Improved xylitol production from olive stones hydrolysates by biological detoxification

Olive stones, currently used as a biosource of heat and power, is gaining research interest as a feedstock for a biorefinery based on olive derived wastes. In this work, pure xylitol crystals were obtained and fully characterized as one of the possibilities of taking full advantage of this by-product of olive oil production. The proposed process includes a two-step pretreatment (water and acid extractions), detoxification of the liquid fractions and fermentation of sugars. After the water extraction, a liquid fraction with antioxidant capacity was recovered; the second step was done with dilute sulfuric acid for solubilization of sugars, resulting in a solution containing more than 60 g/l of xylose as the main sugar. This liquor contained also up to 16 g/l acetic acid, which rendered it not fermentable. Also other compounds, potentially inhibitors in downstream operations, were produced during the pretreatment, requiring a detoxification procedure. In this work, a chemical-free, biological treatment with Saccharomyces cerevisiae proved to be the best detoxification method, as glucose, furfural, hydroxymethylfurfural and acetic acid were consumed. The subsequent inoculation of Candida boidinii produced xylitol concentrations above 38 g/l after 90 h fermentation time and a yield as high as 63%, equivalent to 12 g xylitol/100 g olive stones.