## 3 Results and Discussion
### Main Volatile Compounds in the Different Experimental Beers  
To evaluate the influence of the mixed starters used on the analytical profiles, the different beers produced from PW, WW and AW ports were analysed for the main by-products, such as acetaldehyde, ethyl acetate, acetoin and higher alcohols (\(n\)-propanol, isobutanol, active amyl alcohols). In Table 3, the levels of these compounds, detected after bottle refermentation, are reported. The variance analysis (ANOVA) of these data showed significant differences (\(p<0.05\)) among the different experimental beers for almost all the compounds, except ethyl acetate in WW and AW fermentations, and isoamyl alcohol in Weizen beers, demonstrating the influence of non-_Saccharomyces_ strains on the aromatic characteristics of the experimental beers. Regarding Pilsner beers, the main differences between the single and mixed starters were related to the lowest content of \(n\)-propanol, isobutanol, \(n\)-butanol and acetoin found in beers fermented with the _S. cerevisiae_ P4 strain. Regarding acetaldehyde, the P4, P4-CR1 and P4-LC2-1 beers contained similar amounts, significantly higher than the acetaldehyde content of the experimental beers obtained with the mixed starters including _C. zemplinina_ strains (P4-TSF and P4-CZ3). The level of ethyl acetate, which is responsible for a fruity aroma (2019), was significantly higher in beers produced by mixed fermentation with the P4-LC2-1 and P4-TSF starters (15.42 and 16.94 mg/L respectively). The lowest amount was found in P4-CZ3 beer (7.55 mg/L). Active amyl alcohol levels were very similar in all the samples, with the exception of beer produced by P4-CZ3, which contained a significantly higher level of this compound. The highest level of active amyl alcohol was detected in experimental beer obtained with the mixed starter including the _T. delbrueckii_ strain (P4-LC2-1). In WW, the beer produced by the pure P4 starter was different from beers produced by mixed starters in the content of all secondary compounds, except for ethyl acetate and isoamyl alcohol. The lowest amount of acetaldehyde was found in beer produced by P4-CZ3 (38.29 mg/L), similar to the data reported for PW fermentation. Fermentation with the pure P4 culture resulted in the highest amount of \(n\)-propanol and the lowest amounts of isobutanol, \(n\)-butanol and acetoin. These results, except for \(n\)-propanol levels, were also in agreement with data obtained from the analysis of Pilsner beers. As regards amyl alcohols, similar levels were found among all the samples, except for the experimental beer produced by P4-CZ3, which was characterised by the highest level of active amyl alcohol. This mixed starter showed a similar metabolic behaviour both in PW and WW fermentations, as the beers produced by P4-CZ3 were characterised by the highest amounts of numerous compounds, such as isobutanol, \(n\)-butanol, acetoin and active amyl alcohol. In Amber beers produced by the different starters, significant differences in the level of all secondary compounds were found, except for ethyl acetate. \(n\)-Propanol was produced at varying amounts; the lowest value was in P4-LC2-1 beer and the highest amount was in P4-CZ3 beer. Regarding isobutanol, \(n\)-butanol and acetoin, differences were recorded between beers produced by the P4 single starter and those by mixed starters. The lowest values of these compounds were produced by the single _S. cerevisiae_ P4 starter, as already reported for PW and WW fermentation, whereas the highest values were found in beer produced by the mixed starter P4-TSF for isobutanol and in beer produced by the mixed starter P4-LC2-1 for \(n\)-butanol and acetoin. The beer produced by P4 differed from those produced by mixed starters for the highest content of active amyl and isoamyl alcohols.