A Closed-Loop Biorefinery for Carbohydrate-Rich Macroalgae Gelidium amansii Using Lactic Acid as a Recyclable Pretreatment Catalyst

Abstract Mineral acids (e.g., H2SO4) are commonly employed as catalysts for macroalgae pretreatment but suffer from severe equipment corrosion and poor catalyst recovery and reuse. In this study, a feasible biorefinery strategy for red macroalgae Gelidium amansii was established using lactic acid, a weak organic acid, as the pretreatment catalyst, which features low reactor corrosion and effective catalyst recyclability. The pretreatment conditions were systematically optimized (1% lactic acid, 180 °C, 50 min), achieving overall recovery yields of 80.20% for galactan and 76.25% for glucan. However, the resulting prehydrolysate contained up to 12 g/L of the inhibitor 5-hydroxymethylfurfural (HMF), which significantly suppressed fermentation by Pediococcus acidilactici. To overcome this, a three-stage adaptive evolution was performed on the strain, enabling it to efficiently ferment the prehydrolysate and produce 39.50 g/L lactic acid, with a yield of 0.90 g/g reducing sugars. Corrosion evaluation demonstrated that lactic acid pretreatment caused less damage to the stainless-steel reactor than sulfuric acid. Moreover, the crude lactic acid recovered from fermentation broth was successfully reused for one subsequent pretreatment cycle, exhibiting pretreatment efficiency comparable to that of fresh lactic acid. This study provides a low-corrosion and catalyst-recyclable pretreatment approach for macroalgae biorefinery.