Relating Rough Rice Moisture Content Reduction and Tempering Duration to Head Rice Yield Reduction

摘要

Previous research has indicated that kernels will fissure during the process of drying rough rice using air temperatures above the glass transition temperature (Tg) if a sufficient portion of the kernel surface transitions to a glassy state while the interior remains in the rubbery state, a condition that can result due to intrakernel moisture content (MC) gradients created by drying. State transitions can occur by such extended drying using high-temperature air or when kernels are cooled below Tg immediately after drying and before sufficient tempering has occurred. Two long-grain cultivars, Francis and Wells, at two harvest MCs (HMCs) were used to determine the maximum MC reduction that could be achieved in the initial drying pass, and the associated tempering durations required, without incurring head rice yield (HRY) reduction. Samples were dried with air at either 60°C/17% RH or 50°C/28% RH for various durations to create a range of intrakernel MC gradients and were subsequently tempered at the drying air temperature in sealed bags for durations ranging from 0 to 160 min. After tempering, samples were cooled to cause a state transition, and then slowly dried to 12.2% MC. Samples were then milled to determine HRY. Control samples were dried at 21°C/60% RH. Results showed that the amount of moisture that could be removed in the initial drying pass was directly related to the HMC and the drying air condition. The tempering duration required to prevent HRY reductions increased with the MC reduction in the initial drying pass. The HRY reduction patterns concurred with a hypothesis that explains fissure formation during the drying process based on rice kernel property changes associated with the glass transition temperature