Hot-Air Thermal Pre-Treatment as a Rejuvenation Strategy for Long-Stored Low-Amylose Aromatic Rice: Effects on Drying Kinetics, Microstructure, and Cooked-Rice Texture

Abstract

This study examined whether hot-air thermal pre-treatment can rejuvenate 12-month-aged Khao Dawk Mali 105 (KDML 105) rice by inducing surface cracks that promote water uptake. Aged rice was dried at temperatures of 40, 60, 80, 100 and 120 °C to target moisture contents of 10%, 8%, 6% and 4% wet basis (w.b.), giving 16 combinations, with drying times predicted from thin-layer drying models. The drying kinetics, crack morphology, water absorption, color and cooked-rice texture were compared with those of fresh rice (stored for 2 months) and untreated aged rice. The Two-term and Page models described drying accurately (R2 > 0.982, RMSE < 0.039). Light-transmission imaging revealed a surface crack network in all pre-treated samples. Water absorption followed the Peleg model, with the initial absorption rate (1/K1) rising from 0.026%/s in untreated aged rice to 0.099–0.198%/s, and the effective water diffusivity (Deff) from 0.63 to 1.04–1.35 × 10−10 m2/s. Drying to 10% w.b. reduced the hardness from 25.09 to 16.28–18.53 N, comparable to fresh rice (16.70 N), whereas 4% w.b. increased it again (23.27–25.11 N); stickiness was not restored. Multi-objective optimization identified 10% w.b. as the determining factor at every drying temperature, with T40–M10 scoring the highest (CI = 0.509; relative CI = +0.118). Terminating drying at a target moisture content of 10% w.b., rather than the drying temperature applied, was the primary determinant of successful textural rejuvenation, whereas stickiness remained unrestored under every condition tested.