Valorization of olive stones for xylitol and ethanol production from dilute acid pretreatment via enzymatic hydrolysis and fermentation by Pachysolen tannophilus

•Olive stone, potential lignocellulose material for biofuels and chemicals production.•The aim was to determine optimal conditions to obtain maximum hemicellulose sugars.•Highest yield of d-xylose was found at a temperature of 195 °C for 5 min.•Pachysolen tannophilus metabolized all the sugars, with xylitol yield 0.44 g per g d-xylose.•Overall process led to the production of 9.2 g of xylitol per 100 g of olive stones.

Olive stones are an agro-industrial by-product abundant in the Mediterranean area that is regarded as a potential lignocellulosic feedstock for sugar production. Statistical modeling of dilute-sulphuric acid hydrolysis of olive stones has been performed using a response surface methodology, with treatment temperature and process time as factors, to optimize the hydrolysis conditions aiming to attain maximum d-xylose extraction from hemicelluloses. Thus, solid yield and composition of solid and liquid phases were assessed by empirical modeling. The highest yield of d-xylose was found at a temperature of 195 °C for 5 min. Under these conditions, 89.7% of the total d-xylose was recovered from raw material. The resulting solids from optimal conditions were assayed as substrate for enzymatic hydrolysis, while fermentability of hemicellulosic hydrolysates was tested using the d-xylose-fermenting yeast Pachysolen tannophilus. Both bioprocesses were considerably influenced by enzyme loading and inoculum size. In the enzymatic hydrolysis step, about 56% of cellulose was converted into d-glucose by using an enzyme/solid ratio of 40 FPU g−1, while in the fermentation carried out with a cell concentration of 2 g L−1 a yield of 0.44 g xylitol/g d-xylose and a global volumetric productivity of 0.11 g L−1 h−1 were achieved.