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

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 degrees 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 0.44g xylitol/g D-xylose and a global volumetric productivity of 0.11 g L-1 h(-1) were achieved. (C) 2014 Elsevier B.V. All rights reserved.