Impact of Plasma-Activated Water Pretreatment and Hot-Air Drying on Bioactive Compounds, Drying Kinetics, Structural Characteristics, and Functional Properties of Debittered Sweet Orange Peel Powder

Abstract

Sweet orange peel, a major by-product of juice processing, is recognized as a bioactive-rich material abundant in phenolic compounds, offering substantial potential for valorization and sustainable by-product utilization. This study investigates the effect of plasma-activated water (PAW) pretreatment combined with hot-air tray drying on the drying behavior and quality attributes of debittered sweet orange peel. Drying was conducted under varying temperatures (50°C, 60°C, and 70°C) and air velocities (0.3, 0.6, and 0.9 m/s). The results demonstrated a significant reduction in drying time, from 465 to 225 min in treated samples (PAW-pretreated tray-dried) and from 420 to 165 min in control samples (non-pretreated tray-dried), as the temperature increased from 50°C to 70°C. Among the tested mathematical models, the logarithmic model provided the best fit for describing drying kinetics. The optimal drying condition, identified as 60°C and 0.9 m/s air velocity, resulted in sweet orange peel powder with enhanced phenolic content and improved functional, structural, and physical attributes. Treated samples exhibited significantly reduced oil absorption capacity, water absorption capacity, and swelling capacity compared with control samples. Scanning electron microscopy analysis revealed compact, dense structures in control samples, while treated samples displayed porous structures with visible spacing between particles. Relative crystallinity increased from 18.09% in control samples to 21.45% in treated samples, indicating structural transformation. Fourier-transform infrared spectroscopy confirmed the presence of key functional groups, such as hydroxyl and carbonyl, associated with the peel's bioactive compounds. These findings highlight the synergistic effect of PAW pretreatment and hot-air drying in enhancing the quality and properties of sweet orange peel powder, offering an effective strategy for the valorization of citrus fruit waste.