Extraction of Soluble Polysaccharides from Grape Skins
Obtaining polysaccharides from grape cell walls is an interesting topic for the wine industry given its implications with regards to wine treatments and potential sensory attributes. In the present work, several conditions (i.e., a chelating agent, pH, titratable acidity, extraction time, number of successive extractions, solid to liquid ratio and alcoholic fermentation) were tested to assess their effects on the yield of soluble polysaccharides obtained from grape skins. Among the four tested solvents (i.e., water, tartaric acid, ammonium oxalate, and ethylenediaminetetraacetic acid), aqueous ammonium oxalate was the most effective for polysaccharide extraction but also produced the highest amount of protein impurities. Increasing the number of successive extractions or modifying the extraction time does not increase the amount of total extracted polysaccharides but produces more polysaccharides with a low molecular mass. The extraction yield of polysaccharides does not increase by increasing the tartaric acid concentration; however, the extraction yield increases by increasing the pH of the solvent or decreasing the solid to liquid ratio during the extraction. During the fermentative maceration of the skins, the extraction of soluble polysaccharides is significantly higher than that obtained with aqueous solvents. Thus, it seems that grape skins contain some pectic polysaccharides that are tightly bound to the cell wall matrix that are not immediately solubilized by aqueous buffers and are released under alcoholic fermentation conditions.