## 3 Results and Discussion
### Antioxidant Activity of Beetroot Juices
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**Organic Beetroot Juice**} & \multicolumn{2}{c}{**\(\mathbf{-19\,^{\circ}C}\)**} & \multicolumn{2}{c}{**4 \(\,{}^{\circ}\)C**} & \multicolumn{2}{c}{**20 \(\,{}^{\circ}\)C**} \\ \hline \multirow{2}{*}{**Compound**} & \multicolumn{2}{c}{**Initial**} & **4 Days** & **8 Days** & **4 Days** & **8 Days** & **4 Days** & **8 Days** \\ \hline Betanin & 763 \(\pm\) 25 \({}^{\mathrm{a}}\) & 740 \(\pm\) 32 \({}^{\mathrm{a}}\) & 675 \(\pm\) 31 \({}^{\mathrm{b}}\) & 725 \(\pm\) 30 \({}^{\mathrm{c}}\) & 650 \(\pm\) 25 \({}^{\mathrm{c}}\) & 690 \(\pm\) 27 \({}^{\mathrm{b}}\) & 656 \(\pm\) 19 \({}^{\mathrm{c}}\) \\ Vulgaxanthin I & 413 \(\pm\) 17 \({}^{\mathrm{a}}\) & 393 \(\pm\) 17 \({}^{\mathrm{b}}\) & 360 \(\pm\) 15 \({}^{\mathrm{c}}\) & 371 \(\pm\) 18 \({}^{\mathrm{c}}\) & 341 \(\pm\) 13 \({}^{\mathrm{d}}\) & 350 \(\pm\) 15 \({}^{\mathrm{d}}\) & 312 \(\pm\) 12 \({}^{\mathrm{e}}\) \\ pHBA & 9.00 \(\pm\) 3.47 \({}^{\mathrm{a}}\) & 4.33 \(\pm\) 0.210 \({}^{\mathrm{b}}\) & 4.10 \(\pm\) 0.181 \({}^{\mathrm{b}}\) & 391 \(\pm\) 0.150 \({}^{\mathrm{b}}\) & \(\sim\)LOD & 3.92 \(\pm\) 0.111 \({}^{\mathrm{b}}\) & \(\sim\)LOD \\ Gallic acid & 0.20 \(\pm\) 0.01 \({}^{\mathrm{a}}\) & 0.11 \(\pm\) 0.01 \({}^{\mathrm{b}}\) & 0.12 \(\pm\) 0.01 \({}^{\mathrm{b}}\) & 0.10 \(\pm\) 0.02 \({}^{\mathrm{b}}\) & 0.060 \(\pm\) 0.01 \({}^{\mathrm{c}}\) & 0.10 \(\pm\) 0.02 \({}^{\mathrm{b}}\) & 0.05 \(\pm\) 0.01 \({}^{\mathrm{c}}\) \\ Ferulic acid & 16.2 \(\pm\) 0.60 \({}^{\mathrm{a}}\) & 0.27 \(\pm\) 0.02 \({}^{\mathrm{b}}\) & 0.23 \(\pm\) 0.01 \({}^{\mathrm{c}}\) & 0.30 \(\pm\) 0.03 \({}^{\mathrm{d}}\) & \(\sim\)LOD & 0.25 \(\pm\) 0.02 \({}^{\mathrm{c}}\) & 0.21 \(\pm\) 0.01 \({}^{\mathrm{d}}\) \\ EGCG & 26.1 \(\pm\) 0.904 \({}^{\mathrm{a}}\) & \(\sim\)LOD & \(\sim\)LOD & \(\sim\)LOD & \(\sim\)LOD & \(\sim\)LOD & \(\sim\)LOD \\ Epicatechin & \textFrom the group of flavonoids, various catechins were found. Epigallocatechin-3-gallate (EGCG) in beetroot juices was very unstable and completely disappeared after 2 days, regardless of the storage temperature. The EGCG is thermally sensitive and undergoes epimerization to generate gallic acid, catechin, or epicatechin during the processing of foods, and its stability is influenced by pH value and food components [53]. In contrast to other published data [54; 55], we did not detect other flavonoids (such as kaempfer, quercetin, or apigenin), most probably due to their low abundance in the studied juices or their occurrence in conjugated forms, which are not extracted just by the water. Generally, the observed decrease in antioxidant activity of studied beetroot juices may be caused by the instability and degradation of selenium species, as well as other compounds with antioxidant properties. Regarding selenium species, their antioxidant activity in the DPPH assay decreases in the following order: MeSeCys > SeMet \(\approx\) Se(VI) > Se(IV), and for all species, the values were lower than for \(\alpha\)-tocopherol at the same concentration [56]. Selenomethonine and SeMetO exhibited similar antioxidant properties in DPPH assay. The reducing capacity determined by the CUPRAC method (cupric reducing antioxidant capacity) increases as follows: Se(IV) < SeMetCys < Se(VI) < SeMe, indicating that selenomethonine may be easily oxidized.