2.3. Measurement of Viscosity and Flow Behavior

Viscosity of 200-mL samples, at 25 °C, was measured using a programmable Brookfield digital viscometer (Model LV DV-II, Version 5.0, Brookfield Engineering Laboratories, Inc., Stoughton, MA, USA) fitted with an LV spindle No. 2. A 250-mL low form Pyrex beaker was used as a sample holder, as recommended by the LV DV-II viscometer operation manual. Viscosity readings were taken at six different spindle speeds (10, 20, 30, 40, 50, and 60 rpm). Five readings, at each spindle speed, were taken at three min of running the samples on the viscometer. Flow behavior was deduced graphically and by calculating the viscosity ratio as recommended in the Brookfield labs Inc. guide [17]. The viscosity ratio was the viscosity reading at 10 rpm to that of the 60-rpm spindle speed. Readings at 60 rpm were reported as viscosities of the samples because, under the test conditions, the 60 rpm was equivalent to a shear rate of 50 s⁻¹ [17]. The value 50 s⁻¹ is considered as the shear rate in the mouth [18,19].