Wheat Flour Dough-Rheological and Structural Aspects

Sammanfattning: The thesis presents a rheological study on wheat flour dough, in relation to the structural influence of the different dough constituents, added components and mixing time. Rheological investigations have been focused on measurements at small deformations, including dynamic oscillation and stress relaxation, although complementing measurements have been performed in large deformation viscosity experiments. The dough (flour and water) accommodate two dispersed phases, air cells and starch granules. Due to the insolubility of gluten in water, starch granules and excess water (including solubles) can be considered to constitute a second aqueous phase in developed dough. This suggests that dough can be described as a phase-separated system. Ultracentrifugation of dough was demonstrated to provide a simple tool for studies of the separation properties of wheat flour dough. The advantage of this technique is that information about the components and possible interactions is obtained with only minor influence on the dough constituents. The separation properties were found to be influenced by dough water content, wheat variety, mixing time, and the presence of lipids and ascorbic acid. By studying the corresponding effect on the stress-relaxation modulus, further information useful for the interpretation of the rheological results was obtained. A difference in location of water in overmixed dough and dough mixed with excess amounts of water was found. The gluten phase was only capable of accommodating a certain amount of water (= 55%), independent of the dough water content. Prolonged mixing, however, influenced the water content of the separated gluten phase, i.e. more water was occluded with extended mixing. A considerable effect of starch content on the rheological properties of dough was demonstrated. It was also found that the influence of starch on the rheological properties increased with prolonged mixing. When the starch granule surface was modified, the rheological behaviour of the dough was influenced. This was suggested to be related to an effect on the dough structure, due to the properties of the granule surface. The properties of the gluten phase, which are often masked by the high starch content of dough, were more pronounced in the large-deformation viscosity measurements, than in the small-deformation measurements. The last part of the thesis deals with the gelation of wheat starch in the presence of the non-ionic amphiphilic polymer ethyl(hydroxyethyl)cellulose (EHEC). EHEC may be regarded as a dough improver or a model substance for other polysaccharides present in cereals. The pronounced influence of EHEC on the mechanical spectra was related to a phase separation giving one phase enriched in EHEC and the other rich in starch (granules in an amylose matrix) at 25°C. Complementary rheological measurements, such as dynamic oscillation (frequency and strain sweeps), stress relaxation and viscosity measurements, were demonstrated to be of great value in the interpretation of the results.