A Comparison of Aqueous and Dilute-Acid Single-Temperature Pretreatment of Yellow Poplar Sawdust

Pretreatment of fresh yellow poplar sawdust (<6 mm, 25-30 wt % moisture) with hot liquid water (190-220 °C, 2.0-7.5 min, 2-7 wt % solids loading) or steam (220 °C, 2.0 min) was compared with dilute acid pretreatment (0.8% w/v sulfuric acid, 175 °C, 10 min, 20-25 wt % solids loading). The most favorable conditions for hot liquid water pretreatment (220 °C, 2.0 min, 5 wt % solids loading) performed as well as the dilute acid pretreatment (175 °C, 10.0 min, 0.8% sulfuric acid, 20 wt % solids loading) for fermentation of the cellulosic fraction of the pretreated fibers by simultaneous saccharification and fermentation (SSF), using Saccharomyces cerevisiae and 25 IFPU cellulase/g of cellulose. Ethanol yields of 97 and 92 wt % of the theoretical yield and xylan recoveries of 85 and 70 wt % (92 and 96 wt % solubilization) were achieved with the hot liquid water and dilute acid pretreatments, respectively. Steam pretreatment (220 °C, 2 min) was not as effective as the dilute acid pretreatment, resulting in an ethanol yield of 70 wt % of the theoretical yield, with 88 wt % xylan recovery (54 wt % solubilization). A positive correlation was found between xylan removal and the ethanol yield obtained by SSF. Xylan recovery, however, was inversely proportional to xylan solubilization. Removal of up to 57 wt % of the acid-insoluble lignin in yellow poplar was also possible with hot liquid water pretreatment, with 75 wt % of this acid-insoluble lignin recoverable by simple filtration of the liquid pretreatment products.