Bioconversion of giant reed (Arundo donax L.) hemicellulose hydrolysate to ethanol by Scheffersomyces stipitis CBS6054

Abstract The objective of this study was to evaluate the production of ethanol by Scheffersomyces ( Pichia ) stipitis CBS6054, a native xylose fermenting yeast, from sugars contained in the giant reed ( Arundo donax L.) hemicellulosic hydrolysate. A response surface methodology with two input parameters, severity factor and oxalic acid concentration ranging from 2.87 to 4.05 and from 2 to 8 (% w oxalic acid/w solid dry matter), respectively, was employed to minimize degradation products and maximize sugar release. However, at the optimum condition for sugar release (43.8 g l −1 ), levels of toxic degradation products (acetic acid, furfural, HMF and phenolic compounds) were considered too high for yeast fermentation. The condition to minimize degradation products and maximize sugar yields was judged to be 2.87 severity factor and 5.0% oxalic acid concentration. At this condition 26.0 g l −1 xylose, 5.0 g l −1 glucose and 2.4 g l −1 arabinose were recovered in giant reed hydrolysate fraction. Adjustment of pH to 5.0 with Ca(OH) 2 decreased xylose, glucose and acetic acid, 22%, 8% and 27% respectively. Increasing the initial pH from 5.0 to 5.5, 6.0 and 6.5, respectively, significantly improved the fermentability of the giant reed hemicelluloses hydrolysate; no fermentation was observed at pH 5.0 after 96 h, while 8.20 g l −1 of ethanol was obtained at pH 6.0 after 48 h, with an ethanol yield of 0.33 ( g e / g s ) and a productivity of 0.17 g l −1  h −1 . The optimum pH of acid hydrolysate fermentation for ethanol production was 6.0–6.5.