Environmental effects on mechanical and rheological behavior of parboiled rice

Abstract

Ultimate tensile and compressive strengths, corresponding moduli of elasticity, and compressive stress relaxation properties (relaxation time constants) of parboiled brown rice were investigated. Samples were prepared from two varieties, Lemont (a long grain) and Rico-1 (a medium grain). Rough rice samples were soaked in 55 C water for four hours and subjected to combination of treatments including steaming durations, drying temperatures, storage relative humidities and storage/exposure times. Increase in steaming duration generally increased the ultimate tensile strength (UTS) and tensile modulus of elasticity (TME) especially in Rico-1. Rapid drying rates generally decreased both tensile and compression properties. However, certain combination of steaming duration and drying rate increased the ultimate tensile and compressive strengths and corresponding moduli values, particularly in bold type grain (medium grain). The effects of steaming and drying on the mechanical properties suggested a possible "critical moisture content" in the gelatinized peripheral region of the kernel dictated both tensile and compression properties. Combined effects of relative humidity and exposure time had the most significant influence in decreasing the tensile and compression properties. Strength properties in Lemont were higher than those in Rico-1. Values of compression properties were much higher than those of tensile properties. Hypothetical stress distributions in one dimension in the parboiled kernel were presented corresponding to moisture adsorption and desorption situations and externally applied forces. A four-element Maxwell model best described the visco-elastic behavior of parboiled rice. The first relaxation time constants (t₁), were negatively correlated to deformation rates. An initial loading rate of 40 N resulted in a maximum T ₁ value compared to 20 and 60 N. Relaxation time constant and the intercept values were greater in Rico-1 than in Lemont. Combined effects of relative humidities and exposure times were pronounced on stress relaxation parameters which decreased with an increase in exposure time and the level of RH. Multiple regression models were proposed for the prediction of tensile, compression and stress relaxation properties with respect to processing and storage environments.