Valorising hemicellulosic fraction of corncobs concomitantly into ethanol and xylitol using Candida tropicalis Y6

Abstract

Crop residues are valuable sources of lignocellulosic biomass that will continue to be available as long as food production for humanity continues. Maize, a cereal crop that serves as a staple food in many parts of the world and is cultivated in numerous countries, holds great importance. The surge in maize production generates substantial amounts of corncobs which can be used as feedstock for the production of xylitol, a compound with a rapidly growing market, and ethanol. In this study, hemicellulose was extracted from the corncobs to obtain xylose by subjecting them to acid hydrolysis with 1 % H2SO4. Non-conventional yeast strain Candida tropicalis Y6 isolated from rotten vegetables was tested for growth and xylose utilization potential on synthetic and acid pre-hydrolysate medium supplemented with mineral salts. Candida tropicalis Y6 exhibited high growth and sugar utilization. On synthetic medium with xylose as the sole C source it produced 6.71 g/L and 0.38 gg-1 of xylitol showing maximum conversion efficiency (53 %) at 24 h, and also produced 0.3 g/L ethanol at 48 h. When cultured on undetoxified corncob hydrolysate, C. tropicalis Y6 produced 0.41 g/L xylitol and 0.74 g/L ethanol. Its xylose conversion was severely affected in acid hydrolysates possibly due to the presence of inhibitors. This C. tropicalis Y6 strain was distinct from other reports in producing a higher level of ethanol than xylitol during fermentation of corncob acid prehydrolysate pointing towards the ratio of XR and XDH enzymes activities under studied conditions.