Valorisation of olive stone by-product for sugar production using a sequential acid/steam explosion pretreatment

Olive stones are a lignocellulosic biomass generated as a by-product in the olive oil industry. They are of interest for use as a feedstock in the production of biofuels and other bioproducts in a biorefinery context. The aim of this work was to evaluate the potential of sugar production from olive stones, as a first approach in a valorisation process for this by-product. The process strategy tested consisted of a sequential fractionation process of the lignocellulosic components of olive stones based on acid pretreatment and steam explosion followed by enzymatic hydrolysis. In the first acid step, the study of pretreatment temperature, sulfuric acid concentration and biomass loading led to optimized conditions of 128 degrees C, 10.5 g acid/100 g olive stones and 33 % solids. This yielded a pre-treatment liquor with a xylose recovery of 71 %. In the steam explosion pre-treatment, the influence of increasing temperature from 180 to 225 degrees C at a fixed time of 5 min was evaluated through the assessment of sugar recovery and the enzymatic digestibility of sequentially pretreated olive stones. This permitted the selection of the most adequate steam explosion conditions (i.e., 195 degrees C for 5 min). As a result of the combined pretreatment under the selected operation conditions, an overall sugar production yield of 83 % of the total sugar content in raw olive stones can be obtained, taking into account the different sugar streams generated along the whole process.