## 3 Results and Discussion
### Key Arroma Compounds of Beers Produced with Non-Conventional Yeasts
The higher alcohol, the odour of which was one of the most intense during olfactometric analyses, was 2-phenylethylanol [Table 3]. In peers obtained with non-_Saccharomyces_ yeast--_W. anomalous, Ha. warum_ and _D. bruxellensis_, the intensity of the phenylethanolamine aroma exceeded 2.5 points, making the rose notes of this compound the most intense aroma in the samples. Similar results were obtained by [PERSON] et al. [34] during olfactometric analyzes of lager beers; therefore, phenylethanolamine should be considered one of the most important volatile components affecting the aroma of beers. Our research has additionally shown that the use of non-_Saccharomyces_ cultures in the production of beers can increase the impact of this component on the aroma. Esters were the most numerous group of volatile compounds in the analyzed beers, but they also had the strongest influence on the aroma. Of the 23 esters detected, 16 were odour-active compounds (detected by GC-O), and most of them were ethyl esters [Table 3]. Although volatile esters are only trace compounds in fermented beverages such as beer, they are extremely important for the aroma profile. The most important aroma-active esters in beer are ethyl acetate (apple-like aroma), isoamyl acetate (fruity, banana aroma), ethyl hexanoate and ethyl octanoate (sour apple), and phenyl ethyl acetate (flowery, roses, honey) [35]. Esters occurring in the largest amounts in the samples did not always significantly affect their aroma due to higher aroma thresholds. The best example of this kind of phenomenon was ethyl acetate, the ester formed during ethanol fermentation in the largest amounts [3]. Thresholds of 5 mg/L were exceeded in five beer samples; however, olfactometric analysis enabled the detection of this component only in three samples, with the average intensity not exceeding one unit. This could be related to the co-elution of this ester with a retention time similar to that of the solvent--ethanol. This can be confirmed by the detection of ethyl acetate aroma in beer obtained with the use of _Z. bailii_, containing smaller amounts of the ester compared to other samples, but at the same time smaller amounts of ethanol. The obtained beers differed significantly in terms of the most intensively detected esters, which could significantly affect their sensory characteristics (Table 4). The ester with the highest intensity (top three) in the tested samples was ethyl hexanoate, with a pleasant aroma of red apple. Ethyl dihydrocinnamate with a rum and plum aroma, as well as ethyl 4-methylpentanoate and ethyl 2-methylbutyrate with the aroma of apple and red fruit, were also characterized by a very high intensity of aroma. The significant influence of the above esters on the aroma of beer has already been demonstrated by [PERSON] et al. [34]. The use of yeast culture significantly modified the composition of esters in the obtained beers. The yeast _Saccharomyces_ is not as a significant producer of esters as non-_Saccharomyces_ yeast [4], and this is especially visible in the case of beers obtained with the participation of _S. cerevisiae_ and _S. paradoxus_. A characteristic feature of the beers obtained with the use of _S. cerevisiae_ Safbrew was a high concentration of ethyl 2-methylbutanoate and ethyl pentanoate, and because these compounds are characterized by a relatively low threshold, they belonged to the compounds with the strongest influence on the aroma of these samples (respectively, 1 st and 3 rd compound with the highest aroma intensity). Similar to ethyl hexanoate (2 nd most intense compound in these samples), they had a red apple aroma. Itwas also one of the two analyzed beers, where the esters were characterized by a greater intensity of aroma than phenylethanol. The second was beer produced with _T. delbrueckii_ yeast, where ethyl hexanoate and ethyl dihydrocinnamate were two main odour-active components. At the same time, it was the beer in which the most different esters were detected during GC-O analyses, 12 out of 16 [Tables 3 and 4]. _T.