Carbohydrates recovery from cardoon pretreated by acidcatalyzed steam explosion and enzymatic hydrolysis

Cardoon (Cynara cardunculus) is a promising crop for obtaining oil from seeds and lignocellulosic residues. Lignocellulosic residues are a source of hexose and pentose carbohydrates derived from cellulose and hemicellulose that can be used as feedstock for industrial microbiology processes. In order to obtain fermentable carbohydrates, lignocellulosic residues have to undergo an effective pre-treatment. Among pretreatments, acid-catalyzed steam explosion is a thermochemical process capable of releasing free pentose carbohydrates into a liquid fraction, and at the same time making the cellulose-rich solid fraction easily hydrolyzed by enzymes. The drawback is the production of a high number of carbohydrate degradation products which are present in the liquid fraction and could inhibit microbial growth. This liquid fraction can be further exploited through the optimization of pre-treatment conditions, minimizing inhibitor production while maximizing monosaccharides recovery. In this work the lignocellulosic residues of cardoon, gathered from a dedicated field, were used as raw material for the production of fermentable monosaccharides by employing acid-catalyzed steam explosion. The raw material was soaked with a dilute (1% w/w) sulfuric acid solution and then subjected to steam explosion at three different severity conditions. The recovered slurry was separated into solid and liquid fractions and individually characterized in order to determine total carbohydrates and inhibitor concentrations. The slurry and the washed solid fraction underwent enzymatic hydrolysis to release glucose from cellulose and pentose monosaccharides from hemicellulose. By conducting the pre-treatment at 175°C for 35 minutes and hydrolyzing the obtained slurry a yield of 33.17 g of monosaccharides/ 100 g of cardoon was achieved. At the same conditions, 4.39 g of inhibitors/ 100 g of cardoon were produced.