## 3 Results and Discussion  
### Browning Rates  
The accelerated browning test is a reliable test commonly employed for evaluating the browning development of white wines [14; 26]. The absorbance at 420 nm (A\({}_{420}\)) could be considered as a browning index, since it measures the concentration of oxidation pigments and displays a linear increase with time (zero order reaction). The reaction rate constants (k) (A\({}_{420}\) xdays\({}^{-1}\)) of browning development were calculated from the slope of the regression lines, obtained after plotting A\({}_{420}\) as a function of time according to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample Code** & **FeII (mM)** & **H\({}_{2}\)O\({}_{2}\) (mM)** & **Se (mg/L)** \\ \hline M & – & – & – \\ Fe min & 0.10 & 0.63 & – \\ Fe max & 0.025 & 0.157 & – \\ Se min & – & – & 1.5 \\ Se max & – & – & 3.0 \\ All min & 0.10 & 0.63 & 1.5 \\ All max & 0.025 & 0.157 & 3.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Wine samples used in this experiment. [PERSON] et al. [14]. Table 2 presents k-values and percentages of changes in browning (absorbance at 420 nm) calculated for all samples at 35 and 50 \({}^{\circ}\)C after 12 days. As it can be seen, the calculated k values ranged from 1.0 to 45.5. These values fell within the range obtained from previous studies [14; 26] concerning Greek wines. The wines that showed the lower k values were the control and those that contained Se (both high and low concentrations). In practice, these samples would develop brown color later than the other samples analyzed in this experiment. As far as the authors are aware, this is the first time that Se addition was studied in relation with browning development in wines. It was very interesting to note that at 35 \({}^{\circ}\)C, wine color did not change after 12 days of heating, while at 50 \({}^{\circ}\)C, the rate of browning development was not different than that of the control wine. However, at both temperatures, the simultaneous addition of Se with Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\) resulted in increased k rates. It is possible that Se might have been able to offer protection from oxidation at lower temperatures and in the absence of metals. However, when temperature was increased, and in the presence of oxidant compounds, the inhibition of oxidation was not observed any longer. This might have been probably due to increased contents of ROS which were generated by the increased temperature and were further enhanced by the presence of Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\). Transition metals such as Fe play a key role in ROS production since they react with hydrogen peroxide to generate a hydroxyl radical [20]. The added Se content in wine was probably not adequate to exert its antioxidant properties though ROS scavenging and metal-binding mechanisms. On the other hand, the wines with the higher k values were those made by the addition of Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\). Browning rate was increased 1.4 and 4.7 times at 35 \({}^{\circ}\)C for the low and high concentrations of Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\) while at 50 \({}^{\circ}\)C, browning occurred about two and six times faster, respectively. Se did not affect significantly browning rates when it was added simultaneously with Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\). Hydrogen peroxide, which is formed during oxygen reduction, in the presence of Fe\({}^{2+}\) can be converted to hydroxyl radicals by the Fenton reaction. These radicals together with hydrogen peroxide act as nonselective and strong oxidants, oxidizing most of the wine components [10]. In addition, quinones may be produced by direct oxidation by Fe\({}^{2+}\) at the acidic pH of wine, leading to condensation reactions and further development of brown color [36]. Similar acceleration of browning development after Fe\({}^{2+}\) and H\({}_{2}\)O\({}_{2}\) addition was observed by [PERSON] and [PERSON] [24] in Chardonnay wines during storage for 40 days and nine months, respectively.