When solid food enters the oral cavity, it is masticated, crushed, and mixed with saliva to form a food bolus, which can be regarded as a concentrated solid-liquid dispersion system. In this study, we examined the physical properties of such concentrated solid-liquid dispersion systems and investigated parameters related to masticatory characteristics from the perspective of food science.

Gelatin was selected as the solid-phase material. The solid-phase material was molded into cubes with sides of 10 mm and subjected to compression tests using a creep meter RE2-3305s (Yamaden Co., Ltd.) at a compression speed of 1 mm/s. The Young's modulus, rupture stress, rupture strain, and rupture energy were determined from the resulting stress–strain curves. To prepare the concentrated solid-liquid dispersion samples, the solid-phase materials were crushed using a sieve (6.7 mm mesh, Tokyo Screen Co., Ltd.) and mixed with pure water. The loss modulus (G") of the prepared concentrated solid-liquid dispersion samples was measured at 25 ℃ using a Rheolographsol (, Toyo Seiki Co., Ltd.) under the conditions of ±50 μm amplitude and 3 Hz frequency. In addition, the particle size distribution of the crushed material was measured from micrographs.

The G" of the concentrated solid-liquid dispersion samples followed a power-law relationship expressed as G" = Ks · W^m. Where Ks is the proportionality coefficient, W is the solid weight fraction, and m is the nonlinearity index. Here, Ks represents an index of bolus fluidity, while m serves an index of salivary disintegration. A value of m greater than 1 indicates a high degree of salivary disintegration. Furthermore, the energy required to increase the surface area of fragmented material was evaluated by dividing the work necessary for fragmentation by the corresponding increase in specific surface area (ΔS). This value was regarded as an index of masticatory disintegrability. Assuming that the work required for fragmentation is proportional to the rupture energy (E), the ratio E/ΔS was defined as the apparent rupture surface energy. The results suggest that the apparent rupture surface energy and the nonlinearity index m can be effectively used as indicators for classifying and evaluating the masticatory characteristics of foods.