# Potential Aroma Chemical Fingerprint of Oxidised Coffee Note by HS-SPME-GC-MS and Machine Learning
## 1 Introduction  
The quality of a coffee is related to the aroma and flavour developed because of the chemical reactions that take place during roasting and on the basis of the precursors present in the bean, which are due to various terrori and processing factors. The pleasant aroma of a freshly opened packet of roasted coffee is an additional indicator of quality [1]. Coffee is considered a stable product with a long shelf life, although it is still an active \"chemical reactor\" after roasting, as the components resulting from the pyrolysis of the nonvolatile precursors, the degradation of lipids and sugars, and the Maillard reaction(s) are very reactive. During storage, chemical and physical changes can affect the quality of roasted coffee. The changes in sensory features are generally due to the loss of key aroma compounds and the appearance of oxidation products that can cause unpleasant flavours [2, 3, 4]. The loss of aroma freshness during storage, known as \"staling\", that is _\"a sweet but unlovely flavour and aroma of roasted coffee which reflects the oxidization of many of the pleasant volatiles and the loss of others; a change in the flavour and the acid constituents causing a partial bland tone\"_[5], mainly depends on the temperature, humidity, presence of oxygen, and barrier capacity of the packaging [3, 6, 7, 8, 9]. Although sensory analysis performed by a professional panel is the reference system for assessing coffee quality, several studies have been conducted with the aim of combining sensory and chemical analysis to define the freshness of a product through an objective analytical approach [10, 11, 12, 13]. Available studies have principally identified a number of chemical markers associated with ageing, but most of them have focused on the loss of the main key odorants rather than the formation of new compounds [2, 14, 15, 16, 17, 18, 19]. In particular, low-molecular-weight sulphur-containing compounds, such as methanethiol [_sulphurous_, _putrid_] and 2-furfurylthiol (_roasty_, _coffelike_) [2, 19], and Strecker aldehydesand \(\alpha\)-dicarbonyl compounds, such as propanal (_ethereal pungent_, _earthly_), 2-methyl propanal, 2- and 3-methyl butanal (_malty_), 2,3-butanedione, and 2,3-pentanedione (_buttery_, _pungent_) [3, 8, 17, 20], have been indicated as possible reasons for the loss of freshness in the aroma (Table S1). Although several volatile markers have been proposed as a means of monitoring changes in coffee freshness, the adoption of their absolute amounts can be questioned, as they are influenced by their initial concentrations, which in turn depend on variables such as the blend, degree of roasting, grinding, and other factors [16, 21]. On the other hand, the use of indices calculated using the ratio between the quantities of selected diagnostic components in the headspace is a more robust technique that better reflects the changes taking place in the headspace. Several indices have been proposed as indicators of coffee freshness and, thus, as possible quality indicators. Table 1 lists some of the quality indicators (volatiles and indices) proposed so far, which correlate with the decrease in freshness of the aroma. For a more comprehensive list of quality indicators related to aroma decline over time and under the conditions studied, see Table S1. The 2-methylfuran/2-butanone (M/B) ratio is one of the best known. Its decrease over time can be correlated with the appearance of a sweet, but not pleasant, aroma [3, 15, 16, 21]. Other proposed indices of coffee freshness are those involving sulphur compounds; however, only the 2-furfurylthiol/hexanal (FFT/HE) and, recently, dimethyl disulphide/methanediol (DMDS/MeSH) indices were found to be useful as indicators for staling over long-term storage [20]. Despite their number, studies in this field have always focused on one or two compounds in the expression of coffee freshness loss, rather than on the synergism between the components responsible for the oxidised perception of the overall coffee volatilome [22].