Verification of rice quality during storage after drying with hot air and radio frequency heating

Abstract

The objectives of the study were to observe the milling and cooking qualities of paddy rice dried with hot air (HA) and hot air and radio frequency heating (HA/RF) at 38 °C and 42 °C, and to observe these qualities after 6 months of storage under average temperature and RH of 28.4 °C and 70%, respectively. During storage, the average moisture content of paddy rice decreased from 13.66% to 11.37% in HA, 13.66% to 11.40% in HA/RF38, and 13.58% to 11.63 in HA/RF42. The milling qualities of rice were affected by the RF heating temperature and storage period. Rice dried with HA/RF38 and HA/RF42 could be safely stored for up to 5 months and 4 months, respectively, with satisfactory fissure percentage and head rice yield. The whiteness index significantly increased from 63.37 ± 0.58 to 64.46 ± 0.75 at HA, 62.78 ± 0.97 to 63.75 ± 1.55 at HA/RF38, and 62.77 ± 1.90 to 64.45 ± 0.95 at HA/RF42 during storage. For cooking qualities, elongation ratio (ER) was affected by RF and storage period, while texture profiles and pasting properties were only affected by storage period. The ER increased from month 3, by 10%, 11%, and 11% in HA, HA/RF38, and HA/RF42. The hardness of cooked rice reached its peak value between months 4–5. However, adhesiveness decreased as the storage period increased. The peak viscosity and setback increased with increasing storage period while breakdown conversely decreased. This study showed that hot air drying and RF heating temperatures of 38 °C could replace hot air drying without undesirable changes in milling and cooking properties during storage up to 5 months. Therefore, this is a promising drying technology for the rice milling industry.