Enzymatic process for saccharification of differently pretreated grape stalks.

The grape and wine production system produces a big quantity of solid residues whose management and disposal give environmental problems. These wastes are, generally, not intrinsically hazardous but the high content of organic matter and the fact that the production is concentrated in a particular period of the year, pose severe pollution problems. One of the main solid residue generated is grape stalk. Several alternatives to the traditional disposals have been investigated, such as composting (1) or removal of heavy metals from aqueous solutions (2). However, this residue is rich in carbohydrates and bioactive compounds that can be recovered by suitable processes to produce high-value added products. Grape stalks, as other lignocellulosic materials (3), can contain up to 15% of their dry weight of non-structural carbohydrates (NSC) in addition to cellulose and hemicellulose. Consequently, the saccharification of all these polysaccharides is an ambitious target that can be reached with an enzyme-based process, a promising technology. Grape stalks are mainly composed of one year old woody tissues, and a pretreatment is required to breakdown the cellular organization of the tissues and to modify the structure of the components of the cell wall, in order to make the macromolecules available to the enzymatic action (4). In our work, grape stalks of the CV Sangiovese, were harvested in March, 2010, corresponding to normal pruning time. The average NSC composition (expressed as % of the dry weight ± SE) was as follows: glucose 0.36 ± 0.01, fructose 0.55 ± 0.02, sucrose 2.02 ± 0.07 starch 7.35 ± 0.19 adding up to a total non structural carbohydrate of 10.29 ± 0.18. The material after drying in ventilated oven at 50°C to constant weight, has been subjected to the following pretreatments: a) steam explosion at 180 or 200°C, in the presence or not of 3 % sulphuric acid; b) alkaline soaking with 10% ammonium hydroxide at 70°C for 24 h. To evaluate the most effective method in releasing fermentable sugars, 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. Our preliminary data show that, without biomass pretreatment by action of the optimized enzyme mixture, about 90% of the glucose from starch can be released after 24 h. However 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 the total release of glucose from starch and 73% released of glucose from cellulose. Work is in progress to optimize the process.