Vanillic acid

Phenolic standards, namely, catechin, catechol, syringic acid, chlorogenic acid, cinnamic acid, ellagic acid, kaempferol, ferulic acid, gallic acid, rutin, caffeic acid, naringenin, benzoic acid, o-coumaric acid, p-hydroxybenzoic acid, pyrogallol, p-coumaric acid, quercetin, quinol, rosmarinic acid, and vanillic acid were bought from Merck KGaA (Darmstadt, Germany). The purities of standards were up to 99%. The solvents used were of analytical grade, dimethylsulfoxide (DMSO; Alfa Aesar GmbH & Co KG, Massachusetts, United States), orthophosphoric acid (H₃PO₄), ethanol, methanol, and acetonitrile HPLC-grade (Fisher Scientific International, Inc., Hampton, New Hampshire, United States).

For the chromatographic analysis, methyl alcohol, n-hexane, isopropyl alcohol, and glacial acetic acid, all of HPLC grade, and water, methyl alcohol, and glacial acetic acid, all of LC-MS grade, were obtained from VWR (Milan, Italy). Analytical- and HPLC-grade water was obtained using purification systems. Phenolic compounds such as (p-hydroxyphenyl) ethanol (p-HPEA) and (3,4-dihydroxyphenyl) ethanol (3,4-DHPEA), vanillic acid, caffeic acid, and α-tocopherol were supplied by Merck (Milan, Italy). The isomer of oleuropein aglycon (3,4-DHPEA-EA), the dialdehydic forms of elenolic acid linked to tyrosol and hydroxytyrosol (p-HPEA-EDA and 3,4-DHPEA-EDA, nowadays named oleconthal and oleacein, respectively), and lignans (+)-1-acetoxypinoresinol and (+)-pinoresinol were extracted from a VOO as reported by Selvaggini et al. [33 (link)]. Pure analytical standards of volatile compounds were purchased from Merck (Milan, Italy).

The used standards are protocatechic acid (Koch-Light Laboratories LTD, Bucks, UK), cinnamic acid (Rhône-Poulenc, Paris, France), chlorogenic acid (Sigma-Aldrich, St. Louis, MO, USA), vanillic acid (Merck, Darmstadt, Germany), gallic acid (Prolabo, Paris, France), rosmarinic acid (Extrasynthèse, Genay, France), Coumarin (Behringer, Willich, Germany), apigenin (obtained from Carl Roth, Karlsruhe, Germany), ferulic acid, and caffeic acid (purchased from Sigma, USA) as well as herniarine, luteolin, and myricetin (purchased from Sarsyntex, Merignac, France). Methanol (Carlo Erba Reagents, Val-de-Reuil, France) and formic acid (Carlo Erba Reagents^TM, Cornaredo, Italy) were of HPLC grades. Acarbose, α-amylase and α-glucosidase were purchased from Sigma Aldrich (St. Louis, MO, USA). D(+)-glucose anhydrous, sucrose, starch and phloridzin were purchased from Sigma Aldrich (Riedel-de Haen, Seelze, Germany). Glucose oxidase-peroxidase (GOD-POD) kit was purchased from Biosystems (Barcelone, Spain). All the chemicals used were of analytical grades.

Microfiltered and ultrapure water was used for the preparation of the solutions via the Merck Millipore ZRQS0P3FR Direct system. All solvents and reagents used in the experiments were of a high degree of purity. Ethanol ≥99.9% ACS for analysis, methanol for HPLC, anhydrous 95% n-hexane, chloroform for chromatography, 1-butanol ACS reagent ≥99.5%, hydrochloric acid 37% RPE for analysis, potassium bicarbonate, anhydrous sodium carbonate for analysis, gallic acid ACS for analysis and Sudan IV reagent were purchased from Sigma-Aldrich Chemical Company (Milan, Italy). Folin-Ciocălteu reagent and hypergrade acetonitrile for LC-MS were purchased from Merck Millipore GmbH (Milan, Italy). 2,2-diphenyl-1-picrylhydrazyl (DPPH·) was obtained from Alfa Aesar (from Thermo Fisher Scientific companies in Rodano, Milan, Italy). Polyphenol standards (malvidin-3-O-glucoside, naringin, catechin, quercetin, gallic acid, vanillic acid, caffeic acid, ferulic acid), amino acid standard mixture, methanol, acetic acid, gallic acid, were purchased from Merck (Darmstadt, Germany); 2-propanol and acetonitrile (ACN) from Honeywell (Charlotte, USA), formic acid by J.T. Baker (Rodano, Italy).

Polyphenol standards (malvidin-3-O-glucoside, naringin, catechin, quercetin, gallic acid,
vanillic acid, caffeic acid, and ferulic acid), methanol, acetic acid,
the Folin–Ciocâlteu (FC) reagent, gallic acid, and sodium
carbonate were purchased from Merck (Darmstadt, Germany); 2-propanol
and acetonitrile (ACN) were purchased from Honeywell (Charlotte, USA);
and formic acid was supplied by J.T. Baker (Rodano, Italy).

The reagents sodium carbonate, di-sodium hydrogen phosphate, sodium carbonate, potassium ferricyanide, potassium persulfate, ethylenediamine tetra acetic acid (EDTA) and gallic acid were purchased from Scharlau (Barcelona, Spain). Folin–Ciocalteu’s phenol reagent, absolute ethanol and trichloroacetic acid (TCA) were purchased from Panreac (Castellar del Vallès, Barcelona, Spain). 2,2-diphenyl-1-picrylhydrazyl (DPPH•) free radical, Iron (II) Chloride 4-hydrate, 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylacid (Trolox), (L)-Dehydroascorbic acid and malondialdehyde (MDA) were purchased from Sigma Chemical Co. (Steinheim, Germany, and St. Louis, MO, USA). Propylgallate (PG) was purchased from Acrōs Organics (Fair Lawn, NJ, USA). 2-Thiobarbituric acid (TBA) was purchased from Merck KGaA (Darmstadt, Germany). The following standards were used: phenolic acids—hydroxybenzoic acid, vanillic acid, protocatechuic acid; cinnamic acids—chlorogenic acid, 4-coumaric acid; flavanols—catechin; flavonols—kaempferol, quercetin, kaempferol-3-O-rutinoside, kaempferol-3-O-glucoside, isorhamnetin-3-O-rutinoside, rutin or quercetin-3-O-rutinoside; flavanones—naringenin, eriodictyol, eriodictyol-7-O-glucoside (Merck KGaA, Darmstadt, Germany).

All reagents (vanillin, VA, vanillyl alcohol, vanillic acid, ferulic acid and l‐alanine) were of analytical grade and were purchased from Merck KGaA (Darmstadt, Germany). The antibiotics ampicillin and streptomycin and isopropyl‐β‐D‐thiogalactoside (IPTG) were purchased from Sigma‐Aldrich (St. Louis, MO, USA). Restriction enzymes, T4 DNA ligase and Phusion High‐Fidelity DNA Polymerase utilized for gene cloning were ordered from Thermo Fisher Scientific (Vilnius, Lithuania). All DNA oligonucleotides (primers) utilized for cloning and sequencing were synthesized by Eurofins Genomics (Ebersberg, Germany).