Effects of dispersing media on the shear and extensional rheology of xanthan gum and guar gum-based thickeners used for dysphagia management

Thickening powders are regularly used in various drinks for the effective management of dysphagia and for the improved quality of life of these patients. Rheology of these thickened liquids can change significantly depending on the dispersing media that may lead to some adverse effects. Therefore, the effects of dispersing media, i.e., water, apple juice and low-fat milk, on shear and extensional rheology of thickened liquids prepared with xanthan gum or guar gum-based thickening powders were investigated in this study. Shear rheological results revealed a power law concentration dependence of the relative zero shear viscosity (ηr,0∝cn) for both thickening powders in all dispersing media, i.e., n = 3.41–4.26 for xanthan gum-based and n = 4.55–5.42 for guar gum-based. For a given thickening powder concentration, the zero shear viscosity was mainly affected by the dispersing media, whereas there was no marked effect on high shear viscosity, especially at shear rate 50 s−1 that is a characteristic value traditionally assumed for oral processing. The flow behaviour of all samples could be described effectively by the modified Carreau-Yasuda model, thus, making it more convenient to compare the material parameters among different samples. Additionally, these dispersing media showed drastic effect on extensional rheological characteristics of the thickened samples, particularly on the filament life span and extensional viscosity. For xanthan gum-based powder, the values of both parameters were decreased at least 2-fold when dissolved in apple juice or milk, signifying weakening extensional property of these thickened beverages compared to thickened water. However, for guar gum-based thickener, enhanced extensional properties were obtained when dissolved in milk, whereas partial hydrolysis may occur in apple juice, leading to inferior extensional property compared to that of water. These differences are largely ascribed to molecular characteristics of the active polysaccharides in these thickening powders. Overall, these results provide new insights into the shear and extensional rheology of food thickeners in various dispersing media that can be used to assist in treating individual patients with dysphagia.