Hydrothermal processing of agar waste to levulinic acid and fermentation of hydrolysate to bioethanol

Increasing global energy consumption and depleting fossil-fuel reserves prompted the search for green alternatives. This study focuses on conversion of waste agar using different acids/alkalis (0.5% and 1%) as catalysts under varied temperature and time towards galactose (Gal), 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) production in sequential reactions. The optimized process for agar depolymerisation was achieved using 1% acid (H₂SO₄/HCl) catalysed conditions with a maximum of 11 g/L Gal yield (121 °C; 15 min). Increase in temperature (150 °C) and time (180 min) with 1% HCl/H₂SO₄ catalyst resulted in improved LA production along with Gal and HMF. The hydrolysis process was optimised for the selective production of LA (10 g/L at 175 °C; 180 min). Further, galactose-rich hydrolysates were assessed for bioethanol production using Saccharomyces cerevisiae that resulted in 3 g/L ethanol. Thus, the study comprehensively demonstrates waste agar utilization to yield biochemicals/fuels in a circular bio-based economy approach.