Pelagic Sargassum as a sustainable source of food-grade alginate: selective functional modulation by high-pressure processing and sonication

Pelagic Sargassum spp. represent an abundant and underutilized biomass, and converting this material into functional sodium alginate offers a sustainable pathway for its industrial valorization and food-hydrocolloid applications. This study evaluated the impacts of thermal (autoclave) and non-thermal (high-pressure processing (HPP), sonication) pretreatments on the structural and functional properties of sodium alginate extracted under optimized alkaline conditions (80 °C, 5 h, 0.3 M Na 2 CO 3 concentration). To our knowledge, this is the first systematic comparison of thermal and non-thermal pretreatments on the physicochemical and functional characteristics of sodium alginate from pelagic Sargassum . Physicochemical properties (yield, molecular weight, viscosity, and monomer composition), structural characterization by Fourier Transform Infrared (FTIR) spectroscopy and proton nuclear magnetic resonance ( 1 H NMR), and functional properties (gel strength and emulsification index) were analyzed to elucidate structure-function relationships. Autoclave pretreatment caused pronounced degradation, resulting in reduced molecular weight (< 15 kDa), intrinsic viscosity (< 0.35 dL/g), gel strength (77 g), and emulsification capacity (5-6%). In contrast, the non-thermal pretreatments selectively retained functional properties: sonication (10 min) produced the highest gel strength among treated samples (174 g), whereas HPP (400 MPa) produced the highest emulsification index (17.2%). The control exhibited the highest molecular integrity, and similar FTIR and 1 H NMR spectra across samples indicated that the alginate backbone remained chemically stable despite partial depolymerization. Overall, the selective improvements observed with HPP and sonication demonstrate that tailored pretreatments can modulate alginate functionality without compromising chemical integrity. These findings highlight a promising strategy to valorize pelagic Sargassum by producing functional alginate suitable for clean-label hydrocolloid applications.