Thin-Layer Drying Kinetics and Desiccant Phase Evolution in Sorption Drying of Barley with Granulated Kieserite

Sorption drying removes water from seeds at ambient temperature by contact with a granular hygroscopic solid, thereby avoiding thermal stress. We studied the drying kinetics of barley (Hordeum vulgare L.) using granulated kieserite MgSO4·H2O and characterised the desiccant by SEM, XRD, and DSC–TG. Barley wetted to 19–20 wt % moisture was mixed with the desiccant at mass ratios of 1:2, 1:4, and 1:6 and sampled over 8 h. Ten thin-layer equations were fitted to moisture-ratio data. The Page equation gave the best fit after penalising the number of parameters (R2 = 0.988–0.998, reduced χ2 = 6 × 10−5–3 × 10−4); its exponent was 0.48–0.60 in all series, which describes the shape of the curve as a stretched exponential; being an empirical equation, the Page model is not by itself evidence of a transport mechanism. Apparent moisture diffusivity, obtained with the spherical solution and reported as a lumped coefficient rather than an intrinsic property of the endosperm, ranged from 6.2 to 8.4 × 10−12 m2 s−1. The grain reached the medium-dry category in 6–8 h at ratios of 1:2 and 1:4. Three consecutive batches were dried on the same desiccant without regeneration, removing 101, 92, and 85 mg of water per gram of desiccant. The hydration number rose from 1.09 to 2.81 and the water capacity from 163 to 420 mg per gram of anhydrous salt, as kieserite progressively converted into hexahydrite. The adsorbed water was released below 116 °C. Each ratio was run as a single kinetic series; repeatability was checked at the 1:2 ratio from three independent runs, which agreed to within 3.6% in moisture ratio.