## 2 Materials and Methods
### Free Acidity, Peroxide Value (PV), and Specific Extinction Coefficients  
Free acidity, peroxide value and specific extinction coefficients K232, K268, and \(\Delta\)K in virgin olive oil samples were determined according to the standardized methods of the International Olive Council [39; 40; 41] (ISO, 2017a; ISO, 2017b; ISO, 2019). Deionized water was obtained with the Elix 3 purification system (Millipore, Bedford, MA, USA). Ethanol, 96% p.a., was a product of Alkaloid AD (Skopje, Republic of North Macedonia). Potassium hydroxide, diethyl ether, and chloroform, all of analytical quality, were manufactured by VWR International BV (Leuven, Belgium). Glacial acetic acid, and sodium hydroxide 0.1 mole/L were manufactured by VWR International SAS (Rosny-sous-Bois Cedex, France); sodium thiosulfate 0.02 mole/L was manufactured by Gram-Mol doo (Zagreb, Croatia). 2,2,4-trimethylpentane for spectroscopy was manufactured by Avantor Performance Materials (Glivicice, Poland). Extraction of the Olive Oil Hydrophilic Fraction and HPLC Analysis of the Composition of Phenolic Compounds  
The extraction and analysis of the composition and content of phenols were performed according to the standardized method of the International Olive Council [42], described under \"Method of analysis no.1\". To 2.0 g of virgin olive oil sample, exactly 1 mL of syringe acid at 0.015 mg/mL from Sigma-Aldrich Chemie GmbH (Steinheim, Germany) as internal standard was added and vortexed for 30 s (Multi Speed Vortex, model MSV-3500, Biosan, Riga, Latvia). Next, exactly 5 mL of methanol 80% (\(v/v\)) was added, and the mixture was vortexed for 1 min, sonicated for 15 min (Sonorex Digitec DT 100H; Bandelin, Berlin, Germany), and centrifuged at 4500 rpm for 25 min (Universal 320R, Andreas Hettich GmbH and Co. KG, Tuttlingen, Germany). The clear hydrophilic solution (bottom layer) was extracted with a Pasteur pipette and filtered through a PVDF filter with a porosity of 0.45 \(\upmu\)m.  
HPLC analysis of phenols was performed using an Agilent Infinity 1260 system equipped with a G1311B quaternary pump, a G1329B autosampler, a G1316A column oven, and a G4212B DAD detector (Agilent Technologies, Palo Alto, CA, USA). An octadecyl silica (C18) reverse-phase column (4.6 mm \(\times\) 250 mm, 5 \(\upmu\)m, pore 110 A), Agilent Omnispheric CP27835 (Agilent technologies, Santa Clara, CA, USA) was used, conditioned at 25 \({}^{\circ}\)C. Solvent was water, with 5% by volume of orthophosphoric acid 85% (\(v/v\)) from Sigma-Aldrich (St. Louis, MO, USA) (A), methanol (B), and acetonitrile (C), with a flow rate of 1 mL/min. Acetonitrile, methanol, and water HPLC grade were products of Sigma-Aldrich (St. Louis, MO, USA). 10 \(\upmu\)L of the extracts were injected. Elution gradient was as described in the method, with solvents B (methanol) and C (acetonitrile) always in equal parts, and solvent A (o-phosphoric acid, 85% solution in water 0.2% (\(v/v\))). The fraction 96% at the beginning, decreasing to 50% by the 40 th minute, 40% by the 45 th minute and to 0 by minute 60. The composition was left as isocratic for 10 min and, in two minutes, was rapidly changed to the starting composition. After 55 min, all compounds of interest were eluted but the total run time was 82 min for complete elution of all other compounds present in the extract. Chromotographic elution was followed and recorded at three wavelengths: 280 nm (closest to the maximal absorbance of simple phenols, vanilllic acid, lignans, and secooriridoids), 320 nm (vanillin and p-coumaric acid), and 365 nm (flavonoids). Identification was performed by comparing retention times and typical maximum absorbance wavelengths from the standardized method [42], as well as comparing recorded UV/V is spectra recorded from 200 to 600 nm with those of pure chemical standards when available, or with other literature sources [43].