Abstract
The Effects of Moisture Adsorption on the Tensile
Strength of Rice. (January 1970)
Md. Shahansha-ud-Din Choudhury, B. S. Eng., Dacca University;
M. S., M. E., Texas A&M University.
Directed by: Dr. Otto R. Kunze
Investigations were conducted to determine the tensile strength of individual rice grains at moisture equilibrated conditions and immediately after being removed from a moisture adsorbing environ. ment. Researchers have postulated that fissuring (breaking or cracking) of rice kernels during moisture adsorption may be due to tensile stresses in inner portions of the grain developed from expanding cells in the outer layers. However, they have not studied the change in tensile strength of individual rice kernels at different levels of moisture adsorption.
Research was conducted with brown and polished rice kernels and concerned the parameters of 1) retardation times required to obtain initial fissuring responses when kernels, equilibrated at 74 F and 4 percent relative humidity (RH), were suddenly exposed to 86 and 100 percent RH atmospheres, 2) moisture adsorbed after selected intervals of the retardation times, 3) tensile strength of rice kernels at moisture equilibrated conditions of 4 percent Fli at 74 and 92 F, and 4) tensile strength of rice kernels after selected intervals of moisture adsorption when equilibrated grains at 74 F and 44 percent R were exposed to 86 and 100 percent RH atmospheres.
The retardation times were dependent on variety, form of rice and amount of change in RH (from low to high). Polished rice required less exposure time to commence fissuring than brown rice. Nato required longer time to produce initial fissures than did Bluebelle. Larger changes in RH resulted in shorter retardation
time.
Moisture adsorption depended on the form of rice and the RH change in the environment. Polished rice adsorbed moisture faster than did brown rice. Larger changes in RH caused faster moisture adsorption than did smaller changes at a given temperature.
A large variation was observed in the tensile strengths of individual rice kernels. For example, the highest tensile strength observed for a moisture adsorbing kernel was 2072.29 psi while the lowest tensile strength observed for an equilibrated kernel was 109.09 psi. The variation between groups of kernels was small.
The average tensile strength of moisture equilibrated rice kernels at 41 percent RH and 74 F gradually increased during the first 2 to 3 months of storage in the same environment. Thereafter, the strength was more or less stable and varied normally as in other biological products. The increase in tensile strength was more in polished rice than in brown rice. A hypothetical explanation of the stresses within the kernel indicated a possible presence of residual stresses. These stresses may have been developed due to drying on the stalk during maturation or during artificial drying after harvesting and before storage. Polishing of rice may induce stresses in the kernel or produce surface cracks. These may be gradually relieved or healed during storage in a constant environ- ment resulting in a gradual increase in tensile strength. Research is needed for further explanation of these phenomena.
Rapid moisture adsorption significantly affected the tensile strength of rice. Within retardation times, the tensile strength was related to the time after exposure. Quadratic equations gave the best fit to these data. The maximum tensile strength in kernels was obtained soon after moisture adsorption commenced and was always greater than that observed at moisture equilibrated conditions. This behavior was not anticipated. Additional research is required for an explanation of this phenomenon. dimensional stress analysis for a longitudinal section of a kernel. was developed to give a theoretical explanation of the observed
behavior.