Xylitol production from olive-pruning debris by sulphuric acid hydrolysis and fermentation with Candida tropicalis

The debris of olive pruning is a renewable, low-cost and widely available agricultural waste. Its biochemical conversion by hydrolysis and fermentation was undertaken in the present study. Diluted acid hydrolysis was conducted in a heterogeneous stirred tank reactor at 90 degrees C and at a low sulphuric acid concentration (0.0-1.0 N) for 300 min. To increase the D-xylose/D-glucose ratio into the hydrolysate, in another experiment amorphous cellulose and extracts were removed by means of a pretreatment in an extruder with 1 N H2SO4 at 70 degrees C before the acid hydrolysis. The fermentation of hydrolysates was performed under microaerobic conditions in a batch bioreactor at 30 degrees C and pH 5 with Candida tropicalis NBRC 0618. The controlled fermentation parameters included maximum specific growth rate, biomass productivity, rate of the specific substrate uptake, rates of specific ethanol and xylitol production, and overall yield of ethanol and xylitol. In the presence of 1.0 N H2SO4, the fermentation of the pretreated hydrolysate led to specific xylitol production rates and overall xylitol yield (0.1 g g(-1) h for t=25 h; 0.49 g g(-1), respectively) higher than those achieved without pretreatment (0.03 g g(-1) h for t=25 h; 0.39 g g (1), respectively). Under these conditions, 53 g xylitol kg(-1) of dry olive-pruning debris was obtained from the pretreated culture, whereas without pretreatment 70 g ethanol and 34 g xylitol were recovered.