# Microbial Dynamics in Sour-Sweet Wine Vinegar: Impacts on Chemical and Sensory Composition
## 6 Wine Yeasts and Acetic Acid Bacteria Interactions  
### Biotic Interactions  
In vinegar production, the interaction between wine yeasts and acetic acid bacteria, and even lactic acid bacteria--the so-called biotic factor--plays a crucial role in the conversion of ethanol to acetic acid. The initial fermentation stage is carried out by wine yeasts, such as _S. cerevisiae_ and other non-_Saccharomyces_ species. These yeasts convert the sugars in the wine into ethanol and other by-products, including glycerol, acetaldehyde, and organic acids. The presence of these by-products can affect the activity and growth of AAB during the subsequent acidification stage [6]. As a result, the metabolism of yeasts impacts the flux of accessible substrates for AAB [102]. Moreover, yeast releases free amino acids during extracellular proteolytic activity and/or autolysis, which also benefits AAB [103]. The interaction between wine yeasts and acetic acid bacteria in vinegar production can be either symbiotic or competitive, depending on various factors such as temperature, pH, and nutrient availability. The symbiotic relationship between wine yeasts and acetic acid bacteria is beneficial for vinegar production because it promotes both microorganisms' growth and enhances the fermentation process's efficiency. Wine yeasts produce ethanol, which serves as a substrate for acetic acid bacteria, while acetic acid bacteria consume ethanol and produce acetic acid. This symbiotic relationship has been shown to increase the yield of acetic acid during vinegar production [5, 20, 104]. Furthermore, it was found in Kombucha, a sugared tea fermented by SCOBY (Symbiotic Culture of Bacteria and Yeast), that AAB re-routed the metabolism of _S. cerevisiae_ towards elevated invertase and fermentative activities [102]. [PERSON] et al. [105] evaluated the performance of a consortium of non-_Saccharomyces_ and AAB capable of achieving a complete ethanol-acetic acid fermentation for the production of a Balsamic-styled vinegar in South Africa. Two inoculation strategies were followed: the co-inoculation of yeasts and AAB at the start of fermentation (ethanol 0%, _v/v_) and the sequential inoculation (yeast only at the start and AAB at ethanol 6%, _v/v_). They found that co-inoculation was a faster process, and there were minimal differences between yeast and bacteria growth kinetics, meaning that they could coexist. In the special case of sherry wine vinegar, the microbiology of sherry wine must be considered. Molecular and physiological characterization of the yeasts implicated in the veil of sherry wines has described the presence of different _S. cerevisiae_ strains, mainly var. _cheesiensis, beticus, montuliensis,_ and _Zygosaccharomyces_ _rouxii_, which can grow aerobically in wine [106]. During this first fermentation process, the yeasts produce a variety of aroma compounds, such as esters and higher alcohols, that contribute to the sensory properties of the final vinegar product. Lactic acid bacteria (LAB) are also present in vinegar, albeit in smaller numbers compared to yeasts and AAB. They are implicated in the production of lactic acid and other organic acids, such as acetic and succinic acids, which contribute to the flavor and acidity of the vinegar. Additionally, some LAB strains may also produce antimicrobial compounds (bacteriocins) that may inhibit the growth of other microorganisms [107], including the AAB. Moreover, according to the recent work of [PERSON] and co-workers [108], the production of cereal vinegar LAB and AAB showed a negative interrelatedness during the fermentation process. _L. helveticus_ CGMCC 12062 and _A. pasteurianus_ CGMCC 3089 showed no nutritional competition when they were co-cultured in vitro. However, the growth and metabolism of _L. helveticus_ were reduced during fermentation due to the presence of the AAB, indicating an amensalism phenomenon involving these two species [109].