Effects of Different Drying Techniques on Bioactive Compounds and Functional Properties of SCOBY-Fermented Pomelo Substrate Powders

Drying is a critical post-fermentation process because it influences product stability and the retention of bioactive compounds. The present study evaluated the effects of different drying techniques on the physicochemical characteristics, functional properties, bioactive compounds, and antioxidant activities of SCOBY-fermented pomelo peel substrate powders. Pomelo peel substrates fermented with 6% (w/w) SCOBY inoculum for 25 days were subjected to freeze drying (FD), hot-air drying (HAD; 50, 70, and 90 °C), and radio-frequency drying (RFD; electrode distances of 14, 15, and 16 cm). Drying kinetics, effective moisture diffusivity (Deff), water activity, color, particle size distribution, functional properties, total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activities were determined. RFD showed comparable or slightly higher moisture diffusivity (1.19–2.06 × 10−9 m2/s) compared with HAD (1.03–1.95 × 10−9 m2/s) under suitable drying conditions, suggesting that radio-frequency heating effectively promoted internal moisture migration through volumetric dielectric heating. FD retained the highest antioxidant activity, with DPPH radical scavenging activity of 74.25% and TEAC of 24.85 μmol TE/g. However, moderate thermal treatments enhanced phenolic extractability, and HAD at 50 °C showed the highest TPC (161.65 mg gallic acid equivalents (GAE)/g DW). Among the RFD treatments, RFD at 15 cm exhibited the highest TFC (27.18 mg rutin equivalents (RE)/g DW) and maintained relatively high antioxidant capacity. FD powders showed superior water solubility and swelling capacity, whereas RFD produced finer particle distributions and improved drying efficiency. These findings demonstrate that drying techniques significantly influence the quality attributes of SCOBY-fermented pomelo substrate powders, and RFD represents a promising alternative drying technology for balancing drying efficiency and bioactive compound retention.