Effect of inhibitors formed during wheat straw pretreatment on ethanol fermentation by Pichia stipitis

► Wheat straw can be used efficiently for ethanol production using Pichia stipitis. ► Acetic acid, furfural and 5-hydroxymethylfurfural are the main inhibitors found. ► Acetic acid, furfural and 5-hydroxymethylfurfural show a synergic effect. ► Good ethanol yields are obtained with no detoxified steam exploded wheat straw. The inhibitory effect of the main inhibitors (acetic acid, furfural and 5-hydroxymethylfurfural) formed during steam explosion of wheat straw was studied through ethanol fermentations of model substrates and hydrolysates from wheat straw by Pichia stipitis. Experimental results showed that an increase in acetic acid concentration led to a reduction in ethanol productivity and complete inhibition was observed at 3.5g/L. Furfural produced a delay on sugar consumption rates with increasing concentration and HMF did not exert a significant effect. Fermentations of the whole slurry from steam exploded wheat straw were completely inhibited by a synergistic effect due to the presence of 1.5g/L acetic acid, 0.15g/L furfural and 0.05g/L HMF together with solid fraction. When using only the solid fraction from steam explosion, hydrolysates presented 0.5g/L of acetic acid, whose fermentations have submitted promising results, providing an ethanol yield of 0.45g ethanol/g sugars and the final ethanol concentration reached was 12.2g/L (10.9g ethanol/100g DM).