## 3 Results and Discussion
### Key Arroma Compounds of Beers Produced with Non-Conventional Yeasts
The presence of cis-rose oxides in fermented beverages can therefore be attributed to the reductive yeast metabolism during fermentation [41]. It is generally believed that during fermentation, the geraniol from the hops is converted by the yeast into citronello [39]. In the case of beers obtained with the participation of _S.cerevisiae_, _S. paradoxus_, _D. bruxellensis_ and _T. delbrueckii_ yeasts, such a bioconversionoccurred (Tables 4 and 5). However, in the remaining four, the level of geraniol remained unchanged and even increased, thus remaining above the aroma threshold. The increase in the level of citronellol was therefore not associated with a reduction in the amount of this compound. According to [PERSON] et al. [39], this increase in \(\beta\)-citronellol might be partly explained by an occurrence of glycosylated bound flavor precursor and a glucoside hydrolase activity secreted from yeast. Nerolidol was also a terpene that appeared after fermentation, becoming the odour-active component in three beers. Nerolidol accumulation could be stimulated by the inhibition of sterol biosynthesis. Under anaerobic conditions, meridolol formation would be stimulated, whereas under microaerobic conditions the sterol biosynthesis pathway mostly occurs [42]. The floral-fruity notes in beer were also associated with the presence of aldehydes such as nonanal, decanal and dodecanal. These compounds have a low aroma threshold and are formed as a result of lipid oxidation, and their presence in the amount of several \(\mu\)g/L of beer is its natural feature [43]. All analyzed beers contained the compounds in question in amounts exceeding the aroma threshold. The highest odour intensity of these components was found in samples obtained with _Saccharomycesces_, _T. delbrueckii_ and _D. bruxellensis_ yeasts. Malt and hops contribute to the lipid and fatty acid precursors that can then potentially be transformed by yeast also into other important groups of aroma components of beer--lactones [44]. Most commonly reported lactones in alcoholic beverages are \(\gamma\) and \(\delta\)-lactones (five and six membered rings, respectively) with their potent oily, stone fruit, and coconut aroma qualities. We detected two different \(\gamma\)-lactones in the samples. As in the case of aldehydes, the sweet, fruity aroma of this \(\gamma\)-decalactone was found during olfactometric analysis in beers fermented with _Saccharomycesces_ and _T. delbrueckii_ yeasts. Currently, in the brewing industry, more and more attention is paid to the determination of volatile sulfur compounds due to their low aroma thresholds. However, there is still very little research on this subject, and the available articles date back many years. These compounds, present in beer in higher concentrations, can contribute to the formation of unpleasant flavors and can be used as indicators during fermentation [45]. Three sulfur compounds (methanol, dimethyl disulfide (DMDS) and dimethyl disulfide (DMIS)) were found in the analyzed beers, all significantly affecting the aroma of the beverages. As it was already described earlier, all of them were present in the work. After fermentation, their level decreased but remained above the threshold. The sample in which all these three compounds were detected during olfactometric analyses was beer obtained with the use of _D. bruxellensis_ yeast [Tables 4 and 5]. In low-alcohol beer produced by _Z. bailii_ yeast, metholasal was the main odour-active component [Tables 4 and 5]. As it should be assumed, the ability to reduce this compound during fermentation depends on the available carbon sources. The presence of the discussed sulfur compounds is associated with the microbial decomposition of sulfur amino acids--cysteine and methionine [46]. DMIS is characterized by a high potential for introducing sulfuric notes into beers and it is a component of hops [47]. DMIS is recognized, e.g., through aromas such as onion, boiled cabbage, and garlic. These characteristics also resemble DMS (dimethyl sulfide), but they are more intense [48].