# Potential Aroma Chemical Fingerprint of Oxidised Coffee Note by HS-SPME-GC-MS and Machine Learning
## 3 Results and Discussion
### Extraction of the Informative Volatiles of the Oxidised Note
\begin{tabular}{c} **Upper Bound** \\ **(98\%)** \\ \end{tabular}  & \begin{tabular}{c} **Pearson Cer** \\ **Coeff 0** \\ \end{tabular}  & **Odour Description** & **M** & **B** & **P** & **1** \\ \hline
1-Markov(J) & 1.0788 & 0.0068 & 0.0070 & 1.2866 & \(-\)0.6932 & - & -36.14 & 4.63 & \(-\)1.09 & \(-\)25.06 \\ \hline Limestone & 1.0918 & 0.1573 & 0.8227 & 1.3609 & \(-\)0.6900 & Topic & \(-\)54.26 & 19.10 & \(-\)68.30 & -3.57 \\ \hline Pyridine & 1.0985 & 0.1543 & 0.8751 & 1.3919 & \(-\)0.6886 & Fully & -91.20 & 0.29 & 0.30 & \(-\)29.83 \\ \hline _Feedward_ * & 1.0775 & 0.0068 & 0.9780 & 1.1770 & \(-\)**0.6810** & **Sweet would almost** & **-68.79** & **-38.45** & **-35.34** & **-61.13** \\ \hline \hline \multicolumn{1}{l}{2-Propensity/Iran} & 1.079 & 0.1379 & 0.8056 & 1.3642 & \(-\)0.6800 & Family & \(-\)40.96 & 12.05 & \(-\)2.07 & \(-\)22.46 \\ \hline Rapidly fluctuate & 1.0788 & 0.1853 & 0.7106 & 1.4700 & 0.6787 & Short, family, fresh, diffusive, and free & 178.85 & 38.13 & \(-\)48.61 & 82.24 \\ \hline Thiazole & 1.0715 & 0.2109 & 0.6830 & 1.4850 & -0.6727 & Fabry & 100.00 & 25.64 & 36.27 & \(-\)4.72 \\ \hline \multicolumn{1}{l}{2-Needy(4)-Huff-pyridine} & 1.0895 & 0.1148 & 0.6444 & 1.2546 & \(-\)0.6759 & Burst & -53.74 & 1.68 & \(-\)15.36 & \(-\)48.11 \\ \hline Acodee & 1.086 & 0.2088 & 0.6740 & 1.4611 & \(-\)0.6747 & Efferent, apple, pear & -97.27 & 21.30 & \(-\)98.39 & \(-\)44.09 \\ \hline \multicolumn{1}{l}{2-Baltimore} & 1.0940 & 0.1815 & 0.7036 & 1.4151 & \(-\)0.6583 & Efferent & -98.01 & 53.26 & 11.18 & \(-\)11.23 \\ \hline \multicolumn{1}{l}{2-Postrandom *} & 1.0515 & 0.0663 & 0.9216 & 1.1814 & \(-\)**0.6646** & **Bottesweet, wet  
For example, 2-butanone (_ethereal_) was observed to be present in lower amounts in oxidised samples M and I, as reported by [PERSON] and [PERSON], who noted a reduction over time [3; 20; 34], and on the opposite in B and P samples. Hexanal (_fatty, sweaty_) is a known by-product and marker for the degradation of linoleic acid by autoxidation, which increased over time (Table S1) [3; 14; 35]. Surprisingly, it is present in lower amounts in the oxidised coffees than in the good coffees in our experiments (Figure 3). This can be explained by its high reactivity and the strongly oxidative environment, which leads to the formation of hexanoic acid (_sour fatty sweat_). 2-Acetylfuran (_sweet blasam, caramel_) decreased in amount in the M (100% arabica for moka) and I samples (a blend containing robusta in Eco caps); on the other hand, it increased in B and P (100% arabica in Eco caps) (Figure 3 and Table 2). The behaviour of 2-acetylfuran in I samples confirms the previous results of [PERSON] et al., which cover 6 months of storage [7]. These authors also report that 2-acetylpyrrole (_musty, nutty_) decreases in blends containing robusta, while, in this study, it increased in aged samples (Figure 3 and Table S2). 4-Vinylgluaiocl (_phenolic, smoky_) was indicated as a staling component by [PERSON] and [PERSON] [15; 16] as it decreases with time, as it does in all oxidised coffees in these experiments, although its VIP is below 1 (i.e., 0.8919) and r is \(-\)0.5637 (Figure 3 and Table S2). This behaviour, however, is not confirmed in robusta samples in caps after 6 months' storage [7] (Table S1). VIP cut-off. FFT has a VIP of 0.9713 and a moderate correlation with the oxidation process (r = 0.6323) and decreases dramatically in oxidised samples from the first stages of storage. The decrease in FFT may also be due to evaporation, adsorption onto the solid matrix, and internal reactions. FFT and all thiols in general are highly reactive nucleophiles that can degrade in the presence of hydroperoxides, oxidise (e.g., MeSH to DMDS), and react with phenolic compounds [36]. In particular, [PERSON] et al. showed that FFT decreases because it becomes trapped by low-molecular-weight melanoidins, unlike what happens with mercaptan-aldehydes [37; 38; 39]. More recently, [PERSON] et al.