# Microbial Dynamics in Sour-Sweet Wine Vinegar: Impacts on Chemical and Sensory Composition
## 4 Traditional Balsamic Vinegar Acetic Acid Bacteria (AAB) and Acetic Fermentation  
Acetic acid bacteria (AAB) are a cluster of Gram-negative, obligately aerobic bacteria belonging to the family _Acetobacteraceae_[76] that show an important function in the production of fermented foods such as vinegar. They are rod-shaped, facultative anaerobes, with a size of approximately 0.5-1.5 \(\upmu\)m in width and 1-5 \(\upmu\)m in length. They are also catalase- and oxidase-positive, which means they produce the enzymes that break down hydrogen peroxide and can use oxygen as a terminal electron acceptor in respiration. They can fix nitrogen and use nitrogen gas as a source of nitrogen for growth [77]. AAB grows optimally at a pH range of 4.0-6.0 and a temperature range of 25-30 \({}^{\circ}\)C. They require a source of ethanol or other alcohol for growth and energy production. The metabolism of AAB is highly aerobic, and they require oxygen for the oxidation of ethanol to acetic acid. This oxidation reaction involves the production of acetaldehyde as an intermediate [78]. Many AAB strains can partly oxidize carbohydrates, alcohols, and related compounds to yield industrial products such as acetic acid. The main metabolic pathway of AAB is the oxidative fermentation of ethanol into acetic acid. This pathway involves the following steps [78]: (i) Ethanol is oxidized to acetaldehyde by the enzyme alcohol dehydrogenase. (ii) Acetaldehyde is then oxidized to acetic acid by the enzyme acetaldehyde dehydrogenase. This reaction produces NADH, which is used in the electron transport chain to produce ATP. (iii) The electron transport chain in AAB involves several electron carriers, including quinones and cytochromes. The final electron acceptor in the chain is oxygen, which is reduced to water. These partial oxidation processes of AAB are named AAB oxidative fermentation (AOF) [79]. The PPP (Pentose Phosphate Pathway) and ED (Entner-Doudoroff) pathway provide the necessary energy and reducing power for the AOF process. The PPP generates NADPH, which is used for the biosynthesis of cellular components and the reduction of acetaldehyde to ethanol. The ED pathway generates ATP and NADPH, which are used for cellular  
\begin{table}
\begin{tabular}{c c c c} \hline
**Microorganisms** & **General Metabolic Features** & **Sensory Properties** & **Ref. ** \\ \hline Non-Zygosacchromomyces & & & \\ species & & & \\ \hline \multirow{6}{*}{_Saccharomycescodes_} & Tolerance to sulfur & Sweet, musty odor (isobutyl) & \\  & dioxide (SO\({}_{2}\)) and pressurized & alcohol), buttery odor (acetoin), & \\  & carbon dioxide (CO\({}_{2}\)); & fruity, glue-like aroma (ethyl & \\  & Can produce up to 12\% (\%/\
u) of & acetate), and an oxidative & \\  & ethanol and \(<\)1.0 g/L acetic acid; & Acid/salty taste due to succinic acid & \\  & Resistance to acetic acid & and some softness induced & \\  & and osmotolerant. & by glycerol. & \\ \hline \multirow{6}{*}{_Candida stellate_} & Fructophilic character presenting & & \\  & the Ffz1 transporter, codified by the & & \\ \cline{1-1}  & FFz1 gene; & & \\ \cline{1-1}  & Low fermentation rates and high & & \\ \cline{1-1}  & production of & & \\ \cline{1-1}  & \(\phantom{\text{\text{\small{}}}}\) & softness in the mouth (glycerol); & \\ \cline{1-1}  & \(\phantom{\text{\small{}}}\) & Acid/salty taste due to succinic acid; & \\ \cline{1-1}  & \(\phantom{\text{\small{}}}\) & & \\ \cline{1-1}  & \(\phantom  
metabolism and the reduction of acetaldehyde to ethanol [79; 80]. The AOF process is dependent on various factors, such as the type and concentration of AAB present, pH, temperature, and oxygen availability. AABs are known to be highly adaptable and can thrive in a wide range of environmental conditions [20]. The production of acetic acid results in a decrease in the extracellular pH, which can inhibit the growth of AAB. To maintain pH homeostasis, AAB produces and excretes large amounts of gluconic acid, which can act as a buffer [78].