## 3 Results and Discussion
### Effect of Different Brewing Methods on VOC Composition by HS-SPME/GC Mass Spectrometry Analysis

Table 2: Cont. Aldehydes compounds are extender degradation and autoxidation products of unsaturated fatty acids, which are mainly responsible for the malty coffee flavor. Among these molecules, 2-methylpropanal, 2-methylbutanal, and 3-methylbutanal are classified as coffee key odorants as they have a major influence on the taste of coffee. The first was reported to have the higher RPA in V60 samples with a value of 0.42 \(\pm\) 0.12%, while the others were mainly detected in PB and FP (1.11 \(\pm\) 0.16% and 1.08 \(\pm\) 0.29%, respectively, of 2-methylbutanal and 0.98 \(\pm\) 0.12% and 1.32 \(\pm\) 0.27%, respectively, of 3-methylbutanal). In addition, 5-methyl-2-furan carboxaldehyde, a furan derivative associated with high olfactometric activity [29] and with almond, caramel, sweet, and cooking attributes, was found at the highest concentration in V60, representing 10.38 \(\pm\) 1.11% of the total area. The most detected compound of the aldehyde class was furfural, with a range between 16.46 \(\pm\) 1.48% in FP and 19.51 \(\pm\) 1.41% in V60 samples. Thus, furfural derivatives can be formed by the reaction between monosaccharides and an amino acid at high temperatures. The strong presence of furfural in the analyzed samples was reported to contribute to the cereal- and bread-like aroma in coffee [14]. Moreover, the pyrolysis of carbohydrates produces ketones that impart mushroom-like and even caramelized sweet notes [27]. Among ketones, 2,3-butanedione and 2,3-pentanedione are classified as key odorants, and they can be claimed to be associated with a buttery and creamy taste [7]. The highest RPA values of 2,3-butanedione were found in FP samples (0.31 \(\pm\) 0.15%), while those of 2,3-pentanedione emerged in PB samples (0.94 \(\pm\) 0.03%). A previous study concluded that ketones are particularly concentrated in filtered coffee infusions when compared with espresso coffee beverages [30]. Pyrroles, usually associated with nutty, hay-like, and herbaceous flavors [31], have also been detected in the four groups of analyzed samples. Their highest levels were reported in PB and AP samples. The FP coffee filter had the lowest level. Among the phenolic compounds derived from thermal degradation of CQAs and characterized by spicy phenolic aroma and vanillin note, guaiacol and 4-vinyl guaiacol were the principal volatiles, being also coffee-key odorants [32]. The 4-vinyl guaiacol, known to be associated with spice and clove aromas, reported similar RPA within PB, V60, and FP samples (1.8 \(\pm\) 0.11%, 1.58 \(\pm\) 0.17%, and 1.62 \(\pm\) 0.29%, respectively) and higher values in AP (2.76 \(\pm\) 0.77%). In contrast, guaiacol was detected only in PB and FP (0.44 \(\pm\) 0.13% and 0.24 \(\pm\) 0.08%, respectively). Guaiacol is associated with sweet and medicinal flavors and evokes a burning sensation even at very low concentrations [33]. Pyridine is known to have fishy, roasted, and astringent properties, and can elicit a pungent, burnt taste even at concentrations in the mg/kg range. In coffee samples, pyridine is formed by degradation of trigonelline during the Maillard reactions activated due to the roasting processes [31]. This compound was mostly detected in PB samples (1.49 \(\pm\) 0.11%). Among terpene alcohols, linalool and linalool oxides, _cis_-linalool oxide (furan) and _trans_-linalool oxide (furanoid), were detected. Linalool is related to flowery aromatic notes and was found similarly in all analyzed samples, with a range between 0.44 \(\pm\) 0.12% and 0.59 \(\pm\) 0.1% in FP and AP, respectively. The _cis_-linalool oxide (furan) that was reported to confer sweet-fruity aromatic notes to coffee [34] was mostly abundant in V60 samples (0.44 \(\pm\) 0.12%). In summary, volatile organic compounds (VOCs) are the most quality-determining constituents of coffee, occurring in concentrations ranging from parts per trillion (ppt) to parts per million (ppm). Among the 49 VOC compounds analyzed in this study, 10 are known as key aroma compounds (key odorants), as they can affect coffee taste and produce perceptible odors.