# Aromatic Potential of Diverse Non-Conventional Yeast Species for Winemaking and Brewing
## 1 Introduction  
The most common starter cultures employed in alcoholic fermentation are _Saccharomyces_ species. These species combine several desirable characteristics, such as efficiency in fermenting sugars and producing ethanol, synthesis of aromas, and high ethanol tolerance [1]. However, in the early stages of wine fermentation, as well as in spontaneously fermented wines, several non-_Saccharomyces_ species also participate in the fermentation process. These species may have a positive impact on desirable organoleptic characteristics of the wine such as improved mouthfeel and more complex flavour profiles [2]. Recently there has been an increasing interest in both winemaking and brewing towards non-_Saccharomyces_ yeasts for their potential for aroma enhancement while maintaining the particular _terroir_ of each region or for the development of craft beers. In the case of wine, application studies with a selection of these so-called non-conventional yeasts indeed improve wine aroma; for example, the sequential fermentation of Ribolla Gialla grape juice employing commercial _Saccharomyces cerevisiae_ Lalvin T73 and _Zygosaccharomyces kombuchensis_CBS8849 or _Kazachstania gamospora_ CBS 10400 resulted in positive organoleptic properties and suitable fermentation dynamics, rapid sugar consumption and industrial strain compatibility [3]. In addition, [PERSON] et al. [4] showed that mixed-culture fermentations involving _S. cerevisiae_ and different _Kazachstania_ species have very distinct aroma profiles, whereas [PERSON] et al. [5] showed that wines fermented with a combination of _Metschnikowia pulcherrima_ and _S. warrant_ were characterised by increased concentrations of 2-phenylethanol and 2-phenylethyll acetate, both associated with positive sensory attributes. In simultaneous and sequential inoculations of selected _Rhodotorula mucilaginosa_ and _S.cerevisiae_ strains, both the variety and levels of fermentative aromocompounds were improved, especially that of (Z)-3-hexene-1-ol, nerol oxide, certain acetates and ethyl esters. These changes are associated with an improved perception of citrus, sweet fruit, acid fruit, berry, and floral aroma [6]. In a similar approach, the use of non-_Saccharomyces_ yeasts in brewing has been explored [7]. _Torulaspora delbrueckii_, for example, was studied in single-strain fermentations for the production of low-alcohol beer and for enhancing beer flavour profiles. Some strains fermented all the wort sugars, whereas others did not, but all strains tested displayed improved flavour-forming properties [8]. In another study, the use of pure cultures of _T. delbrueckii_ and mixed cultures of _T. delbrueckii_ and _S. cerevisiae_ for brewing resulted in a well noticeable change in flavour profile due to _T. delbrueckii_. Distinctive sensory profiles with pure cultures of _T. Delbrueckii_ were characterised by 'fruit/citric' notes and 'full-bodied' attributes whereas some mixed cultures enhanced the 'fruity/ester' and 'hop' aromas. At the same time, these beers showed enhanced clarity and a more persistent and compact foam [2]. Another approach involving non-_Saccharomyces_ yeasts to produce low-alcohol beers was carried out by [PERSON] et al. [9]. These authors found that the non-conventional yeasts _Cyberlindnera fubianii_ and _Pichia kudriavzevii_ both in pure and mixed cultures with _S. cerevisiae_ yielded beer with lower ethanol content and higher levels of esters. _Brettanomyces custersii_, isolated from Lambic beer, showed higher \(\beta\)-glucosidase activity than _S. cerevisiae_, thus a higher potential for aroma release from precursors. Fermentation with pure cultures of a _Brettanomyces custersii_ strain or in a co-culture with a _S. cerevisiae_ resulted in enhanced release of volatiles from hop glycosides [2]. In a more recent study, [PERSON] et al. [10] screened 17 non-conventional yeast species in sequential brewing fermentation, followed by inoculation of _S. cerevisiae_.