Hydrolysis of pistachio shell into xylose by using MW-HPCO2 system and bioconversion of xylose to xylitol by Candida tropicalis

Biotechnological xylitol production from secondary agricultural residues is a promising approach in terms of sustainability and the environment. In this study, microwave-assisted high-pressure CO2/H2O hydrolysis (MW-HPCO2) system was employed for the hydrolysis of pistachio shell into xylose. Box Behnken design of response surface methodology was implemented to investigate the effect of temperature, time and liquid-to-solid ratio on xylose yield and to optimize MW-HPCO2 hydrolysis process. A maximum xylose yield of 61.39% and minimum degradation compounds (HMF and furfural) of 11.07% were attained under reaction conditions of 190°C, 30 min and 18 mL/g. HMF and furfural were completely removed from xylose-rich hydrolysate by activated charcoal. Detoxified xylose rich hydrolysate was used for both shake-flask and bioreactor using Candida tropicalis ATCC 13803 for xylitol production. Bioconversion of xylitol in shake-flask was performed at three different initial xylose concentrations (100, 150 and 200 g/L). Highest xylitol concentration (65.15 g/L) and the maximum yield of 0.66 g/g was obtained with xylose concentration of 100 g/L in shake-flask. Box Behnken design was implemented to investigate the effect of temperature, pH and aeration on xylitol yield in bioreactor. The highest xylitol yield was 0.94 g/g under optimised conditions (34.5°C, pH 4.5 and 1.2 vvm). Xylitol was recovered with a yield of 68.4% from culture media by crystallization.