## 3 Results
### Volatile Compounds of Soju  
Based on the above experiment results, CAR/PDMS was used as the adsorbent to perform the experiment. Nineteen samples aged in barrels made of six Korean oak species (GAL, GUL, DUK, SAN, SIN, and JOL) were analyzed. Samples that were not aged were used as controls. These samples were carbonized in three ways: noncharring (1), medium charring (3), and heavy charring (5) (Supplementary Table S1). Acids were detected with a significant result, indicating that their concentration tends to increase with aging and carbonization regardless of the oak species. In the control group, the concentration of acetic acid is 0.68 \(\upmu\)g/mL, and the GAL-5 sample had the highest concentration of acetic acid (6.99 \(\upmu\)g/mL). Acetic acid has a sour taste, which turns bitter with a vinegar-like scent at high concentrations [38]. Among the alcohols, the concentrations  
Figure 2: Comparison of the sum of normalized peak areas for each class of volatile compounds in Soju (GUL-5) using five types of SPME Arrow fibers. of ethanol and 1-propanol tended to become lower than that of their counterparts in the control. In contrast, the concentrations of 2-methyl-1-butanol, described as malty and chocolate [39], and isoamyl alcohol, described as having the taste of red fruit and framboise [40], were higher than those of their counterparts in the control. In particular, 2-methyl-1-butanol and isoamyl alcohol were detected at higher concentrations in GUL, DUK, SAN, and SIN samples than in GAL and JOR samples. Twenty-one types of esters were detected, and the concentration of only isoamyl acetate, described as having a fruity, sweet, and banana-like flavor [41], was lower (3.84-7.29 \(\upmu\)g/mL in all samples) than that in the control (9.57 \(\upmu\)g/mL). However, the esters showed a significant increase in the concentration with aging and carbonization regardless of the oak species used. The concentration of all esters (especially ethyl acetate, ethyl caprylate, ethyl caprate, and phenylethyl acetate), except for isoamyl acetate, increased in all aged samples and became higher than that in the control. Furans, which contain seven volatile compounds, were detected in the samples that underwent carbonization irrespective of the oak species used. However, they were not detected in the control and noncharring samples, and their concentration particularly increased in the samples subjected to medium and heavy charring as these samples were toasted for a longer duration. Furfural and 5-methylfurfural, the major furans, are formed when the wood is baked owing to the breakdown of carbohydrates, and their concentration in Soju depends not only on the age of the barrel, but also on the degree of wood toast [42]. Furfural was described as having a caramel, bready, and cooked meat-like flavor, while 5-methylfurfural was described as having an almond and marzipan-like flavor [43]. The concentration of hydrocarbons tended to become lower than that of the control. Trans-Whiskey lactone and oaklactone were not detected in the control, GAL, and SAN samples, while they were detected in GUL (0.29 \(\upmu\)g/mL), DUK (0.22 \(\upmu\)g/mL), SIN (0.47 \(\upmu\)g/mL), and JOL (0.35 \(\upmu\)g/mL), which smell like toasted, woody, coconut, and vanilla [44] and were present in oak wood [45]. Among the phenols, the eugenol concentration tended to increase with aging and carbonization. The sample with the highest concentration of eugenol is GAL-5 (0.27 \(\upmu\)g/mL). As shown in Figure 3, heatmaps were plotted in order to reveal the difference in volatile compounds between the samples. A color gradient of dark blue (-4) indicates values below the average, while a color gradient of dark red (4) indicates values above the average. The volatile compounds that showed the most significant difference in their concentration based on the oak species and carbonization were ethanol, 1-propanol, isoamyl acetate, isoamyl alcohol, furfural, and 5-methylfurfural.