Monosaccharide yields and lignin removal from wheat straw in response to catalyst type and pH during mild thermal pretreatment

The influence of various low temperature (140 °C) pretreatments, using different acid and alkaline catalysts and different pH values, was studied for enzymatic hydrolysis of wheat straw. The pretreated wheat straw was treated by a standard blend of Celluclast 1.5L and Novozym 188. While pretreatment at pH 1 gave the highest yield of saccharides in the liquid fraction, the solid fraction was more susceptible to enzymatic attack when pretreated at pH 13. The highest yields were obtained after pretreatment with hydrochloric acid at pH 1, and with sodium hydroxide at pH 13 when enzymatic hydrolysis was employed. A two-step pretreatment strategy at pH 1 (hydrochloric acid) and subsequently at pH 13 (sodium hydroxide) released 69% and 95% of the theoretical maximal amounts of glucose and xylose, respectively. Furthermore, this two-step pretreatment removed 68% of the lignin from the straw with only minor losses of monosaccharides and production of only low amounts of inhibitors. Type of catalyst and pH indeed influenced the monosaccharide yields and lignin removal from wheat straw, and need more attention in the choice of pretreatment strategy.