## 3 Contribution of Selected Yeasts to Emerging Winemaking Demands  
### Enhancing the Aroma of Wine  
The overall quality of a good wine is essentially determined by its aromatic and sensory profile, which is strongly linked to its typicity, origin region of production, and above all, by its ability to determine the consumer's purchase choice [59]. In wines, more than 800 volatile organic compounds (VOCs) have been described and detected in concentrations ranging from ng/L to hundreds of g/L; however, only a few of them, at concentrations over their perception threshold, are odor-active compounds [60]. As recently reviewed by [PERSON] et al. [57], the sensory complexity is mainly determined by three aromatic categories: primary aromas, characterized by terpen, thiols, carotenoids, and norisoprenoids, originating from grape varieties that contribute significantly to wine typicity; secondary aromas, constituted mainly by higher alcohols, terpenoids, volatile fatty acids, esters, and phenols; carbonyl compounds and sulfur com  
Figure 4: Phylogenetic tree showing the 41 yeast species of Saccharomyces, obtained on the basis of the 18S ribosomal DNA sequence. Adapted from [51]. _Schizosaccharomyces pombe_ (belonging to Schizosaccharomyces) is represented as the outgroup species. Among the displayed species, the yeast species in the red boxes represent the NS yeasts of enoological interest described in the present review. pounds, produced by yeasts during fermentation, which are involved in the \"bouquet\" of wine; and finally, tertiary aromas that are produced during wine aging (maturation and bottle aging), including volatile phenols, furanic compounds, and acetals. In particular, the main metabolisms involved in aromatic compound production are summarized in Figure 5.  
Each of these compounds is able to influence, in different ways, the aromatic complexity of wine by imparting a specific aroma and/or off-flavor. In detail, higher alcohols can affect the final bouquet in both positive and negative ways; excessive amounts can produce a strong, pungent smell and taste, while appropriate levels yield fruity traits [58]. Volatile phenols, derived from the precursors of hydroxycinnamic acid present in grape must, can enhance the fragrance of wine by conferring the characteristic aroma of spices, smoke, and leather, as well as by enhancing the bitterness and astringency of wine; however, when their levels exceed the threshold, they can induce off-flavors such as \"band-aid\", \"barmyard\", or \"stable\" [61]. In this context, _Brettanomyces bravellensis_ is strongly involved in phenolic off-flavors, primarily linked to 4-vinylphenol and 4-ethylphenol from p-coumaric acid and to 4-vinylguaiacol and 4-ethylguaiacol from ferulic acid. These compounds confer undesired flavors, described as \"horse sweat\", \"rancid cheese\", or \"musky\" [62]. Esters, which are produced by yeast during fermentation, can expressively affect fruity flavors. The most widespread ester found in wine is ethyl acetate, which shows a fruity/solvent-like aroma at a threshold level of 150 mg/L. However, many other esters, including isoamyl acetate (banana aroma), isobutyl acetate (apple/pear aroma), ethyl hexanoate (fruity, strawberry, green apple, anise aroma), and 2-phenylethyl acetate (honey, fruity, flowers aroma), have been detected [63]. Moreover, volatile thiols are another class of aromatic compounds found in wine. They are mostly produced during fermentation and can confer either a pleasant or negative aroma, such as furfuryl thiol, identified in Bordeaux red wines (Petite Manseng variety), that exhibits roasted coffee flavors [64]. Among carbonyl compounds, acetaldehyde is the most commonly detected in wine, at concentrations ranging from 10 mg/L to 75 mg/L and with a sensory threshold value of 100 mg/L. At low concentrations, it can give a fruity flavor; however, it can also provide off-flavors like \"bruised apple\" and \"nutty\", which are also used as indicators of oxidation in wine.