Quality of Milled Rice from Large-Scale Dried Paddy Rice by Hot Air Combined with Radio Frequency Heating

Abstract

A scaled-up process for paddy drying was developed using hot air (HA) combined with radio frequency (RF) heating. The study was conducted using hot air (control treatment) arranged in descending order in four temperature levels, namely 80 °C at moisture content of 25–26%, 70 °C at moisture content of 20–25%, 60 °C at moisture content of 17–20%, and 50 °C at moisture content of 13–17%, as well as with hot air combined with radio frequency (HA/RF) at different paddy temperatures (45–60 °C) by adjusting the appropriate RF energy when passing through RF heating chamber, namely HA/RF45, HA/RF50, HA/RF55, and HA/RF60. Each treatment was performed in three replicates and data were statistically analyzed in a randomized complete block design. The quality attributes of paddies affected by the drying process were assessed: fissure percentage, color, milling quality, and sensory evaluation. The drying efficiency showed that the drying time and the specific energy consumption could be decreased by up to 54.44% and 23.17% at HA/RF60 and HA/RF45, respectively. As the RF heating temperature increased, the fissure percentage of brown rice kernels at HA/RF45 and HA was not significantly impacted. Regarding color evaluation, combining RF heating and convective drying at all given conditions could be statistically applied in terms of the b*, WI, and ΔE* value. Considering the milling yield of HA as the baseline, head rice yield was maximized at HA/RF45, while bran yield reached the maximum at HA/RF60. The liking score of cooked rice after it was dried using the HA method was the highest. This study concludes that the HA/RF45 was the most appropriate drying condition, and this may provide preliminary exposure to the industrial drying of paddies.

Effect of Large-Scale Paddy Rice Drying Process Using Hot Air Combined with Radio Frequency Heating on Milling and Cooking Qualities of Milled Rice

“…In this study, it was proposed that HA/RF42 was the optimal condition for using a continuous flow hot air dryer in combination with radio frequency heating in a large-scale process. Compared to the previous study [ 12 ], there are three advantages of this significant improvement: (1) The fissure percentage on HA/RF42 was lower than on HA/RF45. The fissure percentage was the first parameter for a rice miller to determine the efficiency of the dryer.…”

“…The optimum condition for RF heating temperature was HA/RF42, as no “Moderate” and “Severe” fissure degrees were observed. Referring to the earlier study by Chitsuthipakorn and Thanapornpoonpong [ 12 ], drying paddy with hot air combined with RF heating temperatures of 45 °C, 50 °C, 55 °C, and 60 °C showed that the critical limit of RF heating temperature was HA/RF45 because the least “Moderate” and “Severe” fissure degrees were observed. Moreover, the fissure percentage result was relatively lower in this study because of the modification of the prototype machine, as mentioned in the Introduction.…”

“…In this study, there was no significant difference in HRY, which could mean that all RF heating temperatures were suitable for large-scale paddy drying. With reference to the previous study by Chitsuthipakorn and Thanapornpoonpong [ 12 ], drying paddy with hot air combined with RF heating temperatures of 45 °C, 50 °C, 55 °C, and 60 °C showed that the critical limit of RF heating temperature was HA/RF50 in terms of HRY and the optimum condition was HA/RF45 in terms of HRY, color, and fissure percentage. Therefore, the aim of this study was to achieve the most suitable drying conditions by combining hot air and RF heating through a lower RF heating temperature and a narrower temperature level.…”

“…Based on the vertical operating prototype that supported an RF heating system by Vearasilp et al [ 10 ] and the earlier study by Chitsuthipakorn and Thanapornpoonpong [ 12 ], the electrical voltage control of the RF heating machine was set manually, which proved to be a disadvantage for the authors. There were two different ways to solve this problem: (1) installing an automatic system with a vibration feeder to control the paddy release rate from the RF heating machine to control the paddy temperature in the RF heating chamber, and (2) installing an automatic system to control the electrical voltage to control the RF heating energy for the paddy.…”

Application of emerging thermal and nonthermal technologies for improving textural properties of food grains: A critical review

“…This difference may have been caused by the melting or gelatinization of starch during RF heating (Xu et al., 2023). A recent study reported that cooked rice becomes harder and less sticky after prolonged storage because of enzymatic and nonenzymatic reactions that weaken the cell wall structure, allowing the formation of protein‐rich regions owing to the higher resistance to water absorption; hence, aged rice cooked with less water showed a harder texture (Chitsuthipakorn & Thanapornpoonpong, 2023). The textural characteristics of dried food products affect their taste.…”