## 3 Results
### Identification of Key VOCs in Coffee Samples  
The VOCs of roasted beans, ground powder and brewed coffee were studied and divided into different groups. Previous reports have shown that some VOCs, though not high in concentration, are very important due to their low thresholds [30]. These compounds are summarized and listed in Tables S1 and S2, including the pyridine, pyrrole, pyrazine, alcohol, phenol, furan, and sulfur-containing compounds. Table S1 lists those VOCs identified using G, and Table S2 lists those VOCs identified using GG. The pyridines, pyrroles, and pyrazines, also known as nitrogen-containing heterocyclic compounds, are produced from the Maillard reaction and impart the characteristic roasted or toasted flavors [31]. The concentrations of pyridines and pyrroles were relatively lower than pyrazines in the coffee samples (Tables S1 and S2). The concentrations of pyridine were relatively high in the three brands of coffee in the bean and powder state, ranging from 1.1982 to 4.1309 (Table S1). Pyridine is known to have bitter, burnt, roasted, and astringent characteristics, produced by trigonelline degradation and Maillard reactions [32]. It is especially present in coffee that is subjected to high temperatures or strong roasting processes [24]. Pyrrole was only detected in the coffee beans, and the concentrations ranged from 0.0420 to 0.1319 (Table S1). Pyrroles are usually related to nutty, hay-like, and herb aromas. There were three pyrroles that were identified using G, and most of them were only present in beans. Only one compound, 1-(2-furan methyl)-1H-pyrrole, with hazelnut and coffee aromas [33], was detected in all samples. The other two pyrroles, 1-methyl-1H-pyrrole and pyrrole, were only present in the coffee beans. In a study on the flavor components and quality characteristics of espresso, 1-(2-furan methyl)-1H-pyrrole was also classified as green and had a vegetable aroma [34]. There were eight pyrroles detected using GG, two of which, 2-ethyl-4-methyl-1H-pyrrole and 3-methyl-1H-pyrrole, were only present in the coffee powder. Only one compound, 1-butyl-1H-pyrrole, was detected in the brewed coffee. This result corresponds to the PCA results showing the powder and brew samples located separately on the map. Pyrazines are known to have roasted and earth characteristics in coffee. Pyrazines can mask the odor of other compounds, such as the butter flavor associated with 2,3-butanedione and 2,3-pentanedione levels [35]. There were 23 more pyrazines that were identified using GG than using G. From the list of VOCs in Tables S1 and S2, the concentrations of methyl pyrazine, 2,3-dimethyl pyrazine, 2,5-dimethyl pyrazine, and 2,6-dimethyl pyrazine were the top four pyrazines that were detected in almost all the samples. Methylpyrazine has nutty, toasted, and chocolate aromas [36]. It is formed during the roasting process as a result of the Maillard reaction between sugar and amino acids [37]. One study showed that the formation of pyrazine compounds was affected by the reaction temperature, reaction time, and reactant concentration in glucose and L-alanyl-L-glutamine (Ala-Gln-ARP) reaction systems [38]. Of the remaining three pyrazines, 2,3-dimethyl pyrazine had roasted, nutty, and caramel flavors [39; 40], 2,5-dimethyl pyrazine had nutty and peanut flavors [41; 42], and 2,6-dimethyl pyrazine had sweet, fried, and nutty flavors [36]. For all the identified pyrazines, sucrose is the main carbohydrate precursor. The side products of the Strecker degradation of amino acids can release aroma-active volatile aldehydes, which are crucial for the flavor of coffee [43]. In addition, the fragmentation of the side chains may lead to the formation of pyrazines. Since the pyrazine generation reactions consume free sugars, it has been suggested that they are in competition with other reaction pathways consuming intact sugar skeletons during coffee roasting, such as the formation of 4-hydroxy-2,5-dimethyl-3(2H)-furanone [44].