In this study, we designed a biomass popping pretreatment, system using a fired burner and a horizontal cylinder rotating on an axis, to produce ethanol from mandarin (Citrus unshiu) peel (MP) waste. Popping pretreatment was performed at 150 degrees C for 10 min without chemical treatment. Popping pretreatment reduced the size of particles to less than 1 mm and decreased the concentration of D-limonene, a yeast fermentation inhibitor, from 0.21% to 0.01%. Enzymatic hydrolysis of pretreated MP was performed in 50 mM sodium acetate buffer (pH 4.8) at 45 degrees C for 6 h, and the total saccharification rate was approximately 95.6%. The vacuum evaporation process increased the fermentable sugar concentration to 10% (glucose 7.1% and fructose 2.9%). Subsequent fermentation at 30 degrees C at pH 5.0 for 12 h in a laboratory bio-reactor increased the ethanol yield to 90.6%, compared to 78% at 36 h from raw MP. (C) 2012 Elsevier Ltd. All rights reserved.