Enzymatic and Physical Pretreatment Strategies for Alginate Recovery from Saccharina latissima

Brown macroalgae are a promising substrate for alginate production, yet the conventional acid–alkali route raises environmental and quality concerns due to its high reagent inventory, large saline effluents, and partial depolymerization of the polymer backbone. To address these limitations, we evaluated five extraction strategies on a single Saccharina latissima feedstock: conventional acid–alkali extraction as the reference, enzymatic hydrolysis (HE), extrusion combined with enzymatic hydrolysis (Ex-HE), sonication combined with enzymatic hydrolysis (S-HE), and the sequential combination of extrusion, sonication, and enzymatic hydrolysis (Ex-S-HE). The optimized HE process achieved an alginate yield of 34.08% on dry biomass. This value exceeds the conventional benchmark of 31.08%. Hydrolysis time and biomass loading governed the yield. Enzyme dose showed no significant effect within the studied range. Sonication alone yielded 14.55% under surface-limited kinetics, driven exclusively by acoustic amplitude. Scale-up of HE to a 150 L pilot bioreactor recovered 43% of total soluble solids through lyophilization. Ethanol precipitation at the 5 L scale delivered 26.87% of purified alginate. The two metrics describe distinct end products. They represent complementary outputs of a cascade biorefinery rather than competing routes. The HE-derived alginate matches commercial standards by FTIR, TGA, and viscosity. Its M/G ratio is expected to fall within the published range for S. latissima alginate (1.4 to 1.8). Ethanol precipitation of brown algal hydrolysates typically yields products with 80 to 95% uronic acid content. Quantitative techno-economic and environmental analysis indicates substantial reductions in CO2-equivalent emissions and E-factor relative to the conventional route. Total cost becomes competitive when the cascade biorefinery is monetized through co-products such as mannitol, laminarin, and phlorotannins. These results position enzymatic hydrolysis as the most effective single-step strategy for alginate recovery from S. latissima. To our knowledge, this is the first study to couple face-centred central composite optimization of two pretreatment families on a single S. latissima biomass batch with a 30-fold pilot-scale validation from 5 L to 150 L and a comparative functional characterization of two recovery methods (ethanol precipitation and lyophilization).