Low energy-demanding recovery of antioxidants and sugars from olive stones as preliminary steps in the biorefinery context

Olive stones constitute the main solid by-product of the olive oil extraction process. As a lignocellulosic material, their use as a source of fermentable sugars, antioxidants and other applications has been proposed. In this work the possibilities of a better use of this material through a relatively low energy-demanding operation, e.g. autoclave treatment, are assessed. Dilute acid extraction was used for evaluating the influence of the main variables (temperature, acid concentration and pretreatment time) on the sugar composition and the antioxidant capacity of liquid fractions (prehydrolyzates) issued from autoclave treatment. Results show that the highest production of fermentable sugars, 27 g/100 g initial dry matter, was obtained at the most severe pretreatment conditions (130 °C for 90 min and 2% sulfuric acid), with xylose being 90% of the released sugars, while cellulose degradation was limited. Concerning antioxidant capacity of the prehydrolyzates, the best result was obtained at the highest temperature (130 °C) and time (90 min) but using no acid. This procedure is proposed as a preliminary step of a broader treatment scheme which can also include further steps of pretreatment and eventually enzymatic hydrolysis and fermentation. As a conclusion, a two-step process strategy is suggested to optimize the recovery of antioxidants in the first step, and the production of fermentable sugars in the second step.