## 3 Results and Discussion
### Key Arroma Compounds of Beers Produced with Non-Conventional Yeasts
delbrueckii_ strains are considered good producers of esters, which is why they are proposed as cultures that can enrich the aroma of beer with fruity and floral notes [36]. The highest total ester intensity was found in beers obtained with the participation of _W. anomalous_ and _Ha. uravun_ yeasts. The most fruit and flower aromas were also found in both beers. Among the different cultures of _Saccharomyces_, the _S. kudriavzevii_ yeast introduced the most aromatic-active esters into the beer. This confirms our previous research, in which yeast from this species had a positive effect on the sensory characteristics of semi-sweet white wines, among others due to the relatively large amount of esters produced [37]. In low-alcohol beers produced with the use of _Z. bailii_ yeast, a slightly lower intensity of esters was found than in other beers. The profile of key aroma esters was also different. During the olfactometric analysis, only nine esters were detected, among which, ethyl 2-methylbutyrate, ethyl 4-methylpentanoate and ethyl dihydrocinnamate were the most intense. Esters as well as terpenes introduce fruity and floral aromas to beer [38]. HSMEC-MS analysis showed the presence of 16 terpenes and their derivatives in the samples, six of which were odour-active (Tables 4 and 5). Four of them are characterized by a fruity-floral aroma--cis-rose oxide (rose petals aroma), citronello1 (citrus aroma), geraniol (floral-citrus aroma) and damascenone (plum aroma), other two--\(\beta\)-ionone and nerolidol by herbal-woody scents. Methyl geranate (aroma-active component of wort) and humulene present in unfermented hopped wort were not detected in beer samples, and several others showed significant reductions after fermentation (e.g., \(\alpha\)-farnesene concentration decreased below the aroma threshold). [PERSON] and [PERSON] [38] showed that brewer's yeast can convert some terpenes to others. For example, the monoterene alcohols geraniol is converted to citronello1, and humulene is converted to carryophyllene. The lager yeast also produced acetate esters of geraniol and citronello1. Beers obtained with the participation of yeasts _W. anomalous_ CBS5759 and _S. kudriavzevii_ CBS3774 were characterized by the highest amount of terpenes, as well as the highest aroma intensity of these compounds (Tables 4 and 5). This may mean that they show not only the greatest ability to bioconvert these compounds but also \(\beta\)-glucosidase activity. This enzyme converts terpenes from bound to free form (in which they are found, for example, in hops), thanks to which they begin to affect the aroma [39]. \(\beta\)-Damascenone was identified as the compound with the highest odour activity in wort and beer. It was also one of the terpenes whose content increased after fermentation (over three times) as well as aroma intensity--from 0.6 in the wort to even 1.8 in beers obtained with _W. anomalous_ and _D. bruxellensis_. This compound is characterized by a very low odour threshold in water (20-90 ng/L) and can be found in naturally aged beer in a concentration up to 210 ug/L. \(\beta\)-Damascenone can be produced by some non-_Saccharomyces_ yeasts during the fermentation processes of wine and beer, such as _Cyberlindnera saturnus_, _Debaryomyces hansenii, Hanseniapora uavun_, _Metschnikowia pulcherima_, and _Wickerhamomyces anomalus_[40]. The second most important aroma-active terpene in the analyzed beers was cis-rose oxide. It was present in a rather low concentration (0.5-0.7 ug/L), but due to the low odour threshold, it was detected in almost all samples (the highest intensity of _D. bruxellensis_ beers--1.6). Fermentation studies with a model that contained deuterated water revealed that yeast is capable of reducing the precursor 3,7-dimethyl octa-2,5-dien-1,7-diol (geranyl diol I) yielding 3,7-dimethyl-5-octen-1,7-diol (citronellyl diol I) that gives rise to cis- and trans-rose oxide after acid-catalyzed cyclization.