日本レオロジー学会誌 Vol. 45(2017) No. 2

Food bolus cohesiveness plays an important role to suppress miss-swallowing in swallowing disorder. The cohesiveness of masticated particles can be enhanced by binding liquids, which often represent high spinnability similarly as oral saliva does. Thus, in addition to shear viscosity, the extensional viscosity is important to control the bolus swallowing. Using a capillary thinning extensional viscometer, this study measured the extensional viscosity of natural okra, yam, and kelp mucilage in addition to whole saliva and a thickener solution for liquid care foods. The mucilages showed very high extensional viscosity, which was two or three orders of magnitude higher than the shear viscosity. The rheological characteristics for shear and extensional flow were found to be described by a Giesekus model approximately. The results obtained here can be applied to simulate the bolus swallowing numerically and to improve care foods for swallowing disorder.

For an amorphous polymer, it is well known that annealing under the glass transition temperature introduces changes in physical properties due to entropy relaxation. For casting solutions of amorphous polymers under the glass transition temperature, the molding and solidification history is reflected in their viscoelastic properties (especially tanδ). However, above the glass transition temperature, it has been thought that the conformation of the polymer becomes random. In this study, we evaluated the dependence of the mechanical properties on the duration of melt compression molding by using amorphous polystyrene virgin pellets. From the results, we found that the tensile mechanical properties of virgin PS improved with increases in both the molding duration and temperature. In addition, a new diffraction peak appeared at a low scattering angle in X-ray diffraction measurement.

The solution properties of tunicate cellulose (TC) were investigated with respect to variations in molecular weight. The linear and non-linear rheological measurements showed that the chain-entanglements of TC molecules in solution were stiff enough to be released within a short time scale, and the behavior depended on the molecular weight of cellulose and the solution concentration. However, the concentration dependence became weaker when the molecular weight was lowered. The infrared spectra of TC solutions indicated that many intermolecular hydrogen bonds were formed with higher concentrations. This concentration dependence was consistent with that of the damping function.

Swallowing disorder is a serious worldwide problem. Liquid care foods for the swallowing disorder have shear thinning viscosity like yogurt, and the high viscosity reduces the bolus velocity in pharynx to suppress miss-swallow. However, the thickened bolus brings the post-swallow residue in pharynx, which increases a risk of aspiration. Thus it is an important issue to suppress the post-swallow residue by the rheological modification of foods. This study used the simplified models of pharyngeal recesses, and investigated the influence of shear and extensional viscosity on the liquid residue. The zero-shear-rate viscosity higher than 100 Pa・s induced remarkable residue. By contrast, high extensional viscosity reduced the residue. The suppression of excess zero-shear-rate viscosity and the enhancement of extensional viscosity could serve the liquid care foods safer.

As a molecular weight (M) estimation method for cellulose in ionic liquid solution, fitting procedure of dynamic viscoelastic data to Rouse-Zimm (RZ) model with a correction term (LT term), introduced by Osaki et al. and examined for standard polystyrene samples, is examined in this study. Since the RZ model calculation with and without LT term for loss modulus G" almost coincide, the data are first fitted to RZ model to get the best fit of G" by using M as a single fitting parameter, while fixing other parameters from experimental conditions and using experimentally determined relaxation time τ_RZ. Then the LT term, calculated from intrinsic viscosity [h ] and two adjusting parameters, is employed to judge the appropriateness of the estimation of molecular weight by further fitting storage modulus G'. The measured G' and G" can be well fitted with RZ model plus LT term in the terminal region though small discrepancy in the transition region was observed. The estimated M showed ±10% error due to ±15% error of τ_RZ. It is concluded that this procedure is useful when τ_RZ can be determined within a small error. The estimated M can be used at least for the comparison of cellulose/IL solutions differently prepared.

Flow-induced orientational changes of xanthan gum aqueous solutions and hydroxypropyl cellulose aqueous solutions in a planar channel with an abrupt expansion were examined by measurements of the flow-induced birefringence changes and velocity fields along the centerline of the channel. Four kinds of the xanthan gum (X-gum, M_w >2,000,000) solutions with different concentrations (0.45 0.5 0.7 0.9 wt%) and two kinds of the hydroxypropyl cellulose solutions with different molecular weights (HPC-M, M_w = 620,000 / HPC-H, M_w = 910,000) were tested in this experiment. Characteristic times were evaluated from G' and G" curves respectively. The birefringence changes were delayed corresponding to the characteristic times. Typical sizes of regions where the flow structure of polymers were affected by the abrupt expansion were estimated with the birefringence changes. The normalized sizes have a good correlation with a viscoelastic factor (apparent Weissenberg number) estimated in upstream region regardless of the concentrations of the solutions nor the polymer types.