餅の動的粘弾性

Complex moduli, E* of 3 kinds of mochi cakes prepared with apparatus of stamping, extruding and mixer kneading were measured at frequencies from 3 to 100 Hz and temperatures from 20 to 60°C and 60 to 90°C, respectively. Measurements were carried out with an apparatus for longitudinal vibrations in a cubic specimen at the former temperature range and with another apparatus based on shear mode for concentrated liquid at the latter temperature range. Dynamic Young's modulus, E′ showed large extent of 10⁶-10⁷ Pa and did not change in the frequency range from 3 to 30 Hz, but increased slightly at 100 Hz for all the samples and for every temperature range. Dynamic rigidity, G′, however, was small (about 10³ Pa) in the latter temperature range and appeared positively dependent on frequency. The temperature dependences of E′ and dynamic loss, E″, were very large but those of G′ and G″ were smaller than in the case of E*. The loss energy ratio, ΔK/K′=π tan δ/(2+π tan δ) calcu-lated using the values of G*, showed a peak value at 30 Hz for all the samples and for every temperature range, especially in stamping sample was smaller than another ones. Thus, differences between the preparation methods may explain the different results. The temperature dependence of the apparent specific volume showed the trend Stamp <Extruder≤ Mixer which was confirmed by microscopic observation and by image analysis of the vacuole-size distribution. No difference was found between the samples in the distribution of residual tissue. Rheological evaluation at high temperature or immediately after preparation of mochi was recom-mended as the optimum.