## 3 Results and Discussion
### Volatiles  
The composition and complexity of the volatile profile of vinegar, and therefore its aroma, is directly related to the quality and value of vinegar products. The main factors \begin{table}
\begin{tabular}{c c c} \hline \hline
**Biocatalyst** & **TPC** & **AC** \\  & **(mg GAE/L)** & **(mg AAE/L)** \\ \hline Free cells & 333.10 \(\pm\) 11.98 a & 263.50 \(\pm\) 8.41 a \\ Immobilized cells & 222.14 \(\pm\) 2.86 b & 277.10 \(\pm\) 6.66 a \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Total phenolic content (TPC) and antioxidant capacity (AC) of the vinegar made from FSS sweet wine using free and immobilized cells of the mixed _A. aceti_ and _K. europaeus_ culture. that determine the volatile fraction in vinegar are the raw material and the production process (alcoholic fermentation, acidification, the ageing process, etc.). So far, the effect on the volatile profile of vinegar of the microorganisms involved in the production process has little been studied [25]. Despite the significant changes caused by the biological processes of vinegar production, the products retain a volatile fraction from the raw material. Moreover, the strong air supply during production can lead to loss of volatile compounds, while the ageing processes to which the high-quality vinegar are usually subjected also contributes to the complexity of aroma and the added value of the products [25]. Volatiles in vinegar mainly include alcohols, organic acids, ethyl and acetate esters. Depending on the type of vinegar, various types and amounts of aldehydes, ketones, acetals, lactones, terpenes, volatile phenols and pyrazines may also be found. Typical volatile compounds of vinegar, such as acetic acid, acetoin, and ethyl acetate, are usually found in higher proportions in the volatile fraction than other compounds. In aged products, acetoin and ethyl acetate can reach amounts higher than 1000 and 3500 mg/L, respectively [25; 26; 27]. As in the case of FSS and dry wine made from FSS [10], the aroma profile of the FSS vinegar produced in this study was analysed by SPME GC-MS. A total of 140 compounds were detected, including 26 esters, 7 lactones, 25 alcohols, 13 organic acids, 37 carbonyl compounds, 13 terpenes, 8 hydrocarbons, and 11 other compounds (mainly pyrazines) (Table 3). The origin of these compounds (raw material, fermentation, etc. ), and their effect on aroma were recently discussed in detail by [10]. The differences observed among the different products (FSS, dry wines and vinegar) are mainly quantitative. The three major esters identified were ethyl acetate (30-46% of the total volatile fraction), isoamyl acetate (4-20%) and 2-phenylethylal acetate (1.6-6%), followed by isobutyl acetate, ethyl isovalerate, ethyl hexanoate, and ethyl octanoate at lower percentages. The percentage of esters in the vinegar made by immobilized cells was higher (62%) compared to those made by free cells (41-55%). The main alcohols found in the products were ethanol, 3-methyl-1-butanol, and 2-phenylethanol. In general, alcohols were more abundant in the vinegar produced by free cells of the wild AAB culture (FCW). Of the volatile organic acids, that were generally found at low concentrations (<0.01%), apart from acetic acid, mainly isovaleric, 2-methylbutyric, hexanoic, and octanoic acids (>0.1%) were detected. The major carbonyl compounds were acetaldehyde, diacetyl, acetoin, and furfural. Acetaldehyde is an intermediate in the catabolism of C-sources by yeasts and AAB. Specifically, it is formed by the oxidation of ethanol and pyruvate [28]. Acetoin is also produced in significant quantities during the acidification and is often used as an indicator of the biological origin of vinegar, as its synthesis is related to the metabolism of AAB. The major terpene compounds identified were D-limonene, linalool, \(\alpha\)-terpineol, \(\beta\)-damascenone, p-cymen-8-ol and trans-geralynacetone. Thujone (>0.01%) was also found at higher percentages in the vinegar compared to the raw material (FSS).