The influence of lignin on steam pretreatment and mechanical pulping of poplar to achieve high sugar recovery and ease of enzymatic hydrolysis

•Lignin restricts hydrolysis of refiner mechanical and steam pretreated poplar.•In contrast to RMP, steam pretreatment re-locates lignin to improve hydrolysis.•Integrating sulphonation into steam pretreatment is more effective than post-treatment.•Sodium carbonate improves sulphonation, delignification and enzymatic hydrolysis. With the goal of enhancing overall carbohydrate recovery and reducing enzyme loading refiner mechanical pulping and steam pretreatment (210°C, 5min) were used to pretreat poplar wood chips. Neutral sulphonation post-treatment indicated that, although the lignin present in the steam pretreated substrate was less reactive, the cellulose-rich, water insoluble component was more accessible to cellulases and Simons stain. This was likely due to lignin relocation as the relative surface lignin measured by X-ray photoelectron spectroscopy increased from 0.4 to 0.8. The integration of sulphite directly into steam pretreatment resulted in the solubilisation of 60% of the lignin while more than 80% of the carbohydrate present in the original substrate was recovered in the water insoluble fraction after Na2CO3 addition. More than 80% of the sugars present in the original cellulose and xylan could be recovered after 48h using an enzyme loading of 20mgprotein/g cellulose at a 10% substrate concentration.