Deep eutectic solvent (DES) assisted optimization and characterisation of lignin extraction from apple pomace: A...

Apple waste valorization provides a sustainable pathway to unlock the hidden resources present in the apple processing industry waste. The present research focused on the deep eutectic solvent (DES) pretreatment of apple pomace (AP) for the simultaneous production of cellulose-enriched residue and high-purity lignin. The efficiency of the pre-treatment was systematically evaluated by comparing a synthetic DES (ChCl: CA) (choline chloride and citric acid) with a new, bio-derived DES at 60 °C and 100 °C. To break down the lignin carbohydrate complex (LCC), effective lignin recovery was observed at 100 °C. Synthetic ChCl: CA (1:8) DES exhibited the highest lignin recovery (73.04 ± 0.2%). The associated low pretreatment yield, which dropped to 31.95 ± 0.4%, indicated a lack of selectivity. In contrast, Bio-derived sustainable DES (1:8) exhibited lower selectivity, achieving a high pretreatment yield (43.55% ± 0.6). The DES recovery remained robust across all experimental conditions, consistently ranging between 74.00 ± 0.3% and 85.00 ± 0.7%. Structural validation confirmed the elimination of amorphous components, hemicellulose, lignin and amorphous cellulose and a significant enhancement in cellulose crystallinity index (CrI), rising from 18.6% to 56.1%. The purity of extracted lignin was confirmed by ¹³C NMR. DES ChCl: CA (1:8) was found the most viable pretreatment method for sustainable AP valorization.