## 2 Results  
### Identification of Volatiles  
To compare the profiles of volatiles for the investigated vegetables, the compounds were isolated using headspace analysis, where the volatiles were adsorbed using SPME fiber and then separated by comprehensive two-dimensional chromatography-mass spectrometry (HS-SPME-GC\(\times\)GC-ToFMS). This approach allows for the specific separation and (tentative in this case) identification of volatile compounds, thus, it is widely used in volatilome fingerprinting and profiling [17; 18]. As the peak capacity of GC\(\times\)GC systems can reach 10,000, the number of identified compounds is usually very high. GC\(\times\)GC-ToFMS provides plenty of data, for this reason extracting useful information is often a very challenging task. For our work, only the most abundant compounds with a relatively high signal to noise ratio (S/N > 250), high similarity and retention indexes comparable to literature data were taken into consideration and the comparison was based on the peak area values and the peak area percentage. This serves the assumed comparative purposes well; however, it has to be remembered that SPME is a partition coefficient based extraction method and the area percentage may be different from the data provided by exhaustive extraction methods. Figure 1 shows the differentiation between various _Brassica_ vegetables using principal component analysis (PCA) based on the profiles of volatile compounds. Each point represents a mean of three replicates analyzed for a given cultivar. One can observe obvious differences in the profiles of particular vegetables, both raw and cooked. However, there are also significant differences between particular cultivars of some investigated vegetables, which is especially noteworthy for raw broccoli and Brussels sprouts. Examination of the PCA graphs shows a similarity between caulfilower and broccoli volatiles in raw vegetables, despite the between-cultivar differences for broccoli, while kohlrabi and Brussels sprouts differ substantially in terms of their volatile compound profiles. When the data are compared for cooked vegetables, the similarities between caulfilower and broccoli can still be observed (close distance between clusters), whereas the other vegetables form distant clusters. Interestingly, after cooking, the differences between cultivars are much less pronounced. This would indicate the more uniform profile of the main volatiles. A detailed list of main compounds for all the examined cultivars is provided in Tables S1-S4 in the supplementary files. The compounds corresponding to the number in the PCA loading plots are presented in Table S5 in the supplementary files. As can be observed in the tables, the compounds can be classified into several main groups: Aldehydes (with alkalans, 2-alkenals  
and 2,4-alcaledients), alcohols (mainly unsaturated), isothiyanates (mainly aliphatic, although also aromatic), other sulfur compounds (mainly sulfides and thiophenes) and nitriles. To a certain extent, the classification of volatile compounds into these classes is also associated with the sensory properties and acceptance of these compounds. Some of the identified aldehydes and alcohols may contribute to the \"green\", \"sweet\", \"fatty\", \"soup\" and other notes in the vegetable aroma. Isothiocyanates are products of the enzymatic hydrolysis of glucosinolates, which are widely investigated because of their beneficial effect on human health. However, they are also partially responsible for the specific pungent notes. Nitriles, which are an abundant group in all analyzed samples, are also found as degradation products of glucosinolates, instead of isothiates [1]. Other sulfur components include mainly products of the cysteine pathway, which are sulfides. All the detected sulfides, i.e., disulfides, trisulfides, tetrasulfides, pentasulfides, and others have been collected under the general name of \"sulfides\", simply because of their labile nature and mutual transposition.