Mathematical modelling of bread dough sheeting : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Engineering at Massey University, Palmerston North, New Zealand
Bread dough sheeting is an important operation in the bread making industry. The process involves the passing of a mass of dough between two, or more, rotating rollers. The function of sheeting can be: i) to shape the dough, ii) to laminate layers of product together or iii) to develop the gluten network, which gives dough many of its properties. The model developed, in this thesis, describes the dough sheeting process using a continuum mechanics approach, solved using a perturbation technique. The bread dough rheology is described using the Criminale-Ericksen-Filbey (CEF) viscoelastic constitutive equation. It was thought that this approach may model the process better than the viscous models used elsewhere. The perturbation technique, used in solution, meant that the model remained computationally swift. The CEF equation was used as it is reasonably simple mathematically and measuring the required dough properties was quite straightforward, although, as was discovered, reproducibly measuring the properties of bread dough is never easy, not least because of the history dependent nature of bread dough. Some important assumptions made, in this model, on the basis of literature and preliminary experiments, were that: the process is two-dimensional; the process is at steady state; the process is unaffected by inertia, temperature or gravity; some parts of the process (conveyor belt speeds, for example) are unimportant; and that the dough is incompressible. It was found that such a model can be used to predict the exit height of the dough, the forces and torques experienced by the rollers, and velocity and pressure profiles in the dough. The predictions were qualitatively consistent with validation data gathered on a pilot plant sheeter, but there were some large quantitative inaccuracies, particularly with the exit height prediction (as there is with viscous models). The inaccuracies suggest that such an approach to modelling bread dough sheeting misses some important facet of the process, possibly the compressibility of the dough. That is, a viscoelastic description of the process material will not, alone, lead to a complete model of the dough sheeting process.