*2.1. Chemicals*

Solvents, reagents, standards, silica gel 60 F254 plates, and silica gel 60 were purchased from Sigma-Aldrich (Milan, Italy). Hydroxytyrosol was prepared by the IBX oxidation of tyrosol as already reported [19]. A patented procedure based on membrane technology was applied to obtain hydroxytyrosol-enriched extracts from *Olea europaea* by-products [45]. The vegetal material was extracted with water and then subject to microfiltration (MF), nanofiltration (NF), and reverse osmosis (RO). Microfiltration was carried out with tubular ceramic membranes in titanium oxide; nanofiltration and reverse osmosis were carried out using spiral wound module membranes of poly(ether sulfone) [45]. The collected fraction was concentrated using a heat pump evaporator (Vacuum Evaporators—Scraper Series, C&G Depurazione Industriale Srl, Firenze, Italy) [3,4]. High Performance Liquid Chromatographic (HPLC) analysis of the resulting extracts was carried out at λ = 280 nm using a diode array detector (DAD) to identify the polyphenolic compounds found in the sample. The quantitative data were referred to pure standards (tyrosol, hydroxytyrosol, oleuropein, and caffeic acid).
