Green Valorization of Roselle Residues: Integrated Flavonoid Extraction and Bioethanol Production

Roselle (Hibiscus sabdariffa) is cultivated for its health-promoting properties, yet large quantities of calyces are discarded as rlandfills after beverage preparation, resulting in the loss of a bioresource rich in bioactive compounds leading to environmental pollution. This study employed a green biorefinery strategy to valorize both fresh and spent calyces using deep eutectic solvents (DES). Flavonoids were first extracted with various DES systems, and the residual biomass was subjected to pretreatment, enzymatic hydrolysis, and fermentation with Saccharomyces cerevisiae. Fresh calyces treated with ChCl/EG showed the greatest flavonoid recovery (300.63 mg QE/g biomass), achieving nearly threefold higher levels than untreated controls. In contrast, spent calyces consistently produced higher reducing sugar yields (274.38–744.48 mg/g pretreated biomass) than fresh material, likely due to structural modifications during prior processing that improved hydrolysability. Among the tested systems, the sequence of ChCl/EG for flavonoid extraction followed by ChCl/LA pretreatment resulted in the highest sugar release. Subsequent fermentation yielded ethanol concentrations up to 16.48 g/L from spent calyces, demonstrating the effectiveness of this approach. These findings highlight the potential of integrating DES-based extraction and fermentation to recover both flavonoids and bioethanol, providing a sustainable pathway for converting Roselle residues into value-added products.