# Aromatic Potential of Diverse Non-Conventional Yeast Species for Winemaking and Brewing
## 3 Results and Discussion
### Microbeer
#### 3.2.2 Aroma Profiles  
Vicinal diketones are carbonyl compounds that are undesirable when they appear in high concentrations in beer. Diacetyl adds a buttery flavour to beer if found above its sensory threshold (0.10-0.15 mg/L) and 2,3-pentanedine its known for its toffee flavour (sensory threshold 0.90 mg/L) [14]. However, it is known that other aroma compounds could strongly influence the sensory threshold and perception of diacetyl in alcoholic beverages [29]. Both the adjunct strains, as well as dosage, significantly influenced the final quantity of vicinal diketones at the end of fermentation according to multifactorial ANOVA. The interaction of these two factors made a significant impact, as well. Diacetyl appears above its sensory threshold in most of the resulting microbeers (even in the reference microbeer), contrarily to the case of 2,3-pentanedione, whose quantity was below its sensory threshold in all of the cases (Table 6). However, it has to be taken into account that microbeers did not undergo maturation in this study. In the maturation process, yeast take up diacetyl and reduce it to 2,3-butanediol, which does not have an undesirable flavour [14]. Most of the 5:1 fermentations presented diacetyl levels near its threshold and similar to reference microbeer with the exception of _H. guillermondii_ CBS 2567, _W. subpelliculusus_ CBS 5552, _S. caribaea_ CBS 7692 and _B. californica_ CBS 8860, which showed higher levels (Table 6). This fact was already observed in some of these non-conventional yeasts in previous works [22]. On the other hand, diacetyl levels, on average, increase with increasing dosage of adjunct strain, indicating that these non-conventional yeasts can produce more diacetyl, thus making necessary longer maturation times if used industrially. However, a few strains form an exception to the above and result in lower (_W. subpelliculusus_ CBS 5552) or similar (_H. guillermondii_ CBS 2567, _Z. rouxii_ CBS 9716 and _K. gamospora_ CBS 10400) amounts of diacetyl when higher proportion of non-conventional yeast is employed. Higher Alcohols  
Multifactorial ANOVA showed that the non-_Saccharomyces_ strain participating in the fermentation was a significant factor regarding higher alcohol yield. Contrarily, the pitching ratio did not influence the quantity of higher alcohols, which was in accordance to [30], who compared the brewing performance of _Brettanomyces bruxellensis_ as pure culture at different pitching ratios and the only aroma compound presenting different levels was 4-vinylguaiacol. This pattern was just the opposite as in the case of microwine, evidencing the importance of the fermentative substrate employed to carry out the fermentation process. In general, adjunct cultures resulted in equal or lower concentration of higher alcohols compared to the reference microbeer (Table 6). In all microbeers, isoamyl alcohol was the only alcohol present in concentrations above its sensory threshold (Table 6). [PERSON] et al. [12] obtained similar results when screening 60 non-conventional yeast for aroma production in brewing as they only detected isoamyl alcohol and 2-phenylethanol as higher alcohols produced by all the yeasts. In our data set, strains _Z. rouxii_ CBS 9716 and _W. subpelliculosus_ CBS 5552 resulted in 2-phenylethyll alcohol levels higher than that found in the reference microbeer (Table 6). The high production of alcohols in by _Z. rouxii_ has already been reported by other authors for beer as well as for other food products such as soy source [14]. Except for ethyl acetate, none of the acetate esters investigated reached its sensory threshold (Table 6). In general, levels of ethyl acetate and isoamyl acetate did not vary between the reference microbeer and the adjunct strains nor between pitching ratios, except for ethyl acetate, which appeared slightly above its sensory threshold in just two strains, _W. subpelliculosus_ CBS 5552 and _Z. rouxii_ CBS 9716, not being significant differences between pitching ratios (Table 6). High ethyl acetate synthesis by _Z.