AbstractBACKGROUNDThe recovery of phenolic compounds from olive milling is recognized as strategic for producers. The aim of this work was to evaluate the quality and stability of retentates obtained from olive mill wastewaters treated with a membrane filtration system constituted by a micro‐, ultra‐ and nanofiltration, followed by a final reverse osmosis, over three crop seasons. Efficiency was evaluated in terms of phenolic amount in the retentates and of organic load in the final discarded waters. Phenolic compounds were quantified using tyrosol as external standard.RESULTSOur study highlighted a reproducibility of the process over years and a low organic load in permeates from reverse osmosis. Hydroxytyrosol was very stable in the liquid products at 18–28 °C over 24 months of storage. The retentates from reverse osmosis showed the highest phenolic content (78.6 mg g⁻¹ dry matter in 2015), associated with a potassium content of 22 g kg⁻¹.CONCLUSIONSThe liquid concentrated retentates showed an unexpected stability over time of their bioactive phenolic compounds, particularly of hydroxytyrosol. These samples recovered from olive mill wastewaters can be good sources of natural antioxidants and potassium to guarantee the correct intake and to formulate new food ingredients or food products. © 2017 Society of Chemical Industry