Abstract
In order to determine the stress relaxation modulus of a rice kernel, the uniaxial compression and the stress relaxation tests in the same manner as the previous work were carried out by using the rice endosperm pieces formed into a cylindrical shape under various temperatures and moisture contents of the pieces. The results obtained were summarized as follows:
(1) By applying the compressive force to the length, the width and the thickness directions of the rice grain, it could be considered that the rice endosperm was nearly isotropic.
(2) Within about 1% of the strain, the compressive (tensile) relaxation modulus E(t) and the shear relaxation modulus G(t) were experimentally given by the following equations,
E(t)=1.02×104/Tw{0.3exp[-0.0198exp(0.02T+11w)t]+0.7exp[-0.000833exp(0.02T+11w)t]}
and
G(t)=8.16×102/Tw{0.3exp[-0.0198exp(0.02T+11w)t]+0.7exp[-0.000833exp(0.02T+11w)t]}
respectively, where T: temperature [°K], w: moisture content [kg/kg-dry solid] and t: time [hr]. These equations were valid in the ranges 293<T<323 and 0.10<w<0.35.
(3) By assuming that the rice endosperm was a thermo- and hydro-rheologically simple material, the master curves for the relaxation moduli of the rice endosperm were given by the following equations,
E0(ξ)=205[0.3exp(-45ξ)+0.7exp(-1.9ξ)]
G0(ξ) =164[0.3exp(-45ξ)+0.7exp(-1.9ξ)]
where E0(ξ): the compressive relaxation modulus at the reference temperature T0(=293°K) and the reference moisture content w0(=0.17d.b.), G0(ξ): the shear relaxation modulus at T0 and w0, and ξ: the reduced time [hr] (ξ=4.39×10-4exp[0.02T+11w]t).
