# Microbial Dynamics in Sour-Sweet Wine Vinegar: Impacts on Chemical and Sensory Composition
## 6 Wine Yeasts and Acetic Acid Bacteria Interactions
### Abiotic Interactions
Up until a certain concentration (7-8%; [PERSON] et al. [115]), the presence of acetic acid in vinegar alters the flavor of vinegar and increases the survival advantages of AAB, namely in _Acetobacter_ strains [19]; however, if acetic acid accumulates beyond a certain concentration, it causes acid stress that inhibits AAB growth [116]. For yeasts, 7-8% acetic acid concentrations are considered lethal, and high acetic acid levels are commonly associated with arrested fermentations [117; 118]. Results from [PERSON] et al. [119] indicate that yeast cell wall resistance to lyticase activity increases during yeast population adaptation to sudden exposure to sub-lethal acetic acid concentrations. This response correlates with increased cell stiffness, increased content of cell wall \(\beta\)-glucans, and a slight increase in the transcription of the GAS1 gene, which encodes a \(\beta\)-1,3-glucanosyltransferase that leads to an extension of (1-\(\rightarrow\)3)-\(\beta\)-d-glucan chains. These alterations guarantee an adaptive response that limits the cycle associated with the re-entry of the toxic acid form after the active exclusion of acetate from the interior of the cell. This phenomenon was also observed in _S. cerevisiae_ cells. Individual _Z. bailii_ cells, a yeast species present in TBV, in a population exposed to different weak acids will exhibit variable tolerance to acetic acid, among other weak acids [120]. The most tolerant sub-population presents a lower pH, leading to reduced intracellular dissociation of weak acids and, reduced accumulation of the counterion in the interior of the cell (cytoplasm), thus conferring tolerance to any weak acid, including acetic acid [121]. While, for yeasts, acetic acid is lethal at vinegar's expected acidity, for AAB, the concentration of alcohol (ethanol) in the initial liquid is an essential abiotic factor. AAB bacteria require a certain concentration of alcohol (typically around 5-7%) to carry out the conversion into acetic acid. If the alcohol concentration is too low, fermentation may be slow or incomplete. Conversely, high alcohol concentrations can be toxic to the bacteria, depending on the strain used. [PERSON] et al. [122] found, among 400 strains of AAB isolated from different substrates, six strains (FAGD1, FAGD10, FAGD18, GCM2, GCM4, and GCM15) isolated from apple and grape that can produce acetic acid at 16% (\(v/v\)) of ethanol and can tolerate acetic acid concentrations in the fermentation medium up to 6% (\(v/v\)). Furthermore, the isolated strains are also metabolically active at temperatures up to 48 \({}^{\circ}\)C.  
Regarding nutrient availability, it must be considered that if acetic acid fermentation is carried out after the AF, or in some cases, relatively at the same time, AAB will have a poor nutrient medium or compete for nutrients with the yeasts. Hence, depending on the raw material, the addition of nutrient sources (amino acids and proteins), minerals, and vitamins may be necessary to a significant or minor extent [10]. [PERSON] et al. [123] studied the need for vitamins and minerals to increase cell mass production by AAB isolated from the vinegar industry (086/06) and a standard _Acetobacter aceti_ strain (CCT 2565). Five minerals (Mn, Zn, Mo, B, and Fe) and eight vitamins (niacin, _p_-aminobenzzoic acid, pantothenic acid, thiamine, pyridoxine, biotin, cyanocobalamin, and inositol) were tested in a fractional factorial design. The results showed that the 086/06 strain required a lower number of different micronutrients than the CCT 2565 strain, which may be because of the adaptation of the 086/06 strain to the conditions of industrial production, reinforcingthe fact that knowledge of microbial nutrition is necessary for the nutrients to be supplied in the appropriate form and amount.