Beyond pure subcritical water: A critical review of synergistic technologies for greener and more efficient...

This review critically examines hybrid subcritical water extraction (SWE) systems that synergistically integrate auxiliary energy-based (ultrasound, microwave), solvent-modified (co-solvents, natural deep eutectic solvents (NADES)), and physical/biological (pulsed electric fields (PEF); enzymes) technologies to enhance the recovery of bioactive compounds. It analyzes the underlying mechanisms and comparative performance, demonstrating that these hybrid configurations significantly improve extraction kinetics, yield, and selectivity while operating at lower temperatures to preserve labile phytochemicals. For instance, ultrasound-enhanced subcritical water extraction achieved a maximum polysaccharide yield of 17.34% from Lentinus edodes, while microwave-assisted subcritical water extraction produced total phenolic yields of 211.73 mg GAE/g extract from blueberry pomace. Furthermore, ultrasound-assisted systems exhibit higher asymptotic extraction capacities (S_∞) and rate constants (k) than SWE alone, indicating enhanced mass transfer efficiency. The analysis identifies key scalability challenges related to energy consumption and industrial reactor design. Notably, pilot-scale hybrid SWE systems remain scarcely documented in the literature: while Trigueros et al. successfully scaled subcritical water extraction of red algae residue from 0.5 L laboratory to 5 L pilot reactors with comparable yields (72-93% for various biopolymers), dedicated pilot-scale hybrid configurations combining SWE with ultrasound, microwave, or PEF have not yet been reported. This absence of mature, standardized hybrid SWE equipment at industrially relevant scales constitutes a major knowledge gap. Future pathways are outlined, emphasizing the need for intelligent process control, advanced kinetic modeling, and rigorous techno-economic and life-cycle assessments to validate sustainable, industrial-scale green extraction processes.