Drying Characteristics of Lemon By-product (Citrus limon. v. lunari): Effects of Drying Modes on Quality Attributes Kinetics’

The aim of this work is to establish a comparative investigation of the effect of different drying processes: microwave dehydration (100–600 W), air drying (40–60 °C) and infrared drying (40–60 °C) on drying kinetics and on the main quality attributes (color, total phenols and flavonoids contents, water and oil retention capacities) kinetics’ of lemon (Citrus limon. v. lunari) peels. The experimental drying kinetics’ of lemon peels were fitted to nine thin layer drying models. The Midilli and Kucuk model showed a better fit to experimental drying data for the three drying processes. Microwave drying at 450W preserved the maximum phenols (1.855 ± 0.016 g CAE/100 g DM) and flavonoids (0.802 ± 0.072 g RE/100 g DM) contents respectively. An increase of phenol content was noticed after 9, 13 and 14 min of microwave drying at 600, 450 and 300 W and after 156, 102 and 54 min of infrared drying at 40, 50 and 60 °C. A first-order kinetic model adequately fitted, at the beginning of drying processes, the change of total phenols and flavonoids contents. Total color difference (ΔE) increased progressively with increasing drying time and temperature or microwave power level. ΔE showed a zero-order model whatever the applied drying process. Water retention capacity (WRC) of dried lemon peels decreased during drying processes. However, infrared and microwave drying increased the oil retention capacity (ORC) of lemon peels. The WRC and ORC followed first-order kinetics during air drying and a second order polynomial model during microwave and infrared drying processes.