Comparison of different drying methods on bitter orange (Citrus aurantium L.) peel waste: changes in physical (density and color) and essential oil (yield, composition, antioxidant and antibacterial) properties of powders

Bitter orange peel is acknowledged as a bioactive-rich and valuable residue of citrus processing. However, owing to its high moisture content, bitter orange peel is exceptionally perishable and for the purpose of future utilization its moisture level must be reduced. Drying is among the most common proceedings of moisture diminution, though depending on their type and operating conditions, drying methods could degrade plant heat-sensitive metabolites which would lead to quality degradation. Therefore, the aim of this research was to investigate the effect of nine drying methods (sun, shade, oven 45 degrees C, oven 60 degrees C, vacuum oven 45 degrees C, vacuum oven 60 degrees C, microwave 360 W, microwave 600 W and freeze drying) on the physical (bulk density and color) and essential oil (yield, composition, antioxidant and antibacterial activities) characteristics of bitter orange peel with the intention of determining the superior drying approach. The results showed that regardless of the low amount of bulk density (1.32 g/ml), freeze drying had the highest lightness (75.54), yellowness (77.91), lowest browning index (231.88), highest essential oil yield (6.90%), total phenol content (12.73 mg GAE/100 g), FRAP (0.52% at 80,000 ppm) as well as lowest IC50 (6916.90 ppm), MIC and MBC compared to other treatments. Thus, it could be implied that freeze drying is the most effective drying method in terms of retaining both physical and essential oil properties of bitter orange peel.