## 2 Materials and Methods
### Physicochemical Analysis  
.3.1 Determination of pH and Lactic Acid, Reducing Sugar, and Uronic Acid Contents in Pickled Apples  
Determination of pH. The pH was determined by direct measurement at 0, 7, 14, 21, 28 and 35 days of fermentation, using a benchtop pH meter InoLab 7310 (Xylem Analytics Germany Sales GmbH & Co, Weilheim, Germany). The measurements were performed in triplicate. Determination of lactic acid content. An efficient method to determine lactic acid in biological matrices and cultural liquids is spectrophotometric analysis of the colored product of the reaction of lactate ions with iron (III) chloride at 390 nm, as described by [20]. Briefly, a lactic acid solution with a concentration of 10 g/L was added to iron (III) chloride solutions with concentrations in the range of 0.05% to 0.3%. Spectrophotometric measurements were performed of iron (III) chloride solutions; the absorbance of the solution increased with an increase in the concentration of iron (III) chloride, reaching an optimum value at a concentration 0.2%. The method did not require complex sample preparation. The measurements were performed in triplicate. Determination of reducing sugar content. The reducing sugar content was determined using the 3,5-dinitrosalicylic acid (DNSA) method. The measurement was performed according to [21], with a small change. At 540 nm, the absorbance of the sample extract was measured using a UV-VIS spectrophotometer (Biochrom Ltd., Cambridge, UK). The measurements were performed in triplicate. Determination of uronic acid content. The quantity of uronic acid was measured using the colorimetric method described by [PERSON] et al. [22], with small modifications. Uronic acid  
Figure 1: Simplified method of brine-picked apple preparation. content was established after acid hydrolysis of the sample and reaction with carbazole. Briefly, 15 g of finely ground and well-homogenized apple samples were added to 100 mL using distilled water and allowed to stand for 30 min at room temperature, then centrifuged. Next, 1 mL of the clear supernatant was homogenized in a test tube with 3 mL Borax reagent (0.025 M in concentrated sulfuric acid) and 0.2 mL carbazole reagent (0.125% in absolute ethanol). The tubes were closed with Teflon stoppers and heated for 15 min in a boiling water bath, then cooled in an ice bath. The intensity of the purple-brownish color of the samples was measured on a UV-VIS spectrophotometer (Biochrom Ltd., Cambridge, UK) at 530 nm, in a 1 cm cuvette. A solution of D-galacturonic acid (100 ug/mL in distilled water) was used as a calibration standard. Determination of Sodium, Potassium, and Magnesium Contents  
Quantification of the metallic ions potassium (K), sodium (Na), and magnesium (Mg) was performed using the analytical method of high-resolution continuous source flame atomic absorption spectrometry (HR-CS-FAAS) on a ContrAAA 700 instrument (Analytik Jena, Germany). The xenon short-arc lamp (continuum source) and the high-resolution monochromator select the exact wavelength needed to measure the absorbance of each element. Before each working session, the instrument was calibrated using the ICP-MS multi-element standard solution IV 23 elements in diluted nitric acid. Before performing the final analysis, the ions were extracted in an acidic aqueous solution from calcined apples samples. In brief, approximately 1 g of the sample was weighed and introduced into Teflon vessels to extract the targeted ions in an acidic aqueous solution by digestion. Digestion was achieved using an extraction matrix of nitric acid (HNO\({}_{3}\) 65% Suprapur) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\) 30% Emsure). The acidified samples were placed in a TopWave microwave digestion system produced by Analytik Jena Germany, and a program of 200 \({}^{\circ}\)C temperature for 45 min was applied. After sample disaggregation, dilution with ultrapure water (volume = 50 mL) was carried out in previously rinsed Falcon tubes.