# Potential Aroma Chemical Fingerprint of Oxidised Coffee Note by HS-SPME-GC-MS and Machine Learning
## 3 Results and Discussion
### Chemical Profiling of Good and Oxidised Samples  
Although coffee is a stable product compared with other perishable foods, fresh aromas can be quickly lost over time. After roasting, coffee continues to be an active chemical reactor due to compounds formed from: (i) the pyrolysis of nonvolatile precursors (i.e., pyridine derivatives from trigonelline), (ii) the degradation of lipids and sugars (i.e., acrolein, dihydrofuranones, cyclopent/exenolones, and pyrones), and (iii) the Maillard reaction (i.e., deoxyosone derivatives, 5-hydroxymethylfurfural, Strecker aldehydes, etc.) that are still highly reactive. Its stability over time can therefore also be influenced by the physical form of coffee (grains or powder), the barrier provided by the packaging, and external factors such as temperature, humidity, light, and the presence of oxygen. Appropriate profiling of the volatile fraction enables diagnostic and unbiased mapping of its volatilome [22]. In this study, volatile fraction sampling conditions and tools were set to achieve sufficient sensitivity to recover most of the volatiles describing the main aroma notes, while maintaining the complexity, and thus the informative power, of the coffee volatilome [30; 31; 32]. The analysed coffee samples were described by the 147 volatiles listed in Table S2. Figure 1 is an illustrative pattern of the chemical signatures of the good and oxidised moka samples (M) visualized with a heat map of the normalized volatile responses _versus_ the IS (\(n\)-C13). Samples are clustered using ascendant hierarchical clustering based on Euclidean distances using the average linkage agglomerative method. The samples in the heat map are clustered into two groups: good (MG) and oxidised samples (MOX). A comparison of the two groups allows us to distinguish the pool of components with lower abundance (in orange), that is, those that are lost or degraded with the oxidation of coffee, from the components that are more abundant in the MOX samples (in brown), indicating their possible formation or increase with oxidation.  
The principal component analysis (PCA) of all samples in Figure 2a displays two groups that are separated by the first PC component (F1). The clusters partially overlap because they include different blends, packagings (Eco and standard pack for M), and samples for different brewing modes (moka and caps for espresso coffee). Looking at the most standardised samples, that is, moka (Figure 2b), the clustering of oxidised and good samples is well defined for the first PC with an explained variance (expl. var.) of 77.50% compared with 50.30% of F1 in Figure 2a; this is nevertheless a positive result, indicating that the effect of ageing and oxidation is also when looking at different coffees and packs. The PCA carried out only on the cap samples shows the differences between the blends (Figure 2c); blends P and B are 100% arabica, while blend I is a 50/50 arabica and robusta mix. They also vary in the roasting processes to obtain the desired aroma for the different commercial blends.