Investigating the esterification-induced inhibition in enzymatic saccharification of corn stover pretreated with carboxylic-acid based deep eutectic solvent

Choline chloride-lactic acid, a common carboxylic acid-based deep eutectic solvent (DES), was used to pretreat corn stover (CS). Despite significant lignin and xylan removal, enzymatic cellulose conversion remained below 30 %, only slightly higher than untreated CS. Structural characterization confirmed esterification between the CS's hydroxyl groups and lactic acid during pretreatment. A brief 5-min saponification effectively removed these ester groups, leading to a 1.08-, 2.75-, and 2.50-fold increase in 48-h cellulose hydrolysis for 2-, 5-, and 8-h DES-pretreated samples, respectively. Analysis of chemical composition, surface morphology, crystallinity, and degree of polymerization revealed that these factors did not contribute to the enhanced hydrolysis observed in the saponified samples. This study concludes that saponification-induced de-esterification is the key factor driving the significant increase in enzymatic cellulose hydrolysis, highlighting the detrimental impact of esterification occurring during carboxylic acid-based DES pretreatment on subsequent cellulose saccharification.