Evaluation of the main inhibitors from lignocellulose pretreatment for enzymatic hydrolysis and yeast fermentation

To produce cellulosic ethanol more economically, utilization of whole slurry of pretreated lignocellulose without separating liquid and solid fractions after thermal and/or chemical pretreatment of lignocellulose may be advantageous in terms of process economics. To carry out such processing on mixtures, which contain pretreatment byproducts, quantitative evaluation of the degree of inhibition of enzymatic hydrolysis and yeast fermentation by pretreatment byproducts are important. Therefore, in this study, the inhibitory effect of byproducts, focusing on sugar degradation products including furfural, hydroxymethylfurfural (HMF), acetic acid (AA), formic acid (FA), and levulinic acid (LA), on enzyme and microbial performance was investigated. The experimental conditions for SSF media containing the inhibitors were optimized by response-surface methodology-ridge analysis. The saccharification using commercial cellulase was most remarkably inhibited (approximately 28%) by HMF. The ethanol production by Saccharomyces cerevisiae was nearly completely inhibited (approximately 80%) by furfural. The toxicity was noted as HMF > FA > furfural > AA ≈ LA for enzymatic hydrolysis, and furfural > HMF > FA > AA > LA for yeast ethanol production. The results indicated that the inhibitor accumulation during pretreatment should be controlled for subsequent effective saccharification and fermentation.