Impact of ultrasound-assisted germination and varied drying methods on phenolic biosynthesis, antioxidant capacity, and oxidative enzyme activity in selenium-biofortified black rice

Germinated black rice (GBR) is a nutrition-enriched product with growing consumer interest; however, drying is necessary for shelf life, which may compromise its quality. This study evaluated the effect of ultrasound (US) pretreatment combined with hot-air drying (HAD), infrared vacuum drying (IVD), and freeze-drying (FD) on the drying kinetics, phytochemicals, microstructure, and volatile composition of selenium-enriched GBR. Ultrasound significantly reduced drying time by 16.7 % in HAD and 14.8 % in IVD, with the Hii model providing the best fit for the drying kinetics (R²,1). Ultrasound-assisted IVD improved effective moisture diffusivity (8.11 × 10⁻⁷ m²/s) and lowered energy consumption (12.91 kWh/kg). Phytochemical retention was enhanced, with total phenolic content increased by 28-36 % and total flavonoid content by 15-24 % under HAD and IVD, while FD better preserved individual phenolic acids. Anthocyanin retention was highest in US-IVD samples (cyanidin-3-glucoside: 3075.64 µg/g), whereas carotenoids peaked in IVD without ultrasound. SEM confirmed the presence of microchannels and porosity in US-treated grains, supporting improved mass transfer. Antioxidant activity (DPPH: 1.97 mM TE/g; FRAP: 1.90 mM TE/g) was elevated in US-HAD and US-IVD, while GC-MS showed enrichment of alcohols such as 3-heptanol and suppression of aldehydes, including 2-methyl-2-pentenal. Collectively, ultrasound-assisted HAD and IVD improved drying efficiency, phytochemical stability, and aroma quality of GBR.