Ethanol-based ultrasonic extraction of Vietnamese macroalgae bioactive compounds: a green approach with optimized conditions and antimicrobial effects

Underexploited Vietnamese macroalgae could provide clean-label antioxidant and antimicrobial ingredients for food systems, yet selective and scalable extraction remains challenging. Here, probe ultrasonic-assisted extraction (UAE-H) using aqueous ethanol was benchmarked against soaking, Soxhlet and ultrasonic bath extraction for Porphyra vietnamensis. UAE-H delivered the highest phenolic recovery and antioxidant responses and was subsequently optimized using response surface methodology. The optimal conditions (56% ethanol, 10 min, 1:30 g/mL) produced 11.96 mg GAE/g DW total phenolics while suppressing carbohydrate co-extraction, indicating improved operational selectivity toward phenolics. The optimized extract exhibited broad-spectrum antibacterial activity against common food-borne bacteria (Staphylococcus aureus, Enterococcus faecalis, Escherichia coli and Salmonella typhi) with MIC values of 0.197–0.395 mg/mL. HPLC-DAD-ESI-MS profiling showed a dominant phenolic peak putatively annotated as a methylated A-type proanthocyanidin dimer (m/z 589.68) based on UV characteristics and diagnostic fragmentation patterns. Overall, UAE-H with food-grade ethanol provides a rapid and reduced-solvent route to valorize P. vietnamensis into antioxidant/antimicrobial fractions with potential for clean-label food and nutraceutical applications.