BACKGROUNDHydrothermal pretreatments were applied to improve the cellulose digestibility of sugarcane bagasse (SB). The effect of hydrothermal process parameters, i.e. temperature, pressure and NH3 addition on the enzymatic digestibility was studied. With the objective of understanding the relationship between pretreatment and enzymatic digestibility, the ultrastructure of untreated and treated SB cell wall was observed. RESULTS72 h digestibility was enhanced from 15.7% (untreated SB) to 77.9% for the samples treated with hot water at 180 degrees C, 4 MPa for 20 min, while it reached 98.5% with the addition of 25% NH3 at 160 degrees C, 6 MPa for 60 min. There is no direct relationship between enzymatic digestibility and crystallinity index value, but it has a close relationship with the removal of hemicellulose and lignin. The boundaries among the different layers in the secondary wall of treated SB could not be distinguished exactly resulting from the migration of lignin in the hot water process or removal of lignin in the NH3 addition pretreatment. CONCLUSIONThe enzymatic digestibility of SB was enhanced by the hydrothermal pretreatment. The homogenization of distribution of cellulose and lignin in the cell wall layer resulted from the removal of hemicellulose or lignin contributed to the enhancement of accessibility to enzymatic attack. (c) 2014 Society of Chemical Industry