Treatment of grape juice by osmotic evaporation

Osmotic evaporation (OE) was applied to grape juice to increase the sugar content and thus to improve the quality of wine obtained after fermentation. The use of aqueous solutions of glycerol as extractant avoids corrosion and scaling, which are the main problems in OE with brines. Preliminary experiments were performed on lab scale using water or glucose solutions as feed, and NaCl or glycerol as extractant. The runs with NaCl allowed measurement of the membrane transport properties, whereas glycerol was used to quantify the concentration polarization phenomena involved. The extract side concentration polarization severely limited the flux achievable for glycerol concentration above 50% wt, while the extract side concentration was negligible in the OE with NaCl. The feed side concentration polarization played a minor role for glucose content up to 40% wt. Experiments with grape juice were performed on pilot scale equipped with plate and frame modules. The performance were in good agreement with lab results on glucose model solutions, the lower flux observed was due only to the low temperature of operation. The process was tested on 3 types of grape juice (white, rosé, and red). Treated and control juices were fermented, and corresponding wines were analyzed for selected parameters and evaluated by sensory analysis. The OE treatment removed water from grape juice and increased both the total soluble solids and sugar content of juice, thus ethanol content and dry extract in wine always increased. The osmotic evaporation significantly improved sensory quality of red wine that was judged as having a full body, more structure, and persistence.