Structural characteristics of corncob and eucalyptus contributed to sugar release during hydrothermal pretreatment and enzymatic hydrolysis

Explicitly understanding biomass recalcitrance through the characterization of biomass physicochemical properties may help to develop efficient pretreatment and enzymatic hydrolysis strategies. The lignin of corncob and eucalyptus contain the same main linkage bonds: beta-O-4 aryl ether bonds, beta-beta and beta-5 structures, but the lignin of eucalyptus was of the syringyl (S)-guaiacy (G) type, while that of corncob was SG-p-hydroxyphenyl (H) type, corresponding to lignin S/G ratios of 1.6 and 1.1 respectively. Under the optimum microwave-hydrothermal pretreatment condition of 180 degrees C for 30 min at a 12.5% substrate concentration, the maximum total xylose yield of corncob (64.7%) was lower than that of eucalyptus (79.2%). In contrast, corncob resulted in a greater increase in enzymatic digestibility, from 59.6 to 82.4%, after pretreatments, compared with 16.7 to 74.9% for eucalyptus. There was a positive correlation between the xylose yield and lignin S/G ratio, but the lignin content was negatively correlated with enzymatic digestibility. Furthermore, based on the non-destructive characterization of three-dimensional X-ray microscopy, not only was the increase in the number and size of surface pores beneficial to the accessibility of cellulose to cellulosic enzymes, but the swelling of cell wall could reduce the recalcitrance of sugar release.