To improve the enzymatic digestibility of switchgrass at mild temperatures. lime pretreatment of switchgrass was explored at 50 and 21 degrees C, and compared with that at 121 degrees C. The effects of residence time, lime loading, and biomass washing on the Sugar production efficiency were investigated. Pretreatments were evaluated based on the yields of biomass-derived sugars in the subsequent enzymatic hydrolysis. Under the best pretreatment conditions (50 degrees C, 24 h, 0.10 g Ca(OH)(2)/g raw biomass, and wash intensity of 100 ml water/g raw biomass), the yields of glucose, xylose, and total reducing sugars reached 239.6, 127,2, and 433.4 mg/g raw biomass, which were respectively 3.15, 5.78, and 3.61 times those of untreated biomass. The study on calcium-lignin bonding showed that calcium ions crosslinked lignin molecules under alkaline conditions, which substantially decreased lignin solubilization during pretreatment, but the resulting high lignin contents of the pretreated biomass did not compromise the improvement of enzymatic digestibility. (C) 2009 Elsevier Ltd. All rights reserved.