Optimizing Radiofrequency Exposure Parameters for One-Pass Drying of High-Moisture Paddy Rice

Abstract

Highlights The utilization of standardized Radiofrequency (RF) exposure parameters ensures safety and process consistency. Equations were used to estimate Power Density (S), Specific Absorption Rate (SAR), and Electric Field Intensity (E). One-pass RF drying effectively lowered paddy rice (Cultivar [cv.] XL-753) initial Moisture Content (MC; 20.4% w.b.) to safe levels (=13.5% w.b.) with SARAdjusted of 1.87 × 106 J, E of 46.82 V/m, and S of 300,000 W/m² (6,000 W/kg). Higher SARAdjusted values impacted paddy rice surface lipid content and milling yields, while minor changes in color and viscosity showed inconsistent and subtle practical effects. Abstract. Radiofrequency (RF) heating utilizes high-frequency electromagnetic waves to induce heat within materials via molecular friction. This method offers rapid, even heating, deep penetration, and energy efficiency. Nevertheless, there is a dearth of research on RF exposure parameters concerning food processing. Investigating these parameters is crucial for RF process optimization, setting safety guidelines, and guaranteeing product quality. This research clarifies RF exposure parameters through the one-pass drying of high moisture content (MC) long-grain paddy rice (cultivar [cv.] XL-753) at an initial MC of 20.4% wet basis (w.b.) to a final of MC of = 13.5% w.b. (safe MC for storage). The specific objectives of this research were to: 1. Advocate the utilization of standardized RF exposure parameters (Specific Absorption Rate, SAR; Electric Field Intensity, E; and Power Density, S) of food and agricultural products, aiming to simplify RF processing, fostering consistency and safety in operations. 2. Estimate the RF exposure parameters associated with the one-pass RF drying process. 3. Determine the RF exposure parameters’ influence on rice physicochemical attributes of Milled Rice Yield (MRY), Head Rice Yield (HRY), Color Parameters (L* a* b*), Surface Lipid Content (SLC), and Pasting Properties. Using a pilot-scale parallel-plate RF heating system (6 kW, 27.12 MHz) with a 105 mm product-to-emitter gap, RF exposure parameters were determined: E = 46.82 V/m; EEff = 33.11 V/m; S = 300,000 W/m²; SAR = 2,224.95 W/kg. Rice samples underwent RF exposure durations of 360, 600, and 840 s, resulting in SARAdjusted levels of 0.80 × 106, 1.33 × 106, and 1.87 × 106 J, respectively. Half of the RF-processed paddy rice underwent tempering, which involved immediately transferring paddy rice after RF treatments into sealed glass jars and then into an incubator set to a constant temperature of 60°C for a duration of 4 h. RF processed samples at the highest SARAdjusted tested (1.87 × 106 J) followed by tempering resulted in MC reductions of 6.7% pt. w.b., final MC of 14.0% w.b., MRY, HRY, and SLC of 70.80%, 47.38%, and 0.47%, respectively. Non-tempered samples at the same SARAdjusted level had MC reductions of 7.34% pt. w.b., final MC of 13.1% w.b., MRY, HRY, and SLC of 60.91%, 4.53%, and 0.43%. Tempering and SARAdjusted values slightly affected rice physicochemical characteristics of color and viscosity. However, the differences were minor and inconsistent. To ensure optimal quality in large-scale RF drying of rice, it is crucial to avoid SARAdjusted levels exceeding 1.33 × 106 J. Exceeding this threshold has been shown to cause overheating, adversely impacting the grain’s quality. Moreover, implementing a tempering process post-drying is also essential in mitigating these detrimental effects. Keywords: Color, Electric field intensity (E), Lipids, Milling yields, Power density (S), Radio-frequency (RF), Rice drying, Specific absorption rate (SAR), Viscosity.