## 3 Results  
### Identification of VOCs in the Coffee Bean, Powder, and Brew  
Table 1 shows the number of VOCs identified in the coffee bean, powder, and brew using GC-TOF/MS (G) and GC\(\times\)GC-TOF/MS (GG), respectively. The VOCs were divided into 13 groups, including hydrocarbons, aldehydes, ketones, acids, phenols, alcohols, esters, pyridines, pyrroles, pyrazines, furans, sulfur-containing compounds, and other compounds. The total number of VOCs identified by G was less than 100, but the total number of VOCs identified by GG ranged from 175 to 374. The results were the same as the previous investigation, in which GG could identify more VOCs in the food samples [17]. Figure 1 shows an example of the chromatogram results of ST coffee beans separated using GC-TOF/MS (Figure 1A) and GC\(\times\)GC-TOF/MS (Figure 1B,C). The peak number of GC\(\times\)GC-TOF/MS was higher than that of GC-TOF/MS (Figure 1A,B). The chromatographic peaks of GC\(\times\)GC-TOF/MS were gathered in a two-dimensional (2D) plane by two columns, so each volatile had two retention indexes with a stable position (Figure 1C). The ketone is the top VOC group in CO, PA, and ST in bean, powder, and brew states. Especially, GG identified 88, 95, and 95 VOCs in the three brands of coffee powder. In addition, the numbers of identified aldehydes and ketones in beans, powder, and brews using GG were 4-6-fold greater than those using G. The oxygenated compounds, such as aldehydes and ketones, are generated during high-temperature roasting, when proteins and carbohydrates undergo the Maillard reaction, Strecker degradation, caramelization and pyrolysis [25]. The number of furans identified by G ranked second. Furans can be generated by the heat treatment of Maillard reaction precursors or lipids [26]. Glucose and alanine were important precursors of furan, of which the formation largely relied on the heating temperature [27]. The number of furans identified by GG was almost twice that by G.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & & & & & & & & & & & & & & & \\ \hline \multicolumn{12}{c}{**Geans**} & \multicolumn{1}{c}{**Poped**} & \multicolumn{1}{c}{**Brew**} & \multicolumn{1}{c}{**Beans**} & \multicolumn{1}{c}{**Poped**} & \multicolumn{1}{c}{**Brew**} \\ \cline{2-12}  & **CO** & **PA** & **ST** & **CO** & **PA** & **ST** & **CO** & **PA** & **ST** & **CO** & **PA** & **ST** & **CO** & **PA** & **ST** & **CO** & **PA** & **ST** & **CO** & **PA** & **ST** \\ \hline Hydrocarbons & 6 & 8 & 8 & 6 & 6 & 4 & 8 & 5 & 13 & 21 & 28 & 23 & 28 & 29 & 39 & 13 & 31 & 33 \\ Aldehydes & 4 & 5 & 3 & 5 & 8 & 7 & 10 & 10 & 10 & 21 & 25 & 13 & 26 & 28 & 26 & 17 & 28 & 18 \\ Ketones & 13 & 14 & 15 & 14 & 13 & 15 & 8 & 7 & 10 & 69 & 83 & 41 & 88 & 95 & 95 & 51 & 63 & 50 \\ Acids & 3 & 5 & 10 & 10 & 6 & 7 & 9 & 6 & 8 & 13 & 13 & 5 & 13 & 14 & 16 & 10 & 7 & 6 \\ Phenols & 5 & 7 & 7 & 8 & 8 & 8 & 8 & 10 & 9 & 11 & 10 & 5 & 11 & 8 & 10 & 6 & 10 & 7 \\ Alcohols & 4 & 9 & 8 & 8 & 6 & 7 & 5 & 6 & 5 & 26 & 36 & 19 & 32 & 40 & 30 & 19 & 25 & 19 \\ Esters & 8 & 8 & 7 & 5 & 5 & 6 & 1 & 3 & 7 & 25 & 37 & 15 & 31 & 41 & 36 & 14 & 25 & 19 \\ Pyridines & 2 & 2 & 2 & 2 & 2 & 1 & 1 & 0 & 0 & 7 & 8 & 3 & 7 & 10 & 6 & 2 & 5 & 4 \\ Pyrroles & 2 & 3 & 3 & 1 & 1 & 1 & 1 & 1 & 1 & 4 & 4 & 2 & 7 & 7 & 5 & 5 & 5 & 5 \\ Pyazines & 8 & 9 & 6 & 8 & 8 & 8 & 6 & 5 & 7 & 27 & 31 & 10 & 25 & 23 & 28 & 14 & 17 & 16 \\ Furans & 9 & 10 & 10 & 11 & 11 & 11 & 11 & 14 & 12 & 20 &Amino acids provide aminoketones that interact with aldehydes, and the Strecker reaction to generate pyrazine yields a series of volatile heterocyclic compounds with aromatic activity, such as pyrazine, pyrrole, and pyridine. Sulfur-containing compounds are the most important class of volatile aromatic compounds, originating from the thermal degradation of sulfur-containing amino acids in the presence of sugars [28]. The number of pyridines and pyrroles identified using G was only 0-3, but the number identified by GG ranged from 2 to 10.