## 2 Results and Discussion
We determined the esters in the maximum amount of 22,553.1 \(\mathrm{\SIUnitSymbolMicro g}\)/kg, alcohols up to 4829.3 \(\mathrm{\SIUnitSymbolMicro g}\)/kg, aldehydes up to 3747.6 \(\mathrm{\SIUnitSymbolMicro g}\)/kg, and ketones up to 1306.9 \(\mathrm{\SIUnitSymbolMicro g}\)/kg. Most likely, the discrepancies result from the different standards used for quantitative determinations. In another paper, researchers determined VOCs from only one apple variety based on cyclohexanone as a standard [26]. The results of quantitative analyses showed the presence of esters in the maximum amount of 48,705.41 \(\mathrm{\SIUnitSymbolMicro g}\)/kg, alcohols up to 25,578.92 \(\mathrm{\SIUnitSymbolMicro g}\)/kg, aldehydes up to 31,862.49 \(\mathrm{\SIUnitSymbolMicro g}\)/kg, and ketones up to 1188.32 \(\mathrm{\SIUnitSymbolMicro g}\)/kg. Although, in this case, the results may be more influenced by the fact that the variety itself probably has a different profile than others, the selection of the standard (ketone) itself was probably not without influence. We can assume that depending on the chemical group of the standard used, diametrically different response factors are observed in the chromatograms and, consequently, the quantitative values of the analysis. We are aware that quantitative analyses of each compound separately for hundreds of compounds present in the samples are impossible. However, we believe that in order to maintain the reliability of the results, at least one calibration curve/one internal standard should be performed for each of the main analyzed groups of compounds, i.e., esters, alcohols, aldehydes, ketones, and acids. Regarding the evaluation of whether it is possible to fingerprint apple cultivars or at least find some key characteristics allowing for a distinction between cultivars, two-level statistical analyses were performed. Based on the volatile profile, the preliminary analysis, which separately included all 119 VOCs identified in apples, allowed the old and commercial cultivars to be separated (Figures 1 and 2). The only exceptions were 'Koksa Pomaraczowa' and 'Koszeta' apples from one cultivar, which were matched to commercial cultivars, and 'Lobo' apples from two cultivars, which were matched to the old cultivars. The second-level PCA was conducted using the raw data of apples belonging to commercial cultivars in order to identify the groups of compounds that best represent the variance of the results (Figure 3). Based on the 12 components generated with 70 compounds present in the apples of the commercial cultivars, the cultivars were separated (Figure 3, Table S2 in Supplementary Materials). The varieties were grouped into 7 clusters: (1) 'Gala', (2) 'Melrose', (3) 'Red Prince', (4) 'Lobo' without 'Lobo 7' samples, (5) 'Ligol', (6) 'Szampion', and (7) only 'Lobo 7' samples. A Kruskal-Wallis test and the median test allowed for choosing 34 representative compounds that significantly differentiated the mentioned cultivars (Figure 3, Table S3 in Supplementary Materials). Based on the obtained results, it may be assumed that the volatile profile has a good potential to be used to fingerprint apple cultivars. Considering the genetic diversity of apple varieties [4; 7], which is a direct reason for the differentiation of VOC profiles, it is particularly interesting how well samples from different crops within a variety match each other, especially for such varieties as 'Gala', 'Ligol', or 'Szampion'. It is also worth noting that the geographical factor (location of the orchard) is of lesser importance because otherwise, the samples would be grouped by number, not cultivar. In this case, it would be appropriate to analyze samples from further locations or even other countries to confirm how climate affects the composition of VOCs. On the other hand, taking into account the number of crossbreeds of varieties that are often combined under one name, it may explain why varieties such as 'Gala' or 'Koksa pomaraczowa' match much worse.