# Microbial Dynamics in Sour-Sweet Wine Vinegar: Impacts on Chemical and Sensory Composition
## 3 Traditional Balsamic Vinegar Yeasts and Alcoholic Fermentation
### Non-Zygosaccharomyces Strains  
Interestingly, the majority of yeasts coexisting in balsamic vinegar are fructophilic, a behavior contrary to that of _Saccharomyces cerevisiae_[39]. This fructophilic behavior depends on the pattern of fructose and glucose transporters, which prioritize fructose [40]. Other less osmotolerant species, such as _Saccharomyces ludwigii_, formerly referred to by [PERSON] et al. [18] and [PERSON] [16], seem to be similar to _S. cerevisiae_ and discreetly choose glucose over fructose. _S. ludwigii_ is known for the spoilage of fruit juices, wines, and ciders [41]. This yeast species also presents high tolerance to SO2 [42] and resistance to pressurized CO2, showing the aptitude to deteriorate carbonated beverages [43]. It has also been isolated from sweet wines [44] and balsamic vinegar [27], thus showing its tolerance to high sugar levels. _S. ludwigii_ (strain APRE2) showed the ability to grow at temperatures of up to 43 \({}^{\circ}\)C [39; 45]. Moreover, this yeast is described as a high producer of secondary metabolites such as isobutyl alcohol, acetoin, ethyl acetate, and acetaldehyde [42; 46]. The high production of isobutanol [46] and acetaldehyde [42] can be considered discriminant characteristics of _S. ludwigii_, although this metabolic feature seems to depend on the strain. Some strains also produce considerable amounts of glycerol (up to 11.7 g/L) and succinic acid (up to 1.4 g/L) [47], giving the vinegar interesting sensory properties (Table 1). In some vinegar fermentations, the oxidation of ethanol to acetic acid is an exothermic reaction that triggers a temperature increase as high as 50 \({}^{\circ}\)C [45; 48]. Thus, even though acidification does not increase the temperature very much for the balsamic vinegar process, the use of natural, thermo-resistant yeast strains allows for better fermentative performance. Thermo-tolerant strains can increase the length of the cytoplasmic membrane lipid layer of acyl chains, saturation, branching, and/or cyclization of the fatty acids, leading to the stabilization of the native configuration of the enzymes [48]. Moreover, they improved the use of Na\({}^{+}\) ions instead of protons as coupling ions in energy transduction. Due to the high H\({}^{+}\) permeability, predominantly at high temperatures, the use of a Na\({}^{+}\) motive force is a more energetic advantage than a proton motive force [49]. _S. ludwigii_ also exhibits a significant multi-stress tolerance toward ethanol, acetic acid, furfural, and 5-hydroxymethyl furfural (5-HMF) when using lignocellulosic biomass as feedstock for ethanol production at high temperatures [45]. These features are also interesting for vinegar production, namely, tolerance toward 5-HMF. In the first stage of balsamic vinegar production, the cooked must is obtained after heating the grape juice in an open container over a direct flame. This typical process leads to the oxidation of glucose and fructose, via non-enzymatic ways, to different chemical mixtures [50]. In acidic media, furfural derivatives, 5-HMF, are produced [51], and this compound is proven to be toxic to biological species, including humans. Indeed, [PERSON] and co-workers [52] proposed a grape must heating strategy after 10 h of heating (after the moisture level falls to 70%), consisting of using controlled conditions to prevent the reactions that lead to furfural formation, which are enhanced in acidic media, high sugar concentrations, and high temperatures [51; 53]. Another example of _S. ludwigii_'s resilience and resistance to acetic acid and sugar is the study by [PERSON] [54]. _S. ludwigii_ cells entrapped in calcium alginate beads, together with immobilized _Acetobacter aceti_ cells, were able to perform the continuous production of vinegar using a saccharified rice medium as feedstock, containing 81 g/L of glucose and 11 g/L of acetic acid. The produced vinegar contained 4.7% to 5.3% acetic acid [54]. _Candida_ spp. is a beverage- and food-associated yeast. _C.