Concentration of clarified orange juice through direct contact membrane distillation

The conventional unit operations employed for the concentration of fruit juices are usually thermal evaporation or osmotic membrane distillation (OMD). With respect to thermal evaporation, both OMD and DCMD present the advantage to operate at lower temperature thus preserving the nutritional and organoleptic properties of the fresh juice. With respect to OMD, water and not a stripper is present on the permeate side of DCMD thus further increasing the sustainability of the process. The aim of this study was to evaluate the potentiality of direct contact membrane distillation (DCMD) for concentrating clarified orange juice on laboratory scale. In particular, an integrated membrane based system constituted by a roughly pretreatment (for the removal of seeds, fibres and pectines) followed by ultrafiltration and membrane distillation units was utilized. Suspended solids and proteins were removed by UF in order to obtain a clarified juice (permeate) to be further concentrated through DCMD. The performance of the process in terms of flux and concentration, of both UF and DCMD, was evaluated. Different samples of all the process streams were analyzed in terms of total suspended solids (TSS), sugar concentration (expressed as °Brix), total antioxidant activity (TAA) and phenolic compounds. The clarified juice, with an initial concentration of around 9°Brix, was concentrated up to final values of 65°Brix. The experimental data indicate that the trans-membrane flux decay was attributable to the increasing of the juice viscosity. In fact, trans-membrane flux normalized with respect to the driving force showed an almost stable and constant trend throughout the experimental time. This observation combined with the TAA and sugar content of the MD permeate (equal to 0.2 mM Trolox and 0°Brix) indicate that the membrane module shows no tendency to wetting and fouling throughout the juice treatment. Moreover, analytical measurements proved that all the antioxidant compounds of the clarified orange juice were concentrated through the MD unit. Therefore, the UF/DCMD integrated process allows to produce a concentrated juice preserving the organoleptic and nutritional properties of the fresh juice.