Concentration of marc extracts by membrane techniques

By-products obtained from red currant processing still contain large amounts of useful components, e.g. pectin. Pectin was extracted from red currant marc with water at a solid/liquid ratio of 1:10. To reduce the operating costs of further possessing, we concentrated the pectin solution by membrane separation, i.e. nanofiltration (NF) and reverse osmosis (RO). The objectives of our work were to study the effects of the operating pressure and cross-volume flow rate on the flux and on the membrane separation concentration ratio in order to establish the optimum operating conditions and to evaluate the contributions of the fouling, cake and membrane resistances to the overall resistance. Flat-sheet RO and a spiral-wound NF membrane were applied in the work. The conductivity, the color, the viscosity and the TSS of the permeate and the concentrate were followed during the measurements. Concentration by RO resulted in an increase of the TSS content to 4.28Brix; for NF the corresponding level was 8.88Brix. The membrane resistance and the fouling resistance were the determinant relative to the gel resistance.

These results revealed that the red currant marc extract could be concentrated successfully both by NF and RO; the permeate was clear, and there was neither colorant nor pectin loss. . The optimum running pressure for both NF and RO systems was 3.0 MPa; the J vs. TMP functions were of saturated type in all cases. The maximum total solid content for NF1 was 8.88Brix, but due to the very high total resistance (1.5 )1015 m(1), the permeate flux was approximately four times lower than that for NF2 (average flux for NF1: 12.44 L m(2 h(1 and for NF2: 40.99 L m(2 h(1), while the TSS of the NF2 concentrate was 6.38Brix. The maximum total solid content for RO2 was 6.78Brix and the permeate flux was also higher in this case (32.43 L m(2 h(1) than for RO1 (average flux for RO1: 8.17 L m(2 h(1 and TSS for RO1: 4.28Brix). The membrane resistance and the fouling resistance were the determinants relative to the gel resistance. The RF and RG ratio was less than 1 for the NF2 system only (the values for NF1, NF2, RO1 and RO2 were 19.12, 0.68, 7.69 and 4.80, respectively), due to the smallest SR value being observed for the NF2 system. The small SR value meant less fouling than in the other systems, and hence the ratio of the gel resistance was higher for NF2, but its absolute value was quite the same as in the other cases. For the RO1 system, the measured gel layer resistances were 8.32 )1014 and 1.04 )1015 at cross-volume flow rate of 500 L h(1 and 400 L h(1, respectively. This means that an 80% increase in the recycling capacity caused only a 25% decrease in the gel resistance.