Effect of low-pressure superheated steam drying on moisture characteristics, mechanical properties, and microstructure of paddy rice

Abstract

Low-pressure superheated steam (LPSS) drying of laboratory-rewetted japonica paddy rice (Suijing 18) was investigated using 25 temperature-pressure runs. Five absolute pressures (0.010–0.030 MPa), five temperatures (50–70 °C), and six sampling times produced 150 observations. Moisture content ranged from 8.92% to 20.81% w.b. The quadratic moisture model gave R 2 = 0.919, while grouped cross-validation gave R 2 = 0.892, RMSE = 0.939 percentage points, and MAE = 0.762 percentage points. Drying time and temperature were the main factors affecting moisture removal, whereas pressure had a smaller effect. From 1,499 valid single-grain force–displacement curves, F 0.50 ranged from 23.75 to 67.31 N. Its model gave R 2 = 0.691, and grouped cross-validation gave R 2 = 0.567, RMSE = 6.51 N, and MAE = 4.99 N. The proportion of grains with an identifiable first fracture increased from 16.7% at moisture ≥19% to 91.8% at moisture <11%. Drying at 60 °C and 0.015 MPa for 20 min produced 14.26 ± 0.22% w.b. and F 0.50 of 46.87 ± 4.11 N. SEM images showed qualitative microstructural differences among LPSS-, hot-air-, and shade-dried samples at similar final moisture contents.