Clarification of blood orange juice by ultrafiltration: analyses of operating parameters, membrane fouling and juice quality

Blood orange juice was clarified by cross-flow ultrafiltration (UF) using tubular polyvinylidenefluoride (PVDF) membranes. In experimental tests performed according to the total recycle mode the effect of transmembrane pressure (TMP), axial feed flow-rate and temperature on permeate fluxes was studied. The clarified juice was produced in experimental tests carried out according to the batch concentration mode working in optimal operating and fluid dynamic conditions. A theoretical evaluation of the fouling effects on flux performance was performed by using a modified form of the differential equation used to describe classical dead-end filtration. The fouling mechanism during cross-flow UF was identified by estimation of the model parameters according to a nonlinear regression optimization procedure. The theoretical predictions, in terms of permeate flux as a function of time, resulted in a good agreement with the experimental data. Analysis of the results revealed that, in the fixed operating conditions of TMP and temperature, the fouling mechanism evolves from a partial to a complete pore blocking condition in dependence of the axial velocity. The quality of the samples coming from the UF process was evaluated in terms of: total soluble solids (TSS), suspended solids (SS), total antioxidant activity (TAA), ascorbic acid, flavonones and anthocyanins content. The resulting clarified orange juice was highly similar to the initial juice except for insoluble solids which were concentrated in the retentate stream. In the permeate of the process a low reduction of the TAA (1.5%) was observed with respect to the fresh juice.