## 3 Results and Discussion
A significant cluster of compounds identified was composed of terpenes, with a total of 15 compounds. These compounds are synthesized in plants in response to stress and play a vital role in communicating with their environment [22; 23]. Terpenes are well known for their unique aromas, such as floral, sweet, citrus, camphor, herbal, and spicy, and are  
Figure 1: Chemical families of volatile compounds identified in natural corks via HS-SPME-GC-MS. Different letters (a, b, c, and d) indicate significant statistical differences between samples. characterized by their low olfactory detection thresholds [24]. While all samples contained the identified terpenes, noticeable differences were observed in their concentrations. The NS1 and NS2 samples, obtained from the same geographic location, exhibited a remarkable concentration of total terpenes that differed significantly from the other samples. This differentiation was mainly attributed to their high levels of \(\alpha\)-copaene and limonene. \(\alpha\)-Copaene is a sesquiterpene that imparts woody and spicy notes and is present in various plant species [25; 26]. Limonene, on the other hand, is a monoterpene with a sweet and citrus aroma that occurs in high amounts in the peels of citrus fruits [27]. Earlier studies have identified both terpenes in wine model solution extracts of corks, implying their possible migration to wine during aging and/or storage [13]. While no significant differences in the overall terpene content was noted between the NS1 and NS2 samples, the latter, with lower visual quality, exhibited notably higher concentrations of several terpenes, including \(\alpha\)-pinene, campene, eucalypt, camphor, \(\alpha\)-terpineol and isothymol methyl ether. These terpen, except isothymol methyl ether, have previously been found in natural and conglomerate cork from different origins [11; 13; 14], and some of them have been identified as important odorants in natural cork stoppers using gas chromatography-olfactometry [11]. In addition to isothymol methyl ether and its isomer, thymol methyl ether, other terpen such as \(\alpha\)-muurolene, cadalene and methyl dihydrojasmonate were detected in natural cork stoppers for the first time. Although these terpen have not been previously described in cork stoppers, it is well known that plants, including _Quercus suber_, emit several floral and woody volatile terpenes as defense agents [28]. In contrast, the NS1 and NS2 samples from northeast Spain showed a lower content of acids compared to the SS1 and SS2 samples from south Spain and P1 from Portugal. Out of the seven identified acids, notable ones included acetic acid in SS1, SS2 and P1 corks, hexanoic and octanoic acids in SS1 and SS2 corks, and 2-ethylhexanoic acid in P1 corks. These acids are typically present in the volatile fraction of wines, but due to their high thresholds, their negative olfactory notes do not significantly contribute to wine aroma [29]. Acetic acid was the major acid identified, which could have originated from thermal procedures used during the processing of cork planks and stoppers. This is because its formation from wood constituents, mainly from acetate groups of hemicelluloses, has been reported at high temperatures [30]. Aldehydes and ketones were among the other volatile carbonyl compounds identified in the cork samples. These compounds are produced through the lipid oxidation of free fatty acids found in the wavlike and suberin fractions of the corks [21]. The total aldehyde content was higher in the NS1 samples, primarily due to the greater concentration of nonanal resulting from the oxidation of oleic acid. However, the NS2 corks, which shared the same geographical origin, did not show significant differences from the rest of the samples. Some of the aldehydes identified, such as (E)-2-ocneal, decanal and (E)-2-nonenal, have been associated with the \"sawdust\" aroma of wines aged in new oak wood barrels [31].