# Microbial Dynamics in Sour-Sweet Wine Vinegar: Impacts on Chemical and Sensory Composition
## 4 Traditional Balsamic Vinegar Acetic Acid Bacteria (AAB) and Acetic Fermentation
In addition to the oxidative fermentation of ethanol, many species of AAB also produce other metabolic products, such as dihydroxyacetone, acetoin, and 2,3-butanediol [78]. After ethanol exhaustion, acetate accumulates in the cell (cytosol) and can be utilized via acetyl-CoA synthase and phosphoenolpyruvate carboxylase [81]. AAB, namely _Gluconobacter oxydans_, are also able to metabolize glucose by an oxidative pathway, generating a considerable number of metabolites such as glucono-\(\delta\)-lactone by glucose dehydrogenase (GDH). Glucono-\(\delta\)-lactone is then hydrolyzed into D-gluconate and, depending on the pH of the medium, D-gluconate is oxidized to 2-ketogluconate and 2,5-diketogluconate. Gluconic acid is the main oxidation product of _Acetobacter_ and _Gluconobacter_ strains and is an indicator of TBV authenticity [81; 82]. The acetic acid bacteria involved in traditional balsamic vinegar (TBV) production are mainly of the genus _Acetobacter_, including _Acetobacter aceti_, _Acetobacter pasteiramis_, and _Gluconacetobacter europaeus_[20]. These bacteria are naturally present in the environment and on the surface of grapes, and they play a crucial role in the production of TBV. During vinegar production, the acetic acid bacteria consume the ethanol produced during the initial fermentation process and convert it into acetic acid, which gives TBV its characteristic sour taste and aroma. This acetic fermentation process takes place in wooden barrels, which provide a unique environment that allows the acetic acid bacteria to thrive [20]. Studies have shown that the composition of the acetic acid bacteria community in vinegar can vary depending on factors such as the type of grape used, the production methods, and the location of the production site [83]. However, the presence of certain species of _Acetobacter_ has been identified as a key factor in determining the quality of TBV [19]. [PERSON] et al. [38] analyzed the microbial communities present during the production of TBV using a combination of culture-independent and culture-dependent techniques. They found that _Acetobacter_ was the dominant genus of bacteria present and that certain species, such as _A. aceti_ and _A. pasteiramis_, were consistently associated with high-quality TBV. Apart from acetic acid, AAB produces a range of volatile organic compounds (VOCs) that contribute to the aroma of vinegar: (i) Acetaldehyde, a key intermediate in the conversion of ethanol to acetic acid by AAB. It is a potent aroma compound that contributes to the characteristic pungent and fruity aroma of vinegar. Acetaldehyde is produced in high concentrations during the initial stages of vinegar fermentation and then gradually decreases as acetic acid production increases [84]. (ii) Acetoin, a neutral aroma compound that contributes to the buttery and creamy notes in vinegar. It is produced by AAB as a byproduct of acetic acid production and is present in relatively low concentrations in vinegar [85]. Diacetyl is another neutral aroma compound that contributes to the buttery and creamy notes in vinegar. It is produced by AAB as a by-product of acetic acid production and is present in relatively low concentrations in vinegar [85]. Ethyl acetate is also produced, and it presents a fruity/glue aroma compound. It is produced by AAB as a byproduct of acetic acid production and is present in relatively high concentrations in vinegar [84].