ABSTRACT
Recent studies show that understanding the rheological properties of thickened fluids, such as viscosity and yield stress, is advantageous in designing optimal thickened fluids for the treatment of dysphagia. To date, these studies have focused on the rheological behavior of thickened fluids in shear deformation, while limited information is available on the surface tension of thickened fluids or their rheological behavior in extensional deformation. Knowledge of the extensional properties of thickened fluids (extensional viscosity and cohesiveness) is important to fully understand the behavior of such fluids while swallowing. Our aim in this work, therefore, was to characterize water and skim milk thickened with a commercial thickener (xanthan gum based) to determine extensional deformation and surface tension properties. It was observed that the surface tension decreases as the thickener concentration increases due to the accumulation of the biopolymer at the surface of the fluid when it dissolves in water. In addition, the extensional viscosity of the fluid increased over time as the filament thinned (i.e., as the Hencky strain increased) until it reached a plateau. It was observed that the maximum extensional viscosity, which is related to the cohesiveness of the fluid, increases with the higher concentrations of thickener. However, the cohesiveness of thickened skim milk was lower than that of the thickened water at a given thickener concentration due to lower surface tension. This study confirms that by increasing the concentration of thickener, it will not only increase the shear viscosity (i.e., bolus thickness) of the fluid, but also the extensional viscosity (i.e., bolus cohesiveness).
