Roller extrusion of biscuit doughs

Biscuit doughs are dense solid–liquid pastes which exhibit complex rheological behaviour. The rolling behaviour of sheets of commercial short and hard biscuit doughs was investigated using an instrumented counter-rotating roll mill, which allowed the roll torque, separating force and surface pressure to be monitored. The rheological characteristics of the two doughs were quantified by analysing data from ram extrusion experiments in terms of a quasi-plastic model (following the Benbow–Bridgwater approach) and a power law fluid model. The results indicated that the doughs were not ideally suited to the quasi-plastic analysis. The power law parameters varied noticeably between the doughs but both were strongly shear-thinning (power law shear indices of 0.25 and 0.5), with large extensional viscosities. These rheological model parameters were subsequently used to generate predictions of dough behaviour during rolling. A standard sheet metal plasticity model was modified to include strain rate dependency but neither dough’s behaviour could be adequately described by this model. The power law fluid approach based on the lubrication assumption reported by Levine tended to underpredict the work requirement owing to the significant contribution from extensional deformation. Interestingly, the system could be modelled in terms of an apparent power law rheology by fitting data to Levine’s model. Better agreement was obtained for some parameters but not consistently so, indicating that the power law approach is not adequate for these soft-solid materials.