MEASUREMENT OF BIAXIAL EXTENSIONAL VISCOSITY OF WHEAT-FLOUR DOUGHS

The biaxial extensional viscosity of wheat flour doughs was measured u sing the lubricated squeeze film apparatus. Two disks of different dia meters, 80 and 57.2 mm, were employed, with an extension rate (0.011 s -1) which remained approximately constant with both disks. Using polyd imethylsiloxane a steady state lasted from 40 to 70 s. Obtaining stead y extensional flow with wheat doughs, on the other hand, necessitated 10 to 200 s depending on the magnitude of normal stresses which ranged from 5.01 8 to 0. 361 KPa. Doughs with different protein contents (13 .2%, 16.0%, and 18.8% based on 14% MB) showed different biaxial extens ional viscosities. These increased with increasing protein content. At an extensional rate of 7.3 X 10(-5) s-1 the biaxial extensional visco sity approached 6eta; Trouton's ratio (ratio Of eta(E) to eta) increas es with extension rate. The differences between biaxial extensional vi scosities and shear viscosities become more profound when viscosities are plotted versus stresses.