## 3 Results and Discussion
### Headspace vs. Nosespace
pyrazines, pyridines, pyrroles, sulfur compounds, and phenols), but was different for coffee B (carbonyls, esters/acids, furans/pyrans, alcohols, pyridines, pyrroles, pyrazines, sulfur compounds, and phenols). The relative abundances of pyridines and pyrroles became dominant over pyrazines in coffee B, most probably due to the higher formation of _m/z_ 80.049 (pyridine) and 82.065 (methyl-pyrrole), with increasing roasting degree. Similarly, the relative abundances of these chemical groups in the NS followed the same order for coffees A and E (carbonyls, furans/pyrans, pyridines, pyrazines, pyrroles, esters/acids, and phenols) and a different order for coffee B (carbonyls, pyridines, furans/pyrans, pyrazines, pyrroles, esters/acids, and phenols). Significant changes were observed between the HS and NS profiles of the coffee samples. Not surprisingly, a 2 to 40-fold signal reduction was observed when switching from HS to NS. For most chemical classes, this meant that the most intense HS peaks remained visible in NS, whereas the less abundant were not detectable anymore; this was the case for pyrazines, pyridines, pyrroles, furans, pyrans, and phenols. The situation was different for carbonyl compounds. While carbonyls as a chemical class remained very abundant in NS, the individual distribution did not necessarily reflect HS composition. Some mass peaks, most notably _m/z_ 87.044 (tentatively identified as butanedine/butyrolactone) and 127.076 and (3-ethyl-1-2-cyclopentone) were proportionally increased in the NS of all coffees, whereas _m/z_ 99.081 (hexenol/methyl-pentenone) was proportionally increased in coffee B and coffee E. On the other hand, mass peaks _m/z_ 73.061 (isobutanal/butanone), 87.080 (methyl-butana), and 31.019 (formaldehyde), which were among the ten most abundant carbonyls in coffee HS, were not detectable in the nosespace of the three coffees. A similar situation could be highlighted when looking at the most abundant six esters in coffee HS and comparing this result with NS data; _m/z_ 89.060 (methyl-propanoate/hydroxy-butanone) and 117.091 (hexanoic acid) were proportionally increased, whereas _m/z_ 61.027 (acetic acid/methyl-formate), 75.042 (methyl-acetate/acetol), 103.074 (hydroxy-pentanone/methyl-butanoic acid), and 47.014 (formic acid) disappeared. Finally, alcohols and sulfur compounds were not at all represented within coffee NS, where they were not detectable. Several reasons may lie behind the changes in the intensity of volatile compounds from headspace to nosespace, including dilution of the sample by saliva in the mouth (quite significant for small amounts of liquid samples), absorption of the volatile compounds by nasal epithelia and their interaction with oral mucosa and lung channels [24; 25], the dilution of the volatiles with the exhaled breath [1; 2; 4], and biotransformation by saliva or nasal mucus enzymes [26]. In conclusion, this work presents a complete workflow for PTR-ToF-MS measurement of in vitro and in vivo aroma release, followed by data processing and analysis and including the comparison of the resulting HS and NS datasets. PTR-ToF-MS was able to provide a detailed profile of the three coffees in both analytical modes, differentiating the three products with \(\geq\)92% accuracy. The comparison between HS and NS data highlights that the latter presents a greatly simplified image of the coffee aroma profile. The question is whether retronasal perception is equally simpler than the orthonasal one and more focused on a few key aroma compounds, such as the ones related to the roasting degree. It must be noted that among compounds that are quantitatively well-represented within coffee nosespace, some, such as pyrazines, also have an important role in determining coffee aroma, whereas others, such as some furans, were found to have only a minor sensory impact [21]. Possible follow-up investigations might involve studies involving larger numbers of coffees or panelists or comparisons between instrumental, sensory data, and individual panelist physiological parameters.