Separation and purification of phenolic compounds from pomegranate juice by ultrafiltration and nanofiltration membranes

Pomegranate juice is well recognized for its nutritional and health benefits due to the presence of phenolic compounds, including anthocyanins, ellagic acid, phytoestrogenic flavonoids and tannins. Therefore, the demand for the production of functional foods containing bioactive compounds isolated from the juice has remarkably increased in the last decade. In this study ultrafiltration (UF) and nanofiltration (NF) flat-sheet membranes, with nominal molecular weight cut-off (MWCO) ranging from 1000 to 4000 Da, were tested to purify biologically active compounds from clarified pomegranate juice. The filtration process was evaluated in a crossflow pilot unit equipped with a Sepa CFII Membrane Cell System featuring an effective membrane area of 0.014 m2. A first screening was made in order to evaluate the performance of selected membranes in terms of productivity, fouling index and retention towards sugars, phenolic compounds and total antioxidant activity. Among these membranes the Desal GK membrane, with a MWCO of 2000 Da, displayed higher permeate fluxes, lower fouling index and a good separation efficiency of sugars from phenolic compounds in comparison with the other tested membranes. Therefore further experiments were addressed to evaluate the separation capability and the productivity of this membrane at different transmembrane pressure (TMP) values. Concentration/diafiltration experiments were also performed in order to obtain a retentate fraction enriched in phenolic compounds and a permeate stream mainly containing glucose and fructose. According to the proposed process the yields of polyphenols and anthocyanins in the retentate stream were of the order of 84.8% and 90.7%, respectively. The diafiltration step allowed to obtain a recovery efficiency in the permeate side for glucose and fructose up to 90% and 93%, respectively.