Influence of Drying on Rice Fissure Formation Rates and Mechanical Strength Distributions

Tests were conducted to determine the effects of drying conditions on the rate of fissure formation after drying and the resultant mechanical strength distributions of individual rice kernels. Long-grain varieties Cypress, Drew, and Wells at 21% harvest moisture content (HMC) were dried at 40°C, 45°C, 50°C, 55°C, and 60°C and constant 17% relative humidity (RH) to 12% MC. Immediately after drying, the samples were placed in sealed plastic bags at 21°C. Fissure enumeration, milling tests, and individual kernel breaking force measurements were made at 1, 24, 48, and 120 h after drying ceased. Head rice yields (HRYs) decreased as drying temperature increased. Very few fissures were visible immediately after drying. Most fissures appeared within 24 h of drying cessation and corresponded to HRY reductions. There was no difference in the kernel thickness distributions of fissured and non-fissured kernels taken from the dried samples. The drying treatments produced samples having greater variation in kernel breaking forces than that of control samples. Thus, the breaking force distributions (BFDs) were affected by drying treatment, but were also affected by post-drying durations. The percentage of strong kernels in a sample, defined as those brown rice kernels withstanding a 20 N force in a three-point bending test, was strongly correlated with HRY (R2 = 0.804, 0.835, and 0.915 for Cypress, Drew, and Wells, respectively).