## 2 Results and Discussion
### Non-Targeted Analysis of Two Coffee Groups
#### 2.2.1 Classification Model and Feature Selection
Samples in group III are instrumental triplicates of the same sample, and scCO\({}_{2}\) was used for the decaffeination process. Samples in group II were subjected to decaffeination methods that are different from group I and III. As highlighted by the targeted analysis, the regular coffee samples presented higher peak areas in alcohols, pyrazines and phenols. Their higher presence was previously discussed in Section 2.1. Similar to pyrazines, thiazoles significantly contribute to the aroma profile of coffee. Their presence is attributed to the Strecker degradation [33]. Additionally, they are strongly related to the roasting intensity of coffee [34]. The presence of an oxolane-derived compound was also noticed among the selected top features with higher peak areas in the regular coffee samples. In fact, oxolanes originate from the thermal degradation of melanoidins due to the roasting process and are therefore linked to the roasting intensity of the coffee beans [35]. As previously discussed, decaffeinated coffee  
Figure 3: The heatmap of the top 25 significant aroma-related volatiles selected by feature selection tools. The \(x\)-axis indicates the samples names. The first letter of each sample name indicates the coffee type: regular (R) or decaffeinated (D) and the second letter corresponds to the label of the coffee capsule. The \(y\)-axis indicates the specific aroma-related volatiles (refer to Figure 2). The chemical structures of the volatiles are discussed in the text. is more likely to be less roasted than regular coffee, thus this could explain the higher presence of thiazoles and oxolanes in regular coffee samples. Caffeine was also present among the top 20 features. Its peak areas were evidently higher in regular coffee samples. On the other hand, decaffeinated coffee samples displayed a more prominent presence of furan and pyrrole derivatives, corroborating therefore the conclusions drawn by the targeted analysis. Furans are one of the most distinguishable volatile aroma components in coffee. Their presence is related to the thermal degradation of endogenous components during the roasting process [36]. Based on the selected discriminatory features, furan derivatives seem to be a strong marker of the decaffeinated group. Figure 4: The comparison of the normalized contents of 8 pyrazines which presented a significant difference between R-R vs. D-R, R-F vs. D-F, and R-VO vs. D-VO. The first letter of each sample name indicates the coffee type: regular (R) or decaffeinated (D) and the second letter corresponds to the label of the coffee capsule. Figure 5: _Cont_. For the selected decaffeinated coffees, the manufacturers report their products with the flavor profiles of sweet aroma of almonds, chocolate and dried fruits for the D-L sample, cacao notes for the D-F sample, notes of caramel, chocolate and toasted bread for the D-I sample, and fruity notes for the D-R, D-VO and D-VI samples. Some of these flavors were not reported in their regular coffee counterparts. As for the other samples, i.e., D-DE and D-S, allergens such as peanuts and sesame were reported in the allergen section of the products. Little information regarding the impact of the coffee type on furan presence is available in the literature. However, it has been proven that furans are characteristic flavor markers for roasted almonds [37], roasted sesame seeds [38], chocolate [39] and are also responsible for the fruity aroma [40]. Therefore, this might be a conceivable explanation for their presence in the selected decaffeinated coffee samples. Additionally, furans are associated with a broad range of aroma profiles, ranging from caramel-like, to sweet and fruity, to nutty and meaty. Their prominent presence can also be interpreted as a way to compensate for the thin taste of decaffeinated coffees caused by the removal of caffeine and other aroma precursors during the decaffeination process.