Dynamic Viscoelasticity of Shiratama Mochi Cake in a Range of High Temperature

Mechanical Properties of Waxy Rice Cake, Mochi Part VI

Abstract

Mechanical properties of waxy rice cake, "Mochi", in the high temperature range, 40-100°C, were examined using a coaxial cylindrical rheometer. The dynamic modulus G of shiratama mochi cake decreased rapidly as increasing the apparent specific volume in a relatively lower temperature range, 45-60°C. The activation energy in this range was about 70 kcal/mol. The dynamic viscosity η showed the similar tendancy to G. Repeating extrusion of mochi cake made G lower slightly at high temperatures above 65°C. No effect of aging period was observed during 3 days to 4 weeks. Both G and η of shiratama mochi cake were substantially lower than those of commercial mochi cake. On the basis of these experimental results, the energy balance during oscillation was discussed in relation to texture. The new index, ΔK/K', loss energy ratio, was introduced, which defined by following equation: ΔK/K'=tanδ/(2+πtanδ), where tanδis loss tangent. The equation. is also applied to the material of very high viscosity. ΔK/K' which is defined in the range from 0 to 1, shows the degree of periodic damping. The properties indicated by ΔK/K' agreed well with the actual chewing characteristics of mochi cake.