## 3 Results and Discussion
### Identification of Volatile Metabolites in Analyzed Grape Marc Spirits
Acetic acid (acetic and vinegar-like odor) was the most abundant volatile acid in the analyzed distillates, with varying concentrations among samples, but under its perception threshold. Other studies have presented higher concentrations of acetic acid in European distillates, but in this work, samples were not fresh distillates, and, as is already known, the concentration of acids decreases after storage. Fatty acids have also been detected in grape marc spirits [55]. Octanoic and decanoic acids were the most important components of this compound group, with lower concentrations than acetic acid, being below 1 mg L\({}^{-1}\) in most cases. A high concentration of octanoic acid was detected in Thessally samples, while \(n\)-decanoic acid was more abundant in Muscat and Muscat of Alexandria samples from Thessaly, Thessaloniki, and Limnos distillates at similar levels to Portuguese bagaceiras spirits [54] and Croatian Muscat Blanc and Muskat Ruza Porecki spirits [27]. A group of volatile compounds that is positively involved in the sensory quality of distillates is the group of higher alcohols. 2-Methylpropan-1-ol, amyl alcohols, and 2-phenylethanol were found to be the dominant alcohols in Greek distillates, as well as in other European beverages, such as grappas or orujos. The presence of amyl alcohols reinforces the structure of the distillates in the mouth, while 2-phenylethanol contributes to rosy, sweet, and perfume-like notes of the distillate [9; 25; 54]. 2-Methylpropan-1-ol and amyl alcohol levels increase during aging, and their concentrations were found as similar to other studies for Greek distillates [10; 21]. 2-Phenylethylanol tends to be distilled when the alcohol content is low, typically characterized as a tail product, so it should be present in marc distillates at low levels. However, this depends on both grape variety and distillation techniques employed [25; 31]. Its concentration was higher in Thessaloniki and  
Figure 4: Boxplots of representative flavored compounds resulting from the addition of anise in Macedonia and Thessaly tispuoro samples. Black dots represent the samples’ values and yellow dots are the mean values. some Macedonian samples produced from Muscat grapes; however, it did not present great differentiation among analyzed samples, as illustrated in Figure S2. It increases during the aging of wines and brandies, probably as a result of the transesterification reaction of phenethyl acetate [26]. 1-Hexanol is another alcohol contributing to the aroma of spirits, with positive herbaceous notes in low concentrations, or with coconut-like, harsh, and pungent aromas when the concentration exceeds 20 mg L\({}^{-1}\)[10; 25; 26; 54]. An interesting observation is that the sample of Cyprus was very rich in higher alcohols, especially in isoamyl alcohol, which increased during aging. Aldehydes, which are found in alcoholic beverages as a result of spontaneous or microbially mediated oxidation [26], were detected in tispuoro samples in low concentrations. Acetaldehyde, which is usually the major carbonyl compound in grape pomace distillates [25; 56], was detected in much lower levels than its perception threshold (25 mg L\({}^{-1}\)) and even lower than the concentration in the European distillates analyzed by [PERSON] et al. [54] and [PERSON] et al. [25]. Whilst the concentration of acetaldehyde increases during aging, the levels of this metabolite in the analyzed samples suggest the high-quality of the Greek distillates. Moreover, this is in agreement with the results of [PERSON] et al. [26], who noticed that there was no statistically significant change in the concentration of acetaldehyde in Greek tispuoro after 12 months of storage. Acetaldehyde ethyl methyl acetal, acetaldehyde ethyl amyl acetal, and para-anisaldehyde diethyl acetal were observed to be more abundant in Macedonia and Thessaly tispuoro samples.