Biologically active compounds from Goji (Lycium barbarum L.) leaves aqueous extracts: purification and concentration by membrane processes

Goji (Lycium barbarum L.) leaves and fruits have been described as a valuable source of bioactive compounds with a great potential for the development of health-promoting formulations. The present study was aimed at evaluating the potential of a sustainable process for the recovery of phenolic compounds from Goji leaves through a combination of aqueous extraction and membrane-based operations. Water was used as a safe, cheap and non-hazardous extraction solvent and parameters of extraction of dried Goji leaves were optimized in order to maximize the yield of polyphenols, total soluble solids (TSS) and total antioxidants, simultaneously. The aqueous extract was clarified by ultrafiltration and then processed with three flat-sheet polyethersulphone (PES) membranes with molecular weight cut-off (MWCO) in the range of 0.3-4.0 kDa, in order to remove sugar compounds from polyphenols and improve the antioxidant activity of the produced fractions. Among the selected membranes a 1 kDa membrane exhibited the best performance in terms of purification of polyphenols from the clarified aqueous extract. The rejection of this membrane towards TSS and total carbohydrates was in the range of 15.8-25.3% and decreased by increasing the volume reduction factor (VRF). On the other hand, the retention values for total polyphenols and total antioxidant activity (TAA) were in the range of 73-80% and increased by increasing the VRF.