## 3 Results and Discussion
### Stability of Selenium Species in Beetroot juices  
Two series of both beetroot juices under study were prepared, without and with the addition of reductive thiols, DDT and \(\beta\)ME, respectively. They were stored at different temperatures (20 \({}^{\circ}\)C, 4 \({}^{\circ}\)C, and \(-\)19 \({}^{\circ}\)C) for various periods of time, and then the concentrations of selenium species were determined. The obtained results as a function of time storage for samples without thiol addition are illustrated in Figure 1. Detailed concentrations of selenium species in these samples are presented in Table S1 in the Supplementary Materials. The concentration of SeMet in homemade prepared juice without thiol addition was rapidly decreased, regardless of the storage temperature and, after 2 days of storage, its concentration was below the LOD value. In contrast, the concentrations of SeMetO were increased in this juice samples stored at all temperatures in the following order: 20 \({}^{\circ}\)C \(<-\)19 \({}^{\circ}\)C \(<\) 4 \({}^{\circ}\)C after 8 days. In similar samples of organic beetroot juice, a fast decrease of SeMet concentration was also observed, although after 6 days of storage at 20 \({}^{\circ}\)C still retains about 45% of its initial concentration. \begin{table}
\begin{tabular}{c c c c c} \hline
**Juice** & **Total Se** & **MeSeCys** & **SeMet** & **SeMetO** & **Se(IV)** \\ \hline Purchased organic & 1.85 \(\pm\) 0.003 a & 0.29 \(\pm\) 0.004 a & 0.11 \(\pm\) 0.005 a & 0.94 \(\pm\) 0.008 a & 0.45 \(\pm\) 0.007 a \\ Homemade squeezed & 3.60 \(\pm\) 0.004 b & 0.28 \(\pm\) 0.003 a & 0.08 \(\pm\) 0.001 b & 1.44 \(\pm\) 0.006 b & 1.73 \(\pm\) 0.008 b \\ \hline \end{tabular}
\end{table} 
Table 1: Selenium species in beetroot juices (in mg L\({}^{-1}\)). Figure 1: The changes in concentrations of selenium species in purchased organic and homemade squeezed beetroot juice without the addition of thiols stored at **(A)** 20 \({}^{\circ}\)C, **(B)** 4 \({}^{\circ}\)C, and **(C)** \(-\)19 \({}^{\circ}\)C as a function of storage time. Different letters in each point of the graph indicate a difference at a significance level of \(p=0.05\), showed a decrease in Se(IV) response after storage and that only 7% of this selenium form was present in the water extract after 4 days at 4 \({}^{\circ}\)C [18]. The MetSeCys was also highly unstable, and the biggest losses of this Se compound were observed for squeezed juice, particularly when stored at room temperature. The concentrations of Se(IV) in both juices under study increased with time storage, especially when they were stored in the fridge and freezer for longer than 4 days. This form of selenium may be one of the postulated products of the oxidation of SeMet (as well as other selenoamino acids) [36; 41]. Moreover, it has been shown that SeMet oxidation was more favourable as the temperatures decreased, which might affect the stability of Se-enriched food on storage [17; 35]. From the other side, speciation analysis of Se in buckwheat showed a decrease in Se(IV) response after storage and that only 7% of this selenium form was present in the water extract after 4 days at 4 \({}^{\circ}\)C [18]. Figure 2 shows the chromatograms of selenium species in organic and squeezed beetroot juices after the addition of different concentrations of DTT or \(\beta\)ME at 20 \({}^{\circ}\)C. The results for other temperatures of storage are included in Table S2. The presence of reductive thiols, according to the literature data [21; 39], should decrease the concentrations of SeMetO reversing the oxidation process of selenomethionine. However, in both juice samples, SeMet concentrations decreased below the limit of detection (0.05 \(\mu\)g L\({}^{-1}\)) after the addition of thiols, regardless of their concentration. On the contrary, SeMetO concentration increased, as if the oxidation process of selenomethionine occurs after the addition of thiols.