Rheology of Wheat Flour Doughs at Large Deformations and the Relation to Baking Quality and Physical Structure
Sammanfattning: The effects on dough rheology at large deformations in elongational flow and shear flow have been studied. The results are related to bread making quality in terms of bread volume and to the physical structure of the dough. A new method for evaluation of an instrumented mixograph is reported. A PLS model, including the five most important parameters from the mixogram, explained 90.9% of the variation in bread volume. A new parameter, called buildup, was found to be closely related to bread volume. Two new experimental methods for measurements on dough in extension are presented: squeezing flow at a constant strain rate, and contraction flow at a constant strain rate. Using these methods, the nature of strain hardening in dough could be evaluated. A model for the squeezing of liquid dough film between adjacent growing gas cells is presented and discussed in view of the results from the squeezing flow measurements. The results from the squeezing flow measurements and the contraction flow measurements were compared and related to strain rate and bread-making quality. Parameters from the contraction flow measurements and the following stress relaxation were evaluated with multivariate analysis. A PLS model including five contraction flow parameters explained 94.6% of the variation in bread volume. The protein content proved to be a very poor indicator of bread making quality. The influence of enzymes and chemical additives on the dough and its baking quality was the focus of the last work. Using an established research rheometer, a method of slowly applied large deformation shear stress relaxation measurements proved to be useful. The relaxation process for dough is discussed in terms of two flow processes, one occurring at short times and one during longer times. Additions of oxidizing agents shifted the second flow process to longer times and also decreased the relaxation rate in the region of the second flow process. Protease shifted the second flow process to shorter times and increased the relaxation rate in the region of the second process. Glucose oxidase moved the two flow processes apart by increasing the relaxation time of the second process and by decreasing the relaxation time in the first process.
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