Flow Properties and the Velocity through the Pharynx of Solutions Prepared from Commercial Thickening Agents and Those of Yogurt with Relevance to Liquid-Type Care Foods for Dysphagic Patients

With the advent of an aged society, the incident of dysphagia has increased; dysphagic patients often aspirate foods (foods enter not the esophagus but the trachea) [1, 2]. Thin (low viscosity) liquids flow readily with small cohesion between particles and, therefore, are easily deformed during swallowing. Thus, thin liquid may penetrate the larynx and enter the trachea if laryngeal closure does not occur properly. Many dysphagic patients are not capable of controlling laryngeal closure, and therefore often aspirate such liquid foods of low viscosity [3, 4]. Thickeners or gelling agents are, therefore, used in foods for dysphagic patients to enhance both the viscosity of and cohesion between food particles, and many commercial foods containing thickeners and gelling agents have been developed. However, the appropriate physical properties of foods for patients with difficulty in swallowing have not yet been definitely determined. In 1994, the Ministry of Health, Labour, and Welfare in Japan (hereafter referred to as the Ministry) set the diet criteria for gel and sol foods for patients with difficulty in masticating and swallowing. For both gel and sol foods, the value ranges of “hardness,” determined from the compression texture measured with a rheometer, were set. As for sol foods, “the value of viscosity, μ, of sol measured with a B-type viscometer (a rotational viscometer with a vertical cylindrical rotor) at a rate of rotor rotation of 12 rpm was to be 1.5×10 mPa・s (=1.5 Pa・s) and above at 20±2°C after 2min” [5]. The viscosity μ is physically a more appropriate parameter than the “hardness” as an index for liquid-type (sol) foods, since μ determines the flow rate for simple systems such as viscous flow in a tube [6]. The Japan Care Food Conference organized by various Japanese food companies set a similar criterion for “the universal design foods” in 2003. Many foods for the care of dysphagic patients have been developed on the basis of this criterion. However, the new criteria set by the Ministry in 2009 do not distinguish between sol and gel, not including viscosity as an index. For determination of the appropriate properties of foods for dysphagic patients, an effective approach is to relate the flow aspects of foods (or the bolus) in the human pharynx and the physical properties and/or texture of the foods. The ultrasonic pulse-Doppler method, developed by Nakazawa et al. [7], is safer for the human Flow Properties and the Velocity through the Pharynx of Solutions Prepared from Commercial Thickening Agents and Those of Yogurt with Relevance to Liquid-Type Care Foods for Dysphagic Patients