Mechanisms that can be responsible for the ability of wheat flour doughs to retain gas are discussed. It is concluded that the relevant types of physical instabilities are Ostwald ripening (disproportionation) and coalescence of gas cells. The extent of Ostwald ripening is probably primarily controlled by surface rheological properties; it certainly affects crumb structure, but it cannot explain the differences in gas retention among doughs from various cereals or various wheat cultivars. It is argued that surface forces only provide a stabilizing mechanism for dough films between gas cells, i.e., against coalescence, for films that are much thinner than the diameter of a starch granule. It is concluded that variation in the potential for gas retention among wheat flour doughs is largely due to variation in bulk rheological properties. We propose a new rheological criterion for the extensibility of dough films between gas cells (and thereby for gas retention) that is based on the strain hardening of the dough in biaxial extension exceeding a specific lower limit. The criterion is translated into measurable parameters. Preliminary experiments on doughs with poor and with satisfactory baking performance illustrate its potential importance.
Abstract. Doughs from three flours were sheared between a cone and plate at constant rates in the range 6 times 10‐4‐2 times 10‐2 s‐1. At temperatures between 25 and 40d̀C, the apparent viscosity decreased with increasing temperature and with increasing rate of shear. The effects of the temperature and of the rate of shear were independent one of another, and can be described by an Arrhenius type equation and a power equation, respectively. At temperatures between 45 and 60d̀C, the apparent viscosity increased rapidly with increasing temperature; this is ascribed to starch gelatinization. In this temperature range, the apparent viscosity decreased more rapidly with increasing rate of shear than at lower temperatures. At temperatures between 25 and 45d̀C, the shear modulus decreased with increasing temperature and slightly increased with increasing rate of shear. From the former fact it is concluded that the elasticity of dough has an origin different from rubber elasticity. In this temperature range, the shear modulus can be described by an equation similar to that for the apparent viscosity. At temperatures between 45 and 65d̀C, the modulus increased with increasing temperature, though to a lesser extent than the apparent viscosity. Changes in the rate of stress relaxation are in accordance with the effects of temperature and rate of shear on the apparent viscosity and the modulus.
Doughs were sheared at a constant rate and heated in a cone‐and‐plate viscometer. As the temperature increased, the apparent viscosity first decreased, passed a minimum, and then increased rapidly presumably due to the swelling of starch granules. If the dough was heated more rapidly, the viscosity had its minimum value at a higher temperature. This is explained by the swelling of starch granules being a rate process. The present experiments explain why in previous work with heating prior to shearing, the minima in the viscosity versus temperature curves were found at unexpectedly low temperatures. The temperature at which in a standard amylograph experiment the viscosity begins to rise is a fairly good estimate of the temperature at which viscosity has its minimum value during baking. The agreement is due to compensating effects of the rate of heating and of the starch concentration.
RheoIogical properties of dough are important since they affect the quality of the baked product. In addition, they are interesting since they provide information on dough structure. Instruments used for quality control in mills and bakeries are reviewed. Experiments at constant stress or at constant rate of deformation are discussed as examples of experimental techniques suitable for description of dough properties in physically simple terms. Relationships between dough structure and rheological properties are also covered. The Breadmaking ProcessThree stages can be distinguished in the breadmaking process: mixing the ingredients into a dough, some fermentation periods with punches in between, and baking. Punches are a variety of mechanical treatments of the dough intended to drive out occluded gas and divide large gas cells into smaller ones. Within this framework, large variations are found in the duration and speed of mixing, and in the number and duration of the fermentation periods. These variations are related, among others, to the type of bread to be produced and to the extent of process mechanization.The main ingredients of bread doughs are wheat flour and water; the water con-Journal of Texture Studies 3 (1972) 3-17. AN Rights Reserved Copyright 0 1972 by D . Reidel Publishing Company, Dordrecht -Holland
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