based on the rolling geometry and the rheological data with slip provide a wealth of information regarding yielded/unyielded zones, pressure and stress distributions along the rolls. The results from the simulations are in good agreement with the experimental torque values although underestimate the thickness of the exiting sheets. it is argued that the latter is a consequence of viscoelasticity of the bread dough, which manifests itself in a free-surface flow, where (extrudate) swell becomes significant. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.