Conversion of C6 and C5 sugars in undetoxified wet exploded bagasse hydrolysates using Scheffersomyces (Pichia) stipitis CBS6054

Sugarcane bagasse is a potential feedstock for cellulosic ethanol production, rich in both glucan and xylan. This stresses the importance of utilizing both C6 and C5 sugars for conversion into ethanol in order to improve the process economics. During processing of the hydrolysate degradation products such as acetate, 5-hydroxymethylfurfural (HMF) and furfural are formed, which are known to inhibit microbial growth at higher concentrations. In the current study, conversion of both glucose and xylose sugars into ethanol in wet exploded bagasse hydrolysates was investigated without detoxification using Scheffersomyces (Pichia) stipitis CBS6054, a native xylose utilizing yeast strain. The sugar utilization ratio and ethanol yield (Yp/s) ranged from 88-100% and 0.33-0.41 ± 0.02 g/g, respectively, in all the hydrolysates tested. Hydrolysate after wet explosion at 185°C and 6 bar O2, composed of mixed sugars (glucose and xylose) and inhibitors such as acetate, HMF and furfural at concentrations of 3.2 ± 0.1, 0.4 and 0.5 g/l, respectively, exhibited highest cell growth rate of 0.079 g/l/h and an ethanol yield of 0.39 ± 0.02 g/g sugar converted. Scheffersomyces stipitis exhibited prolonged fermentation time on bagasse hydrolysate after wet explosion at 200°C and 6 bar O2 where the inhibitors concentration was further increased. Nonetheless, ethanol was produced up to 18.7 ± 1.1 g/l resulting in a yield of 0.38 ± 0.02 g/g after 82 h of fermentation.