## 3 Results and Discussion
### Main Physicochemical Characteristic of the Beers  
The data of the physicochemical analysis of the beers are reported in Tables 1 and 2. Significant differences between ale and lager beers obtained with the use of _S. cerevisiae_ 41 CM, _S. pastorianus_ W-34/70, and _S. cerevisiae_ S-04 were found, except for pH and final density. At the end of fermentations and maturation, the ethanol content in the beers obtained with the use of _S. cerevisiae_ 41 CM strain was, respectively, 4.20 % _v/v_ in ale and 4.05 % _v/v_ in lager beers, significantly different with respect to beers obtained using _S. cerevisiae_ S-04 (4.47 % _v/v_) and _S. pastorianus_ W-34/70 (4.51% _v/v_) strains. Acetic acid is the main component of volatile acids in beer and an excess of it can give an unwanted hint of vinegar. The concentration of this acid found in ale and lager beers, fermented using _S. cerevisiae_ 41 CM strain were, respectively, 60.86 mg/L and 70.37 mg/L, significantly lower with respect to beers obtained with the use of _S. cerevisiae_ S-04 (80.89 mg/L) and _S. pastorianus_ W-34/70 (90.88 mg/L) strains. This compound is formed during yeast metabolism through the oxidation of acetaldehyde or the reabsorption and metabolization of pyruvic acid [25]. Regardless of the starter used, in the beers produced in our tests, the quantities of acetic acid remained below the threshold value of 100 mg/L [26]. Glycerol is a desirable alcohol and positively influences beer flavor intensity, fullness, and body [27]. In ale and lager beers where the _S. cerevisiae_ 41 CM strain was used, the glycerol content detected was 1556.66 mg/L and 1289.33 mg/L, respectively. Significantly different were the amounts of glycerol found in beers obtained by the use of _S. cerevisiae_ S-04 (1685.33 mg/L) and _S. pastorianus_ W-34/70 (1196.01 mg/L) strains. As regards the concentration of DL-lactic acid, the _S. cerevisiae_ 41 CM strain produced amounts of this acid of 20.73 mg/L (lager beer) and 47.90 mg/L (ale beer), significantly lower compared to the beers obtained with the use of _S. pastorianus_ W-34/70 (28.23 mg/L) and _S. cerevisiae_ S-04 (57.80 mg/L) strains. This acid is mainly derived from yeast metabolism and was reported to have a threshold flavor of about 400 mg/L [26]. Therefore, in all the beers obtained in our tests, this acid does not exert a direct effect on the flavor, while it contributes to the pH of the beer. Diacetyl (2,3-butanedione) is a vicinal diketone product of yeast metabolism and is more characteristic of ale than lager beers; its concentration is usually used as an indicator of word fermentation or maturation quality [20]. Diacetyl, mainly in lager beer, is considered an undesirable volatile compound above the taste threshold of about 0.10-0.15 mg/L since it could give the beer a stale milk aroma and flavor [28]. Our results showed a high variability regarding the diacetyl production; in particular, the _S. cerevisiae_ 41 CM strain used both for ale and lager beer produced lower amounts of this compound (0.10 and 0.11 mg/L) compared to the beers obtained by commercial _S. cerevisiae_ S-04 and _S. pastorianus_ W-34/70 strains (0.12 and 0.29 mg/L, respectively). Yeast cells hold the enzymes required to reduce diacetyl to acetoin and then to 2,3-butanediol as well as those required to reduce 2,3-pentanedione to 2,3-pentanediol. This elimination of diacetyl and 2,3-pentanedione occurs at the end of the conventional main fermentation period and during the maturation of beer. These reduced compounds have a much higher flavor threshold and their presence is acceptable at the concentrations usually found in beer. The detected amounts of acetaldehyde, both in ale and lager beers, were between 1.33 and 2.40 mg/L, below the threshold perception values (5-15 mg/L). Acetaldehyde is