Optimized sequential alkaline–hydrothermal xylan recovery from EnZolv-pretreated cotton stalk and enzymatic production of xylo- and cello-oligosaccharides

Biovalorisation of lignocellulosic biomass is gaining interest in the field of bioprocess technology for the production of high value biochemicals. This study aimed to develop and optimize a process for xylan recovery from EnZolv-pretreated cotton stalk (CSPT), followed by enzymatic production of xylo- and cello-oligosaccharides. First, a conventional alkaline extraction was performed on untreated and EnZolv-pretreated cotton stalk using NaOH (2–12% w/v), at 35–50 °C for 8–24 h. A maximum experimental xylan recovery of 38.5% was achieved at 12% NaOH, 45 °C, and 24 h whereas the Box-Behnken design (BBD), predicted 22% recovery at same factor combination. Second, a hydrothermal-assisted alkaline process (12% NaOH, 121 °C for 1 h) resulted in xylan recoveries of 20.6% and 34.9% from untreated and EnZolv-pretreated CS, respectively. Third, a sequential process was implemented in which untreated and EnZolv-pretreated CS were first subjected to alkaline extraction under optimized NaOH conditions followed by hydrothermal treatment at 121 °C for 1 h. This two-step strategy increased xylan recovery to 30.9% (untreated CS) and 54.3% (EnZolv-pretreated CS). The recovered xylan and the residual cellulose-rich cotton stalk were subsequently used as substrates for enzymatic hydrolysis, yielding 12.365 mg mL −1 XOS from semi-purified xylan and 1.759 mg mL−1 COS from the cellulose fraction, thereby demonstrating an integrated route for valorizing both hemicellulosic and cellulosic components of cotton stalk.