Effect of temperature on drying kinetics and quality attributes of coffee beans during hot air drying

Abstract

This study systematically investigates the thin-layer drying kinetics of coffee (Coffea arabica) cherries during hot air drying, with experiments conducted at four distinct temperatures: 40 °C, 60 °C, 80 °C, and 100 °C. A comprehensive analysis was performed using ten widely recognized mathematical models: Newton, Page, Modified Page, Henderson and Pabis, Logarithmic, Two-Term, Exponential Two-Term, Diffusion Approximation, Midilli, and Singh to model the experimental moisture ratio data and characterize drying behavior. The drying process was characterized by a predominant falling rate period, indicating that internal moisture diffusion governs the kinetics. Among the models evaluated, the Midilli model best fits the experimental data, achieving the highest R2 (0.9952 at 40 °C, 0.9973 at 60 °C, 0.9994 at 80 °C, and 0.9995 at 100 °C) and the lowest RMSE values (0.0793% at 40 °C, 0.2092% at 60 °C, 0.0624% at 80 °C, and 0.0868% at 100 °C). Furthermore, the effective moisture diffusivity increased significantly with temperature, ranging from 1.154 × 10⁻⁷ m²s-1 at 40 °C to 8.939 × 10⁻⁷ m²s-1 at 100 °C. The corresponding activation energy for the drying process was calculated as 32.36 kJ mol-1. The L* value (lightness) decreased consistently as the temperature increased. The findings of this study advance the understanding of coffee cherry drying kinetics under hot air conditions and offer practical insights for optimizing drying parameters. This research contributes a robust framework for designing and controlling industrial drying processes, ultimately enhancing product quality and energy efficiency in coffee production. The implications of these results are significant for scaling up operations and reducing energy consumption in drying systems.