Rapid organic acid mechanochemical extraction of alginate from Laminaria digitata

Reducing process time and energy demand in alginate extractions is critical for the sustainable valorization of brown macroalgae. Blade-based systems are simple scalable methods widely used to enhance extraction efficiency, however, their application in alginate extraction remains underexplored. This study evaluated the effects of mechanical blade disruption (200 - 2000 revolutions per minute) and pretreatment duration (2 - 120 min) during an organic citric acid pretreatment, with the aim of shortening processing time. Yield, intrinsic viscosity, M/G ratio, and impurities (ash, acid-insoluble ash, protein) were quantified, and block structure was assessed by ¹H NMR spectroscopy. Crude alginate yield was time-dependent, peaking between 10 and 30 min, with no significant effect of blade speed. Reducing pretreatment time to 6 min did not affect yield, M/G ratio (1.10-1.17), or block structure. In contrast, higher disruption at 2000 revolutions per minute significantly reduced ash (25.83 ± 1.42% to 12.39 ± 3.97%) and protein (7.55 ± 0.16 to 4.25 ± 0.23 mg/g), thereby improving extract purity. Mechanical input influenced rheological properties, with intrinsic viscosity and viscosity-average molar mass varying significantly with speed and time. At 200 revolutions per minute, the highest molar mass was obtained at 120 min (649.35 ± 12.75 kDa), whereas at 2000 revolutions per minute, a peak occurred earlier (20 min) followed by degradation, indicating depolymerization. These findings demonstrate that extraction is dependent on time, while blade-induced disruption only influences purity and molecular properties. This concept can be applied industrially, enabling a low-energy, simple, scalable extraction design for macroalgal biorefinery processing.