Polyol-based deep eutectic solvent/cellulase-assisted water extraction of RG-I-enriched pectin from apple cell wall: Insight into enhanced enzyme activity, structural characteristics, synergistic mechanism and gelling property

To efficiently recover rhamnogalacturonan I (RG-I) entrapped within the recalcitrant apple cell wall matrix, the present study established a two-step strategy combining polyol-based deep eutectic solvent (DES)/cellulase (CE) pretreatment and extraction using water. The highest CE activity was achieved in choline chloride (ChCl)/propylene glycol (PG) at molar ratio of 1:2 with water content of 23.57 wt% among twenty polyol-based DESs tested. The Kamlet-Taft parameters (α and β) of hydrated ChCl/PG showed positive correlations with CE activity (r = 0.85 for α, 0.88 for β), whereas viscosity and polarity (π∗) exhibited negative correlations (r = −0.56 for viscosity, −0.73 for π∗). Extraction using the ChCl/PG/CE-assisted water method yielded the highest recovery (17.8%), significantly exceeding those achieved with CE alone, ChCl/PG/CE alone or ChCl/PG-assisted water extraction. Compared with extracted control pectin (E-P, ChCl/PG-P), ChCl/PG&E-P contained the highest RG-I content with more/longer neutral sugar side chains (RG-I = 81.28 mol%, (Ara + Gal)/Rha = 4.50). Efficient hydrolysis of amorphous cellulose, coupled with enhanced interactions between ChCl/PG and exposed RG-I, synergistically promoted the dissociation of RG-I from cellulose-RG-I matrix and its subsequent dissolution in water. Finally, ChCl/PG&E-P was capable of forming stable gel in low concentration and a small amount of Ca 2+ . Due to the higher content of side chains, ChCl/PG&E-P gel exhibited higher gel strength and rheological moduli (G′ and G″) across varying pectin/Ca 2+ concentrations and pH conditions. This work offers a novel strategy for the efficient recovery of RG-I-enriched pectin from apple cell wall, which exhibits good characteristics as a gelling agent.