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.