## 3 Results and Discussion
### Volatile Compound Characteristics  
The complex flavor of the beers can largely be attributed to biochemical activities within the yeast cell during fermentation processes. Flavor-active substances produced by fermenting yeast cells can be divided into five main groups: sulfur-containing molecules, organic acids, higher alcohols, carbonyl compounds, and volatile esters [29]. In Table 3 are reported the volatile compounds identified in the beers brewed using the different starter yeasts. \begin{table}
\begin{tabular}{r c c c c c c c c} \hline
**Peak No. ** & **Compound** & **R.T.** & **RI\({}_{\text{LLL}}\) 2** & **RI\({}_{\text{Exp. }}\) 3** & **41 CM Ale** & **S04 Ale** & **41 CM Lager** & **W-34/70 Lager** \\ \hline \multicolumn{2}{c}{Alcohols:} & & & & & & & & \\
1 & Ethyl alcohol & 2.63 & - & \(<\)500 & 42.85 \(\pm\) 0.55 & 48.73 \(\pm\) 0.68 & 50.54 \(\pm\) 1.01 & 37.65 \(\pm\) 0.42 \\  
& Decanoic acid, & 28.16 & 1594 & 1597 & 0.55 \(\pm\) 0.04 & 0.15 \(\pm\) 0.02 & 0.66 \(\pm\) 0.05 & 0.09 \(\pm\) 0.01 \\ \hline \multicolumn{2}{c}{Total esters} & & & & & & & & \\
15 & Nonanal & 15.33 & 1102 & 1105 & 0.04 \(\pm\) 0.00 & 0.04 \(\pm\) 0.00 & 0.09 \(\pm\) 0.01 & 0.08 \(\pm\) 0.01 \\
19 & Decanal & 18.35 & 1203 & 1201 & 0.05 \(\pm\) 0.00 & 0.09 \(\pm\) 0.01 & 0.12 \(\pm\) 0.01 & 0.17 \(\pm\) 0.02 \\ \hlineAlcohols represented the major group in all the beers, followed by esters, terpenes, and aldehydes. The ale beer obtained with the use of _S. cerevisiae_ 41 CM showed the lowest percentage amounts of total alcohols (57.75%), concerning the beer obtained with the use of _S. cerevisiae_ S-04 (69.37%). Conversely, in lager beers, _S. cerevisiae_ 41 CM contributed to obtaining a percentage of total alcohols (68.87%) higher than that obtained with the use of _S. pastorianus_ W-34/70 (49.27%). After ethyl alcohol (Peak No. 1), phenethyl alcohol (Peak No. 16) had the highest relative percentage, ranging from 7.26% (W-34/70 lager) to 13.07% (S-04 ale). _S. cerevisiae_ 41 CM contributed to obtaining a higher relative percentage of phenethyl alcohol in lager (11.67%) than in ale beers (9.11%). In particular, phenethyl alcohol is an important aromatic compound which gives the beer an elegant and delicate rose fragrance [31]. The other most present alcohols were 1-butanol, 2-methyl, and 1-butanol, 3-methyl which give the beer hints of whiskey, malt, and burntness [32]. The production of higher alcohols is directly associated with the metabolism of amino acids, which are the main source of nitrogen in cereal-based word [33]. Alcohols play a direct role in taste perception or through interactions with other beer components due to the synergistic and antagonistic effects these compounds induce on taste perception [34]. Moreover, higher alcohols are precursors of esters that contribute to the impression of a fruity aroma. In fact, higher alcohols and esters are necessary for the aromatic profile of a high-quality beer [12]. The flavor profiles of beer can principally be attributed to the enzymatic activities during fermentation within the yeast cell, where sugars are converted to ethanol and volatile compounds such as higher alcohols and esters, which are simultaneously formed as by-products of yeast metabolism [35]. Esters are a class of very important volatile compounds that generally characterize beer with fruity-floral aromas [36]. These compounds are mainly produced by yeast metabolism in the intracellular space by condensation between the acetyl-CoA derivatives of aliphatic acids with alcohols and are rapidly diffused through the yeast cell membrane in the fermentation medium due to their lipid-soluble characteristics. In addition, transesterification of natural hop-derived methyl esters into ethyl esters by yeast can occur [35]. In ale beer produced with _S. cerevisiae_ 41 CM, there was a higher relative percentage of total esters (40.94%) than in beer obtained with the use of _S. cerevisiae_ S-04 (28.14%). On the other hand, in the production of lager beer at 12 \({}^{\circ}\)C, _S.