The effect of ultrafiltration on the subsequent concentration of grape juice by osmotic distillation

The potential for osmotic distillation flux enhancement in grape juice concentration by ultrafiltration pretreatment has been investigated using ultrafiltration membranes with a range of pore diameter/MWCO values. Juice viscosities were determined over the concentration range to assist with interpretation of the flux data. Ultrafiltration using membranes with pore diameters of 0.1 mu m or less resulted in appreciable osmotic distillation flux increases over that observed for juice not subjected to ultrafiltration. These flux increases have been attributed to a reduction in the viscosity of the concentrated juice-membrane boundary layer as the result of protein removal. HPLC measurements showed that the normal concentration of fermentable sugars in standard 68 degrees Brix concentrate can be achieved at a lower Brix value with UF permeate, thereby providing a possible means of reducing the handling of highly viscous streams. Ultrafiltration also resulted in an increase in juice surface tension with a consequent reduction in the tendency for membrane wet-out to occur. (C) 2000 Elsevier Science B.V. All rights reserved.