Changes in chemical composition and biological activity of essential oil from Thomson navel orange (Citrus sinensis L. Osbeck) peel under freezing, convective, vacuum, and microwave drying methods

Thomson navel orange peel is a by-product of citrus processing, which contains high levels of bioactive compounds advantageous to human health, nevertheless due to its high moisture content it is exceedingly perishable. Drying is among the most common preservation methods, which could prolong the plants shelf-life via reducing their moisture value. Taking this into account, depending on their type and conditions, drying techniques could degrade plant heat-sensitive metabolites and lead to quality decline. Therefore, the goal of this paper was to investigate the influence of seven drying methods named sun, shade, oven, vacuum oven, microwave, and freeze-drying with different drying conditions on the physical properties, for example, bulk density and color (L*, a*, b*, ΔE, and browning index (BI)) and essential oil characteristics such as extraction yield, chemical composition, antioxidant (total phenolic content (TPC), DPPH, and FRAP essays), and antimicrobial (MIC and MBC) activities of Thomson peel and determine the superior drying procedure. Results showed that freeze-dried sample had the highest retention of L* (48.54) and b* (49.00) values, lowest BI (216.11) as well as highest EO extraction yield (6.90%), TPC (60.10 GAE/100 g), FRAP (0.52% at 80 mg/ml), and lowest IC50 (5.00 mg/ml), MIC and MBC compared with other drying treatments. Therefore, it could be inferred that freeze-drying is the most efficient drying approach in respect of preserving both physical and EO attributes of Thomson peel