Deconstruction of Pine Wood (Pinus sylvestris) Recalcitrant Structure Using Alkali Treatment for Enhancing Enzymatic Saccharification Evaluated by Congo Red

Abstract

The changes of pine wood structure were evaluated based on Congo red (CR) adsorption in this study. Cellulose free –OH group act as electron acceptor and CR amino or azo group as electron donor that interacts each other to form H–bonds. This interaction of CR with cellulose is considered in this study to analyze the structural changes of pine wood. Pine wood samples were treated with different concentrated NaOH solution at 130 °C for 30 min. CR adsorption of treated samples were assayed in terms of absorbance decrease of CR solution measured by spectrophotometer. Field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) were used to evaluate the microstructural changes of treated samples. Finally enzymatic saccharification was carried out using commercial cellulase enzyme. Treatment of pine wood with alkali degrades the intra and/or intermolecular H– bonds of cellulose leading to an exposure of free –OH groups. These free –OH groups bind with CR molecule while in contact with CR solution. Structural change of pine wood caused by different NaOH (0 to 20%) treatment at 130 °C for 30 min was evaluated in terms of CR adsorption. Scanning electron microscopy and Fourier transform infrared spectroscopy studies showed that 10% NaOH treated sample exposed more cellulose fibers compared to other treatments leading to more CR binding. The cellulose crystalline index was increased with increasing NaOH–treatments and lowered by the 20% NaOH–treatment due to degradation of cellusose fibres. Moreover, after 72 h, reducing sugar yield was 76.5% and 70.7% using enzyme loading of 15FP U/g and 30CB U/g from 10% and 20% NaOH treated pine wood samples, respectively. Reducing sugar yield was decreased from samples while treating more than 10% NaOH solution. 10% NaOH treatment can be considered as an effective concentration for pine wood treatment at 130 °C for 30 min. These results suggest that CR approach is supposed to be helpful for selecting the treatment condition.