Experimental investigation and modeling of date drying under forced convection solar dryers

This paper presents the drying behavior of fresh dates at three ripeness stages (khalal, rutab, and tamr) in three drying techniques, namely, direct sun drying (DSD), greenhouse-like solar drying (GSD), and indirect convective solar drying (ISD) and validated with drying models. The experimental data was fitted to 11 thin-layer drying models to identify the best model to describe the drying behavior of the dates in the three solar drying methods. The results showed that using DSD, the safe moisture level of 35% on dry basis was reached in 86 h for khalal, 103 h for rutab, and 103 h for tamr. However, GSD has the faster drying rate, reaching the final moisture content in 53 h for khalal, 64 h for rutab, and 61 h for tamr, compared to 75 h, 70 h, and 70 h in ISD for the respective stages of ripeness. The Midilli and Kucuk, Diffusion approach, Logarithmic, Two-term, and Verma models were found to be the most suitable for representing the drying process of fresh dates.