Radiofrequency (RF) Drying of Paddy Rice and Its Effects on Fissure Formation and Milling Quality
Authors: Smith Deandrae , Wason Surabhi , Atungulu Griffiths , Bruce Rebecca 
Published: December 2024

Highlights

Elevated SARAdjusted levels led to temperature increases during RF treatment, resulting in significant reductions in paddy rice moisture content (MC) to 13.1% wet basis (w.b.) for non-tempered and 14.0% w.b. for samples tempered for 4 h at 60°C (Control: Initial MC = 20.4% w.b.; MRY = 69.64%; HRY = 66.56%).

MC gradients and structural changes within rice kernels contributed to fissures and decreased milling yields. However, tempering effectively mitigated fissures and improved milling yields.

The optimal SARAdjusted for drying high mc rice while preserving milling yields is 1.33 × 106 J (Tempered: Final MC = 16.6%; Milled Rice Yield (MRY) = 72.58%; Head Rice Yield (HRY) = 68.49%. Non-tempered: Final MC: 15.8%; MRY = 70.51%; HRY = 41.16%.)

Further controlled low-temperature drying at 40°C to 50°C is needed to achieve 13.5% w.b. MC for safe storage.

Abstract. Efficient drying methods like radiofrequency (RF) are vital for preserving high-moisture paddy rice quality and maximizing milling yields (Milled Rice Yield [MRY] and Head Rice Yield [HRY], impacting market value and consumer preferences). This study explored the influence of RF exposure parameters and RF drying stages on fissure formation and milling yields in high MC paddy rice kernels. The objectives were to: 1. Investigate the impact of RF exposure parameters (SAR, SARAdjusted, E, EEff, and S) on rice milling yields (MRY and HRY). 2. Evaluate using X-ray imaging the effect of SARAdjusted levels on rice kernel internal structure and its relationship with fissure formation. 3. Using X-ray imaging, evaluate how RF drying steps (RF drying, tempering, and cooling) influence rice kernel internal structure and fissure formation. This study employed a pilot-scale parallel-plate RF heating system (6 kW, 27.12 MHz) with a 105 mm product-to-emitter gap size to achieve one-pass drying of high-moisture long-grain paddy rice (cv. XL-753) from 20.4% wet basis (w.b.) to =13.5% w.b. moisture content (MC) for safe long-term storage. After RF drying, half of the paddy rice samples were tempered at 60°C for 4 h, then conditioned to 13.5% w.b. in an equilibrium MC (EMC) chamber. SAR and S were notably high at 2,224.95 W/kg and 300,000 W/m2 (equivalent to 6,000 W/kg). E was calculated at 46.82 V/m. Higher SARAdjusted levels led to increased temperatures and more significant reductions in MC. Tempering promoted uniform moisture distribution with increased humidity and heating, leading to a slightly higher final MC. Increased SARAdjusted levels induced MC gradients, weakening kernel integrity and inducing fissures, primarily after RF drying and cooling/equilibrating in an EMC chamber. Tempering improved moisture distribution, reduced fissuring, and boosted milling yields. As SARAdjusted rose from 0.80 to 1.87 × 106 J, non-tempered samples saw MRY drop from 71.67% to 60.91% and HRY from 68.04% to 4.53%. Tempered samples followed a similar trend but maintained higher MRY (71.58% to 70.80%) and HRY (69.22% to 47.38%) at the same SARAdjusted levels. The optimal SARAdjusted level for drying high MC rice while preserving milling yields is 1.33 × 106 J with tempering. At this SARAdjusted level, tempered samples achieved a final MC of 16.6%, with MRY at 72.58% and HRY at 68.49%. Non-tempered samples had a final MC of 15.8% w.b., MRY of 70.51%, and HRY of 41.16% (Control MRY = 69.64%; HRY = 66.56%). Further controlled low-temperature drying at 40°C to 50°C is needed to achieve 13.5% w.b. MC for safe storage. Keywords: Cereal drying, Dielectric heating, Drying, Electromagnetic radiation, Grain drying, Milling yields, Radiofrequency, Rice drying, Rice quality.