Abstract
In the present paper, the mechanism of the stress cracking in a rice kernel was studied by considering the experimental results obtained by the previous papers, rheologically and dynamically. By assuming that the rice endosperm had a viscoelastic property because a tensile stress was produced at the center of the rice kernel where a stress cracking was generated, it was found that the change of the cracked rice percentage as shown on the previous papers could be well explained for each process, as follows:
(1) In a short period of drying process, a tensile stress was produced tangentially at the surface layer of the rice kernel and a compressive stress was produced at the center of the kernel. As no tensile stress was produced at the center of the kernel within the short period, no cracking was caused in the kernel by that stress. Then the surface layer of the kernel was subjected to stress relaxation or creep deformation. When the moisture content at the center of the rice kernel began to reduce after a few hours, the compressive stress at the center would change to tensile stress and the stress cracking might be caused at the center of the kernel.
(2) In the preserving process after a drying-period, the moisture content decreased at the center of the kernel while it increased at the surface layer of the kernel. Then it was easy to increase the tensile stress at the center of the kernel and to cause the stress cracking there.
(3) In the moisture adsorption process, as the moisture content increased at the surface layer of the rice kernel, the tensile stress was produced at the center of the kernel. Then the stress cracking could be easily caused at the beginning of the process.
(4) In the preserving process after an adsorption-period, the moisture content increased at the center of the kernel while it decreased at the surface layer of the kernel. Thus it was difficult to produce the tensile stress at the center of the kernel and to increase the stress cracking.
