# Microbial Community and Functional Analysis of Regionally Produced Traditional Korean Grain Vinegar
## 3 Results
### Volatile Flavor Components  
The volatile flavor compounds of the grain vinegars were analyzed to identify aromatic substances, and their correlations among samples were visualized in a heatmap (Table 3, Figure 5). A total of 16 flavor compounds were detected and classified into alcohols, aldehydes, esters, acids, and ketones. As fermentation progressed, the concentration of alcohols (e.g., ethanol, 2-methyl-1-propanol, etc.) gradually decreased, as acetic acid bacteria consumed ethanol to produce acids through acetic fermentation. In addition to ethanol, aldehydes, such as nonanal and decanal, were also produced during fermentation. However, these aldehydes are more likely derived from the oxidative degradation of fatty acids rather than ethanol oxidation. They are known to contribute fresh, fruits, and natural aromas [45].  
Esters, which are secondary metabolites formed through the reaction of alcohols and acids, increased in concentration over time, with six types detected [46]. Among them, acetic acid, ethyl ester was the most abundant, followed by 3-methylbutyl acetate and methyl salicate, with particularly high levels observed in YO, which was produced from a multi-grain blend and had the highest acidity. According to previous studies, esters are the most influential class of flavor compounds, even though they are typically present in small amounts. Despite their low concentrations, they contribute significantly to flavor [47; 48]. For instance, fruity and sweet aromas reminiscent of banana were detected in the grain vinegar, largely due to 3-methylbutyl acetate. This is consistent with earlier findings reporting that vinegars made from grains like rice, brown rice, and barley contain similar aroma-active compounds, which are mainly produced during microbial fermentation [49; 50]. Acids (e.g., ethyl 2-hydroxypropanoate and acetic acid) and ketones (e.g., 3-hydroxy-2-butanone, 6-methyl-5-hepten-2-one, etc.) also contributed to the sweet and pleasant aromas and were found in higher concentrations in YO, reflecting the influence of this variety's more diverse grain composition.  
Figure 5: Heatmap of volatile flavor compounds in grain vinegars produced via different fermentation processes across four regions and over three fermentation stages.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multirow{2}{*}{**Order Description**} & \multicolumn{2}{c}{**HB**} & \multicolumn{2}{c}{**SB**} & \multicolumn{2}{c}{**JR**} & \multicolumn{2}{c}{**VO**} \\ \cline{3-10}  & & \multicolumn{2}{c}{_p-Value_1 _} & \multicolumn{2}{c}{_p-Value_1 _} & \multicolumn{2}{c}{_p-Value_} & \multicolumn{2}{c}{_VIP_2 _}  
Based on these findings, we suggest that differences in aroma during vinegar fermentation are influenced by variations in the dominant microbial communities, which in turn are affected by the type of raw materials used. Previous studies on traditional Chinese grain vinegas have reported that environmental factors during fermentation significantly impact microbial dynamics and the production of key metabolites, including organic acids, amino acids, and volatile compounds. In particular, the acetic acid fermentation stage has been identified as a critical period for the accumulation of flavor compounds [51]. Accordingly, the results of this study are expected to serve as valuable foundational data for understanding how volatile flavor profiles in traditional grain vinegas are shaped by grain type and fermentation conditions.