Enzymatic process for saccharification of differently pretreated grape cane waste

The wine industry produces a great quantity of solid residues whose management and disposal are environmental problems. These wastes are generally not intrinsically hazardous but the high content of organic matter, and the fact that production is concentrated in a particular period of the year, poses potentially severe pollution problems. One of the main solid residues generated is grape stalk, and several alternatives to the traditional disposals have been investigated, such as composting (1) or removal of heavy metals from aqueous solutions (2). It must be taken into account that this residue is rich in carbohydrates and bioactive compounds that can be recovered by opportune processes to produce high-value added products. Grape stalks can contain up to 15% of their dry weight of non-structural carbohydrates (mainly starch) besides cellulose and hemicellulose. Consequently, the saccharification of all these polysaccharides is an ambitious target that can be reached with an enzyme-based process, at present considered one of the most promising technologies. Since grape stalks, like many other agricultural wastes, are of a lignocellulosic nature, a pretreatment is required to alter the structural and chemical composition of the biomass. A number of physical, chemical, physico-chemical and biological pretreatment strategies have been developed to improve the accessibility of the enzymes to the cellulose and hemicellulose fractions (3). In our work, grape stalks have been previously subjected to the following pretreatments to evaluate the most effective in releasing fermentable sugars in the successive enzymatic saccharification step: a) steam explosion: 180 or 200°C, in presence or not of 3 % sulphuric acid; b) alkaline soaking with 10% ammonium hydroxide at 70°C for 24 h. After, the pretreated biomass has been subjected to enzymatic saccharification at 50°C and pH 5.0 for 48 h by addition of the following enzymes: cellulase, beta-glucosidase, amylase and glucoamylase, and the amount of glucose released has been compared with the amount obtained from untreated grape stalks. Our preliminary data show that about 90% of the glucose from starch can be already released after 24 h from untreated biomass, while a pretreatment is required to improve the hydrolysis of the cellulosic fraction. Ammonium soaking allowed better hydrolysis of the polysaccharides than steam explosion with almost total release of glucose from starch and 73% of glucose released from cellulose