Extensional Flow Studies of Wheat Flour Dough. II. Experimental Method for Measurements in Constant Extension Rate Squeezing Flow and Application to Flours Varying in Breadmaking Performance

Abstract

An experimental method for biaxial extensional measurements on wheat flour doughs in a squeezing flow geometry is reported, in which the speed is controlled so as to be proportional to the gap between the disks, resulting in a constant extensional strain rate in the sample. In such measurements, the stress was obtained directly as a function of Hencky strain at constant pre-set strain rate. This allowed a straightforward evaluation of the strain hardening phenomenon, its onset and strength. Measurements were made at three strain rates and at Hencky strains varying from 0 to 3 for 15 winter wheat cultivars, five of high (H), five of intermediate (M) and five of low (L) protein content and baking performance. It was concluded that all three groups showed strain hardening, which started before a Hencky strain value of 1 was reached. All three groups, at all three strain rates satisfied the stability criterion n>εHat all values of Hencky strain (n is the strain hardening index in the power-law representation σ=KεHnwhere σ is the stress, εHis the Hencky strain, and K is a constant). Differences between the groups were seen most clearly at the lowest strain rate: (i) the onset occurred at a lower Hencky strain, the higher the protein content and baking quality; (ii) the strain hardening index reached a maximum just before a Hencky strain of 2, where the H group had a 25% higher value than the M and L groups.