# Adding Flavor to Beverages with Non-Conventional Yeasts  
[PERSON]  
1 Carlsberg Research Laboratory, Yeast & fermentation, J. C. Jacobsens Gade 4, DK-1799 Copenhagen V, Denmark; [EMAIL_ADDRESS] (D.R. ); [EMAIL_ADDRESS] (A.W.) [PERSON]  
2 Fondazione Edmund Mach, Research and Innovation Centre, Food Quality and Nutrition Department, Via E. Mach 1, I-38010 S. Michele all'Adige, Italy; [EMAIL_ADDRESS] (S.C.); [EMAIL_ADDRESS] (U.V.) [PERSON]  
3 Westerdijk Fungal Biodiversity Institute, 3584 CT Utrecht, The Netherlands; [EMAIL_ADDRESS] (T.B. ); [EMAIL_ADDRESS] (M.G.) [PERSON]  
3 Westerdijk Fungal Biodiversity Institute, 3584 CT Utrecht, The Netherlands; [EMAIL_ADDRESS] (T.B. ); [EMAIL_ADDRESS] (M.G.) U.V.L., V.V.L. Top-fermenting yeasts generally are _S. cerevisiae_ strains. At the end of fermentation, these yeasts rise to the surface of the fermenter, creating a thick cell layer. Bottom-fermenting yeasts belong to two distinct groups of lager yeasts [1]. Lager yeasts are hybrids between _S. cerevisiae_ and _S. eubayanus_[2]. Lager yeasts can be divided into two groups, group I or _Saz_ type, and group II or _Frohberg_ type, that are distinguished at the molecular level by ploidy differences, characteristic chromosomal rearrangements and chromosome losses [3; 4]. In contrast to lager beers, lambic beers with sometimes exceptional flavor compositions are based on a larger biodiversity, including acetic and lactic acid bacteria and various yeasts, e.g., _S. cerevisiae_, _S. pasorianus_ and _Brettanomyces bruxellensis_[5]. Beer is a complex product consisting of volatile and non-volatile components that form the final aroma. The contribution of ale yeasts to the final flavor bouquet is generally higher than that of lager yeasts. This has been attributed to the greater diversity of ale yeasts compared to the limited diversity of the two groups of lager yeast [6]. Other process parameters also influence volatile-compound formation. Specifically, sugar concentration of the wort and different aeration regimes influence the production of flavor-active esters. These esters mainly contribute to the fruitiness of the product. Dominant esters are acetate esters such as ethyl acetate (fruity), isoamyl acetate (banana) and 2-phenyl acetate (rose) and ethyl or medium-chain fatty acid esters such as ethyl hexanoate, ethyl octanoate and ethyl decanoate, which provide a fruity apple- or wine-like flavor to the beer [7]. Among the higher alcohols, n-propanol, isobutanol, 2-phenylethanol and isoamyl alcohol are most abundant. Higher alcohols such as isobutanol can contribute a rum-like aroma which gives a warm mouth feeling, while 2-phenylethanol and isoamyl alcohol are prevalent for their sweet/rose and fruity/banana-like aromas, respectively [8]. Not all flavors are desirable. Strecker aldehydes (aged flavour), aldehydes of the _Maillard_ reaction, e.g., furfural, and aldehydes of fatty acid oxidation, e.g., trans-2-nonenal, are regarded as off-flavors in beer [9]. The ketone diacetyl (2,3-butanedione) is monitored during the lager beer brewing process, in particular, as it imparts an undesirable buttery flavor with a low flavour threshold [10]. Given the low concentration and volatile nature of these aroma compounds, gas chromatography coupled with mass spectrometry (GC/MS) offers an optimal technique to analyze the flavor profile of beer. Recently, non-conventional yeasts or non-_cerevisiae_ yeasts have gained importance for fermented alcoholic beverages [11; 12]. They produce various mixtures of volatile compounds and so contribute to the aroma profile of beverages [6; 13]. Mixed fermentations using _S. cerevisiae_ in combination with a non-conventional yeast strain e.g., belonging to the genera _Lachancea_, _Pichia_ or _HanseniNpora_, could provide novel beverages with improved ester profiles [14; 15]. On the other hand, the synergistic effects on aromatic compound production were observed in co-cultures with _Metschnikowia pulcherrina_ and _S. cerevisiae_[16].