## 3 Results and Discussion
### Volatile Compound Characteristics
pastorianus_ W-34/70 contributed to obtaining a higher percentage of total esters (48.80%) than with the use of _S. cerevisiae_ 41 CM (28.49%). It has been demonstrated that the esters diffuse amongst the cells and fermenting medium depending on the yeast species used and the temperature, since most esters are retained at lower temperatures. The higher proportion of esters produced remains inside cells of lager yeasts, and in the fermenting medium for ale yeasts [29; 37], this explains the different relative percentage of total esters in ale beer (40.94%) compared to lager beer (28.49%) obtained with the use of _S. cerevisiae_ 41 CM strain. In particular, octanoic acid, an ethyl ester (ethyl octanoate; Peak No. 18), was the most abundant in lager beers, with relative percentages ranging from 8.19% (41 CM lager) to 31.12% (W-34/70 lager). Other more detected esters, with relative percentages ranging between around 1.0% and 6.0%, were: 3-methylbutyl ester (Peak No. 5); hexanoic acid, an ethyl ester (Ethyl hexanoate; Peak No. 9); 9-Decenoic acid, an ethyl ester (Ethyl 9-decenoate; Peak No. 26), and Acetic acid, a 2-phenylethyll ester (phenyl ethyl acetate; Peak No. 21). Generally, esters have low odor thresholds. However, the presence of different esters can have a synergistic effect on the individual flavors, which means that esters can also affect beer flavors well below their individual threshold concentrations [38]. Ethyl hexanoate has a low concentration threshold at 0.005 mg/L, ethyl octanoate at 0.5 mg/L and ethyl decanoate at 1.5 mg/L [35]. In particular, ethyl hexanoate, ethyl octanoate, and ethyl decanoate are also considered as highly positive flavor attributes of beer aroma related to flowery and fruity descriptors [39]. The aroma of 3-methylbutyl ester is described as banana and pineapple, while the phenyl ethyl acetate is described as roses and honey [39]. The different enzymatic activities, as esterase, are influenced by temperature and are strain-specific; the temperature has a major impact on beer fermentation and maturation, in fact, and as the fermentation temperature increases, there is a decrease in acetaldehyde and proximal diketones and an increase in esters and higher alcohols [40]. Among the volatile compounds, the presence of aldehydes such as nonanal and decanal with total relative percentages between 0.09% (_S. cerevisiae_ 41 CM ale) and 0.25% (_S. pastorisenius_ W-34/70 lager) was detected. Aldehydes constitute oxidized counterparts of the corresponding alcohols and play a very important role in beer flavor, being responsible for the fresh and slightly green notes of barley. Many of the aldehydes present in word are converted by reduction reaction due to the action of yeasts during fermentation by reduction to their saturated ethanol counterparts [41]. On the other hand, the re-oxidation of higher alcohols to carbonyls during the aging of beer may render them flavor-active again. Aldehydes are also synthesized by yeast as intermediates in the formation of ethanol through the decarboxylation of keto acids. The presence of active yeasts in the maturation phase is a prerequisite for satisfactorily low aldehyde levels in the final product. The aliphatic aldehydes have a green, grass-like aroma, a quality that might be considered desirable at low concentrations; in particular, nonanal and decanal have been related to citrus and orange-peel descriptors. During beer storage, increases in levels of staling aldehydes coincide with the sensory perception of undesirable aging off-flavors [29]. In beers, terpenic compounds are mainly derived from the hop essential oils which are added to the word during the boiling process. These compounds are related to pleasant aromas such as citrus, flowers and lilac [42; 43]. The terpenes detected in the beers produced in our tests were linalool and citronello, with relative total percentages ranging from 0.61% to 1.18%. Recently, [PERSON] and [PERSON] [44; 45] reported the biotransformation of monoterene alcohols by yeast metabolism.