## 2 Materials and Methods
### Quantitative Determination of Proanthocyanins
#### 2.2.7 Determination of Antioxidant Action
Determination of antioxidant action by the electrochemical method [56; 57; 58]  
Volumetric studies were performed with a 273 A EG&G (potentiostat/galvanostat) electrochemical system (Princeton Applied Research/EG&G, USA) with a type KO264 three-electrode microcell with a total working volume of 3 to 15 mL. Millielectrodes (effective electrode area: 3.14E-2 cm\({}^{2}\)) of solid gold (Au) were used as working electrodes, depending on the electroactivity manifested by each compound investigated, with a Pt wire auxiliary electrode and a Ag/AgCl (3M KCl) reference electrode. The used electrochemical techniques were cyclic voltammetry (CV), linear potential sweep voltammetry (LSV), and square wave voltammetry (SWV). The inert electrolyte was H\({}_{2}\)SO\({}_{4}\) 0.12 mol L\({}^{-1}\) in methanol solution. The used reagents were methyl alcohol, sulfuric acid, Trolox, and 2,2'-diphenyl-1-picrylhydrazyl (DPPH). They were of analytical purity and obtained from Sigma-Aldrich, Darmstadt, Germany. All voltametric measurements were made at 25 \({}^{\circ}\)C in an inert atmosphere after deaeration of the analyzed solutions. The Trolox stock solutions were 10\({}^{-3}\) mol L\({}^{-1}\) methanolic solutions freshly prepared and maintained at 4 \({}^{\circ}\)C and in the dark so as to avoid the formation of oxidative degradation compounds. The stock solutions of DPPH were 3 \(\times\) 10\({}^{-3}\) mol L\({}^{-1}\) alcoholic solutions. UV-VIS spectrometric determinations were performed using a JASCO V-530 spectrometer (JASCO INTERNATIONAL CO., LTD., Japan). In the first step, a calibration curve was made for Trolox alone, without free radicals in the system, in the concentration range between 10\({}^{-6}\) mol L\({}^{-1}\) and 10\({}^{-4}\) mol L\({}^{-1}\) to ensure the accuracy of the method. The square wave voltammetry (SWV) investigation technique and the Trolox peak response with a potential of +0.785 \(\pm\) 0.015 V were used. The obtained calibration line (Figure S1 in the Supplementary Materials File), with a good correlation coefficient value (0.9698), defines a linear range of the response in two intervals between 3 \(\times\) 10\({}^{-6}\) mol L\({}^{-1}\) and 1 \(\times\) 10\({}^{-4}\) mol L\({}^{-1}\). The subsequent experimental step was the electroanalytical characterization of the free radical DPPH, which has a peak potential of around 0.915 \(\pm\) 0.015 V and whose signal response is extremely sensitive to concentration (Figure S2 in the Supplementary Materials File). Once the characteristic electroanalytical parameters of the compounds were established, the antioxidant capacity of Trolox was determined against the DPPH free radical. For this, the linear dependencies between the decrease in the current intensity of the DPPH peak and the increase in the Trolox concentration in the working cell were obtained and considered the calibration lines for the antioxidant activity. A total of 5 mL of inert electrolyte and 0.12 M H\({}_{2}\)SO\({}_{4}\) in CH\({}_{3}\)OH were deaerated for 4 min by purging Argon in the system; then, 1 mL of DPPH from the 10\({}^{-3}\) M stock solution was introduced, again purging the Argon for 30 s. The corresponding DPPH voltammogram was registered, then volumes between 10 \(\mu\)L and 70 \(\mu\)L of the Trolox stock solution were added to the system in order to obtain the dependence between the decrease in the anodic peak current intensity of the DPPH and the Trolox concentration in the system. The corresponding voltammogram was displayed 2 min and 4 min after the addition of Trolox. The obtained lines are shown in Figure S3 in the Supplementary Materials File. In Figure S3, it can be observed that for 2 min the sensitivity of the slope was satisfactory and, also, the correlation coefficient that was obtained for the linear dependence, which gives the equivalent value of the antioxidant capacity, was satisfactory. Values are the average of three determinations.