Enhancing fermentable sugar yield from cassava residue using a two-step dilute ultra-low acid pretreatment process

Cassava residue (CR), a fibrous material, is a cassava processing by-product. The complex structure of the lignocellulosic material in CR limits effective saccharification; therefore, pretreatment is an essential step in biomass fuel production. This study investigated the hydrolysis of CR lignocellulosic components, using amylase to remove the starch from the CR to produce remaining starch-free CR (RSFCR). Ultra-low acid (ULA) CR pretreatments (a one-step ULA using 0.01% w/w H2SO4 or a two-step ULA using 0.05% H2SO4) were applied for durations of 0 to 50 min and at temperatures of 160 to 200 °C. The one-step ULA pretreatment achieved maximum glucose, xylose, and total sugar recoveries of 7.57%, 39.11%, and 15.14%, while the two-step ULA pretreatment attained 9.82%, 58.62%, and 22.08% recoveries, respectively. ULA RSFCR pretreatment degraded a small percentage of the hemicellulose sugars into the inhibitors and formed xylose-rich liquid prehydrolysate and a cellulose-rich solid. The hydrolytic yields of glucose, xylose, and total sugar from the two-step ULA pretreatment with enzymatic hydrolysis were 7.43, 1.41, and 3.32 times higher, respectively, than those from the two-step ULA pretreatment and 1.02, 1.13, and 1.04 times higher than those from the one-step ULA pretreatment with enzymatic hydrolysis. Fermentation inhibition was not detected in the pretreatment methods. These results indicate the feasibility of using two-step ULA combined with enzymatic hydrolysis for RSFCR pretreatment.