Measurement of biaxial extensional viscosity of wheat flour doughs

Abstract

The biaxial extensional viscosity of wheat flour doughs was measured using the lubricated squeeze film apparatus. Two disks of different diameters, 80 and 57.2 mm, were employed, with an extension rate (0.011 s−1) which remained approximately constant with both disks. Using polydimethylsiloxane a steady state lasted from 40 to 70 s. Obtaining steady extensional flow with wheat doughs, on the other hand, necessitated 10 to 200 s depending on the magnitude of normal stresses which ranged from 5.018 to 0.361 KPa. Doughs with different protein contents (13.2%, 16.0%, and 18.8% based on 14%MB) showed different biaxial extensional viscosities. These increased with increasing protein content. At an extensional rate of 7.3×10−5 s−1 the biaxial extensional viscosity approached 6η; Trouton’s ratio (ratio of ηE to η) increases with extension rate. The differences between biaxial extensional viscosities and shear viscosities become more profound when viscosities are plotted versus stresses.