Optimization, Chemical Profiling, and Bioactivity of Essential Oils from Citrus Peels Subjected to Different Drying Processes

Abstract

Citrus processing generates large quantities of peel by-products that represent an abundant and renewable source of essential oils and other bioactive compounds. However, drying, a necessary pretreatment for storage and extraction, can significantly affect essential oil yield, chemical characteristics, and biological properties. This study aimed to investigate the effects of different drying methods and operating conditions on the extraction yield, physicochemical properties, and biological activities of essential oils obtained from peels of three Tunisian citrus species: Citrus sinensis (E1), Citrus limon (E2), and Citrus aurantium (E3). Physicochemical and thermophysical properties of the peels, including moisture content, porosity, and volumetric shrinkage, were first determined to evaluate their influence on drying behavior and oil extractability. Drying kinetics were then investigated using infrared, forced-convection, and open-air drying methods. A full factorial experimental design was applied to optimize the drying process by evaluating the effects of temperature, drying time, and air velocity on essential oil yield. The results showed that the three citrus species exhibited distinct physicochemical characteristics that influenced drying performance and extraction efficiency. Essential oil yield increased significantly with drying temperature, time, and air velocity, and the optimum conditions were identified as 70°C, 180 min, and 2 m s -1 . Biological evaluation revealed that essential oils extracted from fresh Citrus limon peels exhibited the strongest and broadest antimicrobial activity, whereas all drying treatments caused a noticeable reduction in antimicrobial effectiveness. Similarly, antioxidant activity, expressed as IC₅₀ values, was highest in fresh samples and progressively decreased after open-air, convective, and infrared drying. Overall, the findings demonstrate that Tunisian citrus peels constitute a valuable source of bioactive essential oils and highlight the importance of selecting appropriate drying conditions to maximize extraction yield while minimizing losses in biological activity. These results provide useful guidance for the sustainable valorization of citrus processing by-products.