## 3 Results and Discussions
### VOC Tracing Using GC\(\times\)GC  
Beer production involves a considerable number of important steps, which presents many variables that can affect the final composition of the beverage if they fluctuate from the expected values. A small fluctuation during beer production can be responsible for relevant changes between the ratio of its components or the presence of unique ones in the perceived flavor, altering the volatile organic compounds and organoleptic profile of the beer. The chromatograms obtained for BR and ENG beer samples, exhibited in Figure 2, shows the presence of approximately 210 compounds in the beer aroma. Main chemical classes of analytes, see Table 1, include alcohols, terpenoids, organic acids and esters, which are corroborated by previous findings [3; 4]. Esters are important contributors to flavor and in general are present in concentrations around their threshold, which means that minor changes in their concentration may dramatically affect beer flavor [27; 28]. Usually they are compounds that impart a sweet-fruity aroma to beers and among them the ones with proven aroma activity can be synthesized intracellularly by fermenting yeast cells. The volatile portion of these compounds are produced in an enzyme-catalyzed condensation reaction between an activated fatty acid (acyl-Coa) and a higher alcohol [28]. Esters detected in the beer samples include 2-phenylethyl acetate and perillyl acetate, with first one contributing to honey attributes and the second one associated with woody and raspberry notes. Figure 2 also reveals a region, indicated by the dashed area between 40-46 min, with notable differences between the samples, with BR sample exhibiting compounds with higher intensities than ENG; among them are aromadendrene (analyte #2), which is associated with woody notes. Important terpene hydrocarbons also identified are \(\gamma\)-muvrolactone (analyte #8), responsible for herbal and spicy attributes, and \(\alpha\)-humulene (analyte #3) a hop oil compound that impart woody notes in the aroma profile. Another chemical class that plays an important role in the aromatic profile of beers are volatile phenols. 4-vinylguaiacol and 4-vinylphenol are the main phenols that impart flavor in beer. Usually their presence must be controlled, otherwise surpassing a specific concentration they are considered off-flavors that negatively affect beer quality [9; 29]. 4-vinylguaiacol (analyte #1) exhibits a higher intensity in the BR sample, imparting a sweet smoky attribute to the organoleptic profile. Conversely, ENG sample exhibits higher intensities for methyl 2-nonynoate (analyte #4), a flavoring related  
Figure 2: GC\(\times\)GC-QMS chromatograms for two craft beer samples using HS-SPME and sampling with DVB/CAR/PDMS (**a**) BR sample; (**b**) ENG sample. BR corresponds to the beer brewed with the _Brazilian Ale 02_ yeast and ENG corresponds to the beer brewed with the _Nottingham_ yeast. substance that contributes with floral and tropical fruity notes, and 2-phenylethyll acetate (analyte #5) that imparts honey and floral rosy attributes. These findings are corroborated by previous studies of the VOC profile of beers [16; 23; 30]. As a matter of fact, it is important to highlight that although approximately 210 peaks were detected in the chromatograms, only 79 (\(\sim\)38%) were successfully identified based on the selected qualitative parameters (80% of similarity and \(\pm\) 25 LTPRI deviation from NIST Web Book). This highlights the need to couple GC\(\times\)GC with high accuracy mass spectrometers to complement qualitative analyses. It should be noted that hexanoic acid (analyte #6), heptanoic acid (analyte #9) and octanoic acid (analyte #7) were successfully identified, although their peaks exhibited deviations higher than 25 LTPRI units. Identification was confirmed by evaluating their peak shapes in the chromatograms and similarities between their experimental mass spectrums and NIST database.