## 3 Results and Discussion
### Fermentation Process and Product Characterization
#### 3.3.2 Second Fermentation Stage  
Acetic Fermentation  
During acetic fermentation, samples were collected from the reactor and analyzed for their total acidity and alcohol content. The results are presented in Figure 8.  
As observed in Figure 8, during the final stage of fermentation, the alcohol produced in the alcoholic fermentation step was converted into acetic acid. As a result, the alcohol content decreased while the total acidity increased. The process was considered complete when the total acidity reached the desired level, which in this case was 5.42 \(\pm\) 0.24%. The total acidity began to rise sharply after 72 h and reached its final value after 200 h (on the ninth day) of fermentation. This finding was consistent with the results from [PERSON] et al. (2013), who reported an acetic acid concentration of 5.6% in the final tomato  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Properties** & **Value** \\ \hline Lycopene content (mg/100 g) & 10.02 \(\pm\) 0.64 \\ Initial sugar content (\({}^{\circ}\)Brix) & 12.21 \(\pm\) 1.05 \\ Final sugar content (\({}^{\circ}\)Brix) & 3.73 \(\pm\) 0.91 \\ Alcohol content (\%) & 5.58 \(\pm\) 0.21 \\ pH & 4.20 \(\pm\) 0.10 \\ TDSs (g/100 g) & 3.81 \(\pm\) 0.84 \\ \hline \hline \end{tabular} Values are presented as means \(\pm\) standard deviations (\(\mu\) = 3).  
\end{table}
Table 4: Properties of the alcoholic fermentation product.  
Figure 7: Changes in the sugar (\({}^{\circ}\)Brix) and alcohol (\(\%\)) contents during the alcoholic fermentation stage.  
vinegar after the acetic acid fermentation process [22]. Similarly, the alcohol content declined significantly after 72 h and continued to decrease gradually until the ninth and final day, when it was nearly depleted. The final total acidity value of 5.42 \(\pm\) 0.24%--the maximum achievable under these conditions--was attained by adjusting the initial sugar concentration to 12-13 \({}^{\circ}\)Brix and was reached after a total fermentation period of 13 days.  
Measurements were conducted on the final product of acetic fermentation to characterize its properties, with the results summarized in Table 5. Based on the lycopene content of the acetic acid product (9.19 \(\pm\) 0.70 mg/100 g) and considering that the initial lycopene concentration in the tomato juice before fermentation was 12.15 \(\pm\) 0.75 mg/100 g, it was concluded that the final product retained most of the lycopene from the raw material. This means that the product also preserves its beneficial antioxidant properties. Furthermore, since the minimum acidity required for a product to be legally classified as vinegar is 5.0%, the tomato vinegar produced using this method met this standard and could be marketed accordingly [37]. Comparing the acidity (5.42 \(\pm\) 0.24%) and pH (2.85 \(\pm\) 0.10) values of the final product with those of commercial white vinegar (6% acidity and pH 3), a high degree of similarity is observed. [PERSON] et al. (2017) developed a tomato vinegar through a two-stage fermentation process, achieving a pH value of 3.08 and a low alcohol content, which aligns with the results of the present study [38]. Finally, the final total acidity fell within the expected range, as it was indirectly adjusted at the beginning of the process through the initial sugar content of the juice (12.2 \({}^{\circ}\)Brix).  
Figure 8: Changes in total acidity (%) and the alcohol (%) content during the acetic fermentation stage.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Properties** & **Value** \\ \hline Lycopene content (mg/100 g) & 9.19 \(\pm\) 0.70 \\ Total acidity & 5.42 \(\pm\) 0.24 \\ Alcohol content (\%) & 0.05 \(\pm\) 0.00 \\ pH & 2.85 \(\pm\) 0.10 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Properties of the acetic fermentation product.