M. Meerts, R. Cardinaels, F. Oosterlinck, C.M. Courtin, P. Moldenaers
Keywords: food materials
Summary:Although bread making has been practised for millennia, the fundamental understanding of the relations between dough microstructure and dough behavior/rheology is surprisingly limited. The complex dough matrix consists of a hydrated gluten network that is intertwined with starch granules. During bread making, dough is subjected to a wide range of deformations, combining both shear and extension. Hence, the rheology of dough plays a pivotal role in the bread-making process. Whereas empirical rheological devices have proven their worth with regard to quality control, the rheology of dough can be characterized more systematically by turning to fundamental rheological techniques, in which a homogeneous shear or extensional deformation is applied. To investigate the dough behavior under well-defined small and large shear deformations, oscillatory resp. creep-recovery tests are implemented on a rotational rheometer. Dough behavior in extension is explored by means of an extensional viscosity fixture. To elucidate the contribution of each of the major flour constituents (gluten and starch) to the overall behavior of (unfermented) dough we investigated the rheological behavior of mixtures of different gluten-to-starch ratios as well as doughs made from weak versus strong flour. In the non-linear regime (i.e. for large deformations), dough behavior appears to be primarily determined by the response of the gluten network. Surprisingly, this conclusion does not hold for the linear dough behavior, which is also strongly affected by the starch granules. Consequently, only non-linear rheological tests are able to distinguish strong from weak flour dough. We found the quality differences between different wheat flours to be revealed most clearly in the value of the strain-hardening index (SHI) in extension. Subsequently, the effects of water content and mixing time (which are both important process parameters in bread making) have been investigated. Water appears to act mainly as a lubricant, affecting mostly the starch-starch and gluten-starch interactions. The mixing time, by contrast, has a strong impact on the gluten network itself. This implies that there is no direct interaction between water content and mixing time, whereas in the past the opposite has been hypothesized in literature. Long mixing times have a detrimental effect on the stiffness of the gluten network. Yet this loss of network cohesiveness appears to be partially reversible, as dough exhibits a self-healing ability. We found the network breakdown during overmixing, as well as the (partial) recovery during subsequent rest, to be clearly reflected in the value of the SHI, for which a maximum is reached at a mixing time close to the so-called ‘optimum’ that had been determined with the empirical Mixograph. Finally, we set out to assess the potential of fundamental rheological techniques to study the rheology of fermented dough. Apart from the carbon dioxide gas, other yeast metabolites such as ethanol and succinic acid also have a significant impact on the behavior of dough. Both components strengthen the gluten network, and tailoring their concentration might therefore constitute a promising tool to optimize the dough performance.