Effects of Multi-Frequency Ultrasound-Enhanced Subcritical Water on the Extraction of Phenolic Compounds from Rapeseed Meal

Conventional extraction methods for rapeseed meal polyphenols are limited by low yields and co-extraction of anti-nutritional factors. This study systematically evaluated the effects of subcritical water extraction (SWE), single-, dual-, and triple-frequency ultrasound-assisted extraction (SUE, DUE, TUE), and their combinations (SUSWE, DUSWE, TUSWE) on dry-extract yield, the composition and in vitro antioxidant capacity of the purified extracts, and the microstructure of the solid residues. Among all methods, DUSWE demonstrated superior performance. DUSWE achieved the highest dry-extract yield (4.60%), a 33.54% increase relative to conventional hot water extraction (HWE). Compared with the purified HWE extract, the purified DUSWE extract showed significantly higher total phenolic, total flavonoid, and anthocyanin contents but lower measured phytic acid and glucosinolate contents (p < 0.05). Scanning electron microscopy (SEM) showed DUSWE induced a porous microstructure favoring polyphenol release. Furthermore, it exhibited markedly higher in vitro antioxidant capacity. In summary, dual-frequency ultrasound (20/40 kHz) combined with subcritical water extraction significantly improves the yield and in vitro antioxidant capacity of rapeseed polyphenols, offering an effective strategy for high-value utilization of rapeseed meal.