Conventional solvents remain the most used media for lignocellulosic biomass valorization. However, these solvents exhibit many limitations and have a negative environmental impact. In the last decade, Deep Eutectic Solvents (DESs) have emerged as a multifaceted tool in biomass valorization, with a promising perspective in the application of lignocellulosic biomass valorization. DESs have gained attention in the last decade as an alternative solvent in biomass valorization and biorefinery processes due to their high efficiency; eco-friendliness; low cost; and numerous other advantages, such as recyclability, non-volatility, and stability. This paper discusses the latest research on the potential applications of DESs in the valorization of secondary lignocellulosic biomass.

This study investigates the potential of sugar beet pulp (SBP), a lignocellulosic by-product of sugar production, as a low-cost substrate for biohydrogen and biomass generation using Escherichia coli under dark fermentation conditions. Two strains—BW25113 wild-type and a genetically engineered septuple mutant—were employed. SBP was pretreated via thermochemical hydrolysis, and the effects of substrate concentration, dilution, and glycerol supplementation were evaluated. Hydrogen production was highly dependent on substrate dilution and nutrient balance. The septuple mutant achieved the highest H₂ yield in 30 g L⁻¹ SBP hydrolysate (0.75% sulfuric acid) at 5× dilution with glycerol, reaching 12.06 mmol H₂ (g sugar)⁻¹ and 0.28 mmol H₂ (g waste)⁻¹, while the wild type under the same conditions yielded 3.78 mmol H₂ (g sugar)⁻¹ and 0.25 mmol H₂ (g waste)⁻¹. In contrast, undiluted hydrolysates favored biomass accumulation over H₂ production, with the highest biomass yield (0.3 g CDW L⁻¹) obtained using the septuple mutant in 30 g L⁻¹ SBP hydrolysate without glycerol. These findings highlight the potential of genetically optimized E. coli and optimized hydrolysate conditions to enhance the valorization of agro-industrial waste, supporting future advances in sustainable hydrogen bioeconomy and integrated waste biorefineries.