Beoordeling van de rol van de belangrijkste bloemcomponenten en van de fermentatie in de reologie van tarwebloemdeeg aan de hand van een nieuwe reologische toolbox

Although bread-making has been practised for millennia, our fundamental scientific understanding of this process is still surprisingly limited. For the preparation of bread dough, only four ingredients are essential: wheat flour, water, salt and yeast. Wheat flour contains mostly starch and gluten proteins. During mixing, which involves substantial shear and extensional deformations, the gluten proteins tend to form a network, thereby encapsulating the individual starch granules. During the fermentation stage and oven rise, this gluten-starch matrix experiences an additional extensional deformation, as it is leavened by the expansion of carbon dioxide gas cells. A very delicate balance between material flowability on the one hand, and material stiffness on the other hand, is required to not only ensure a proper leavening of the dough but also to guarantee shape stability of the baked product afterwards. The rheological properties of dough are thus intrinsically linked to the final quality of the baked product. Hence, an in-depth understanding of how the different flour constituents determine the rheological behaviour of dough is highly desirable. To elucidate the individual contributions of gluten and starch to the overall dough behaviour, the rheological properties of dough and mixtures of different gluten-starch ratios were studied systematically in shear and extension, as both deformation types are frequently encountered in the bread-making process. The dough response in shear was studied by means of linear small-amplitude oscillatory tests and non-linear creep-recovery tests. The behaviour of dough under uniaxial extensional deformations was investigated with an extensional viscosity fixture mounted on a rotational rheometer. The starch component turned out to play a pivotal role in linear dough rheology. With increasing starch content, the linearity limit observed in oscillatory shear tests decreased as a power-law function. Starch also clearly affected the extensional viscosity at small strains. Consequently, in the linear region differences between different gluten systems may become obscured by the presence of starch. As bread-making qualities are known to be linked to the gluten network, it is imperative to probe the non-linear behaviour of dough in order to expose differences in flour quality. The quality differences between a strong and a weak flour type were revealed most clearly in the value of the strain-hardening index in uniaxial extension and the total recovery compliance in non-linear creep-recovery tests. The rheological properties of wheat flour dough are known to be very sensitive to small changes in water content and mixing time. At sufficiently high water levels, a free-water phase exists in dough, which attenuates the starch-starch and gluten-starch interactions. Increases in the water content were found to result in a parallel, downward shift in the dynamic moduli and the extensional viscosity at small to moderate strains, and a concomitant increase in the linear creep compliance. The impact of changes in the water content can thus be captured by a simple scaling law. Dough characterisation after different mixing times showed that overmixing may cause a disaggregation or even depolymerisation of the gluten network. The network breakdown, as well as the subsequent (partial) recovery, were clearly reflected in the value of the strain-hardening index, for which a maximum was reached at a mixing time close to the optimum as determined with the mixograph. Finally, the gluten proteins turned out to be much less susceptible to overmixing in an oxygen-lean environment, which demonstrates the significant role of oxygen in the degradation process. To improve the bread-making performance of wheat flours, enzymes such as glucose oxidase and transglutaminase are frequently included in the dough recipe, as these enzymes have the ability to considerably alter the viscoelastic nature of the gluten network. To evaluate the impact of these enzymes on a flour's bread-making performance, the rheological implications of adding glucose oxidase or transglutaminase to wheat flour dough were investigated with the adequate rheological toolbox developed previously. The enzymes enhanced the elastic character of dough until saturation was reached. In the bread-making process, the use of excessive amounts of enzyme turns out to be counterproductive. Whereas the dynamic moduli did not show a maximum as a function of enzyme content, the strain-hardening index clearly revealed this overcross-linking effect. Besides enzymes, the gluten network can also be reinforced by adding supplementary gluten proteins, which were indeed found to enhance the extent of strain-hardening as well. In this project, we also aimed at revealing the mechanisms responsible for the changes in the rheological properties of dough as a result of fermentation. Despite the obvious importance of the fermentation step in the bread-making process, the number of (fundamental) rheological studies dealing with fermented dough is surprisingly limited. By adding the main yeast metabolites (besides carbon dioxide) to unfermented dough at the concentrations observed in fermented dough, the associated rheological changes could be determined with our fundamental rheological techniques. Glycerol was found to have a softening effect on dough similar to water. Ethanol equally led to decreased values of the moduli, but its effect was not merely diluting: ethanol fundamentally altered the configuration of the gluten network, resulting in a decrease in the dough's extensional viscosity and extensibility. The stiffness and the extensional viscosity of the gluten network were also negatively affected by succinic acid and glutathione. Subsequently, the impact of these metabolites on the rheology of dough was also investigated in situ by examining the rheological behaviour of the dough matrix after fermentation had been completed. Compared to unfermented control dough, the fermented dough matrix exhibited reduced extensibility and a lower maximum extensional viscosity. The storage modulus was also negatively affected, but only at low frequencies. The observed changes could partially be accounted for by the yeast metabolites, yet it was clear that the rheological behaviour of the fermented dough matrix did not merely resemble a superposition of the rheological changes associated with the main yeast metabolites. The differences could perhaps be attributed to other rheologically active components released by yeast during fermentation, or might reflect the time-dependent accumulation of metabolites in an already expanding gluten network during fermentation. Characterisation of the rheological properties of fermenting dough, including the carbon dioxide gas bubbles, is essential to understand the real dough behaviour during processing and to develop a firm understanding of the kinetics of dough fermentation. However, the rheological study of fermenting dough poses great challenges as the dough samples are extremely fragile and their properties change considerably over time. In order to track the time evolution of the dynamic moduli and the density of fermenting dough, a parallel-plate rheometer add-on with adjustable gap was developed. Overfilling effects were taken into account by establishing a calibration curve with unfermented dough. Over the course of two hours, both dynamic moduli exhibited a sharp decline, eventually reaching a steady-state value. As yeast produces several other metabolites besides carbon dioxide gas that are able to alter the viscoelasticity of the gluten-starch matrix, the decrease in the dynamic moduli with increasing fermentation time did not match exactly the time evolution of the dough density. Frequency sweep snap-shots at specific points in time were obtained in multiwave mode and indicated that already early on in the fermentation process, substantial changes occur in the rheological response of dough. The available level of salt (NaCl) and sugar (sucrose) had a clear impact on the rheological behaviour of (unfermented) dough and the fermentation kinetics. To study the latter, the results of the linear oscillatory tests were combined with gas production data obtained with a rheofermentometer. The presence of salt resulted in a stronger gluten network and a slower (and therefore better controllable) fermentation process. Following the addition of sucrose, the dough became softer as the free aqueous phase expanded in volume. The total amount of gas produced increased, even though initially a dip in the gas production rate could be observed as the yeast required time to adjust to the osmotic stresses induced by the high sucrose concentration. The combined operation of fundamental and empirical rheological techniques clearly constitutes a valuable means to study the rheological behaviour of wheat flour dough and to assess the impact of yeast fermentation thereon. The developed rheological methodology can be used further to obtain a deeper understanding of the role of the minor flour components (e.g. arabinoxylan, albumins and globulins, etc. ) in determining the rheological properties of dough and hence the final product quality. In addition, the procedures outlined in this dissertation allow to quickly screen a multitude of yeast strains (each strain having its own metabolic profile) in order to identify those yeast strains that have the potential to improve the stiffness and extensibility of the gluten-starch matrix via their excreted metabolites. This dissertation is thus part of the ongoing effort to further improve the bread-making process, as also in the 21st century bread is still considered to be the staff of life.nrpages: 264status: accepte