Development of industrial scale continuous flow drying machine using radio frequency and hot air for Paddy rice

Abstract

Paddy rice cultivation plays an important role in the Thai economy. It is usually harvested at a moisture content of 18-28% (w.b.) and immediately dried to below 14% for safe storage. Paddy is dried with hot air (HA) at a temperature of (50 °C -80 °C) to achieve the shortest drying time. RF heat treatment, as a type of dielectric heating method. involves the generation of heat as a result of the interaction between an electromagnetic field and polarized molecules in the crops. The effect of various RF heating temperatures combined with hot air (HA/RF) (38, 42. 45, 46, 50, 55, and 60 °C) in an industrial-scale process compared with hot air drying (HA) was studied. The drying characteristics and the moisture diffusivity showed that the higher the combined RF heating temperature, the shorter the observed drying time. The specific energy consumption and energy cost decreased when the RF heating temperature increased. Although the HA/RF60 had the shortest drying time, the fissure percentage and milling quality data showed that the optimum conditions were HA/RF42. In addition. there were no significant differences in head rice yield and white rice color determination. amylose content, texture profiles, pasting properties, and sensory difference tests compared to HA. This study proposes the successful development of industrial-scale continuous flow drying machine using RF and HA for paddy rice and shows the great potential of RF technology for industrial-scale drying for the rice industry. Afterwards, the changes in milling and cooking qualities of milled rice after 2 months of drying by hot air combined with radio frequency heating at low temperatures (38 °C, 42 °C, 46 °C, and 50 °C) under ambient conditions were compared with hot air drying (HA). The fissures in white rice kernels significantly increased with an increase in RF heating temperature and storage period starting from month 1. The HRY of milled rice was without adverse effect after 2-months of storage. All color values were visualized as acceptable values. There was no change in the AC after storing. Regarding pasting properties, the rising of PV, BD, FV and reduction of SB and PT in paddy drying with hot air combined with RF heating beneficially increased the elongation ratio of cooked rice while the texture properties of cooked rice were not altered from fresh rice. Overall, it was proposed that the milling and cooking qualities of milled rice after being stored for 2 months are still in accordance with the qualities of fresh rice and rice dried by HA. Lastly, the financial analysis was studied. The NPV of HA and HA/RF42 on a commercial scale 5 ton/batch) showed positive values of 2,559, 131.11 and 3,830,801.49 Bahts, respectively. The NPV for industrial scale dying (450 kg batch) was negative because the amount of paddy rice to be dried in the dryer was too small. The IRR of the project after 10 years of HA and HA/RF42 at commercial scale had IRRs of 27.01% and 22.78%, respectively. The BCR of HA and HA/RF42 at commercial scale were 1.31, and 1.31, respectively. The PB of HA and HA/RF42 on a commercial scale was 3.36 and 3.83 years, respectively. It can be concluded that commercial-scale drying processes for paddy rice using hot air and hot air combined with radio frequency heating at 42°C are worth investing in.