Biological evaluation of olive mill wastewater polyphenol-enriched in concentrated fractions by membrane operations: comparison of direct and sequential processes

The valorisation of olive mill wastewaters (OMWs) is a challenging opportunity for the green, sustainable, and competitive development of olive oil industry [1]. In this view, the combination of conventional and innovative membrane operations represent an useful approach for the recovery of phenolic compounds of OMWs contributing to the sustainability of olive waste sector, reducing its environmental impact and promoting the development of innovative formulations of interest for pharmaceutical, nutraceutical and cosmeceutical applications [2,3]. The present study evaluated, for the first time, sequential and direct membrane-based processes such as nanofiltration (NF), reverse osmosis (RO) and membrane distillation (MD) to produce concentrated polyphenol-enriched fractions that have been analysed for their chemical profile as well as for their antioxidant and hypoglycaemic activities. OMWs were at first pre-treated by microfiltration (MF), in order to remove suspended solids, and then concentrated directly by membrane distillation (MD) or submitted to a combination of NF/RO or NF/MD in a sequential design. The performance of direct and sequential operation modes were compared in terms of productivity (permeate fluxes) and efficiency in the concentration of compounds of interest and in the preserving the biologically activity of concentrated fractions. Experimental results showed that, at the same volume reduction factor, MD retentates exhibited the highest content of hydroxytyrosol, tyrosol, oleuropein, coumaric acid and caffeic acid. In particular, a content of hydroxytyrosol of 1.9 mg/L, about five times higher than the MF feed, was found. On the other hand, sequential MD retentate, showed the most ABTS and DPPH radicals scavenging activity with IC50 values of 0.9 µg/mL and 6.25 µg/mL, respectively. Moreover, the same fraction inhibited ?-amylase and ?-glucosidase with IC50 values of 41 and 32 µg/mL, respectively. In terms of productivity, direct and sequential MD process showed higher initial and steady state permeate fluxes in comparison with sequential RO. The whole results indicate that MD can be applied successfully to concentrate microfiltered OMWs or MF/NF pre-treated OMWs in order to obtain polyphenol-enriched fractions effecting in scavenging radicals and protecting lipid-oxidations and in inhibiting key enzymes such as lipase, ?-amylase, and ?-glucosidase, useful therapeutic targets for the development of functional products for obesity and diabetes type 2 prevention.