Relationship between Non-Oral Sensory Evaluations, Oral Sensory Evaluations, and Viscosity of Commercial Thickening Agents -Consideration to the Difference of Shear Rate Dependence-

Abstract

In medical facilities, thickening agents are used to avoid the risk of aspiration. However it is problematic that clinical staff is often unsure how to adjust the liquid consistency for each individual dysphagic patient. We need to determine a common index of consistency when we use thickening agents. In Japan, the value of viscosity for patients with difficulties in masticating and swallowing had been measured using a B-type viscometer at a rotor rotation rate of 12 rpm and 20 oC as decided by the Ministry of Health , Labor and Welfare. This rotor rotation rate, calculated as an approximate shear rate of 2-3 s , is a much lower shear rate than that stated for liquid flows through the pharynx. Recently the Japanese Society of Dysphagia Rehabilitation proposed a new criterion for thickened solutions for elderly patients with swallowing difficulties measured by using a cone-and-plate viscometer at a shear rate of 50 s. The rate of 50 s had been adopted as the criterion on the grounds that Wood found that a power function described the relationship between shear stress measured at 50 s and the oral perception of viscosity, so that a shear rate of 50 s was related to the oral perception of viscosity. Other studies of oral shear rates of thickened liquid samples 15-18) have been reported, showing that the correlation between the shear rate of oral perception and the shear rate of objective viscosity is a matter of opinion. In a previous study, we reported a measurement for evaluating the physical properties of thickened solutions and liquid food to examine the correlation with non-oral sensory properties (by tilting the containers, stirring the contents with a spoon, or dripping the thickened solution from a spoon) to establish an index (model foods) of thickened solutions such as honey-like solutions. However, this suggested that the food was not appropriate for the index. The reason was most thickened solutions contain thickening agents that are non-Newtonian liquids, their apparent viscosity varies with shear rate, while other liquid foods, like honey and syrup, are Newtonian. Therefore, flow properties and non-oral sensory properties, e.g., dripping from a spoon, indicated difference between Newtonian and non-Newtonian flow. Recently many kinds of commercial thickening agents have been developed using major ingredients of xanthan gum, guargum, modified starch, etc. Thickened solutions with these ingredients are nonNewtonian; thus, their apparent viscosity varies with shear rate, and they show different dependence on the shear rate of the solution due to the added thickening Relationship between Non-Oral Sensory Evaluations, Oral Sensory Evaluations, and Viscosity of Commercial Thickening Agents –Consideration to the Difference of Shear Rate Dependence–