## 2 Results and Discussion  
### Targeted Analysis of Aroma-Related Volatile Compounds  
#### 2.1.1 Aroma-Related Volatiles  
Coffee is a complex matrix. During the roasting process, the precursors in the green beans are transformed into compounds that determine the aroma and taste of coffee. Major chemical reactions are the Maillard reaction, Strecker degradation, caramelization, degradation of chlorogenic acids and lipid oxidation [19]. Although there are hundreds of volatile compounds identified in roasted coffee, only dozens of them significantly contribute to the aroma profile [19]. Summarizing the Arabica coffee marker list from previous research [20], the volatile compound list in roasted coffee and the main odor-active compound list from two review articles [8; 21], then searching in the coffee samples measured in this study, a total of 52 aroma-related volatile compounds were finally selected for target analysis (Table 2). This aroma-related volatile list included 4 acids, 3 alcohols, 6 aldehydes, 6 ketones, 9 furans, 1 lactone, 3 phenols, 13 pyrazines, 2 pyridines, 3 pyrroles, and 2 sulfur-containing compounds. Three pairs of coffee (R-R vs. D-R, R-F vs. D-F, R-VO vs. D-VO, Table 3) were used to investigate the difference in aroma-related volatiles between regular and decaffeinated coffee. These three pairs were selected because in each pair, the origin of the coffee beans, the species of the coffee beans (_Coffea arabica_ L. (Arabica) or _Coffea canephora Pierre_ (Robusta)), and the roasting process were identical. The only difference was the presence or the absence of the decaffeination process. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Chemical Family** & **Compound** & **Aroma Description** & **CAS** \\ \hline Acids & \begin{tabular}{c} Acetic acid \\ Propanoic acid \\ Butanoic acid \\ Butanoic acid, 3-methyl- \\ \end{tabular}  & \begin{tabular}{c} Pungent, sour, acidic, vinegar \\ Pungent, acidic, cheessy, vinegar, sour milk, \\ butter-like \\ Sour, rancid, butter-like, sweaty, rubbish \\ Acidic, sweaty, rancid, cheese, herbaceous \\ \end{tabular}  & \begin{tabular}{c} 64-19-7 \\ 79-09-4 \\ 79-09-4 \\ 107-92-6 \\ 503-74-2 \\ \end{tabular}  \\ \hline Alcohols & \begin{tabular}{c} 2,3-Butanediol (isomer) \\ 2,3-Butanediol (isomer) \\ 2-Furanmethanol \\ \end{tabular}  & \begin{tabular}{c} Fruity, creamy, buttery \\ Caramellic, burnt, smoky, sweet, coffee \\ \end{tabular}  & \begin{tabular}{c} 513-85-9 \\ 513-85-9 \\ 98-00-0 \\ \end{tabular}  \\ \hline Aldehydes & \begin{tabular}{c} Acetaldehyde \\ Butanal, 2-methyl- \\ Butanal, 3-methyl- \\ Benzaldehyde \\ Benzeneacetaldehyde \\ Propanal, 2-methyl- \\ \end{tabular}  & \begin{tabular}{c} Fruity, pungent, ethereal, coffee, wine, acid \\ Malty, fermented, buttery-oily \\ Almond, fruity, buttery-oily, malty, pungent, \\ acid, apple-like, sweaty \\ Fruity, almond, bitter \\ Sweet-fruity \\ Grassy, fermented, buttery-oily \\ \end{tabular}  & \begin{tabular}{c} 75-07-0 \\ 96-17-3 \\ 590-86-3 \\ 100-52-7 \\ 78-84-2 \\ \end{tabular}  \\ \hline Furans & \begin{tabular}{c} Furan, 2-methyl- \\ 2-Furfurylthiol 1 \\ Furural \\ \end{tabular}  & \begin{tabular}{c} Funar, 2-methyl- \\ 2-Furfurylthiol 1 \\ \end{tabular}  & \ \begin{tabular}{c} Pungent, fruity \\ Smoke roast, caramel, burned matter, fresh coffee \\ Sweet, bread-like, caramel, \\ cinnamon-almond-like, bitter \\ Smoke roast \\ Floral \\ Sweet, balsam, almond, cocoa \\ Ethereal-floral, herbal-spicy, green \\ Spice, caramel, maple \\ Sweet, caramel \\ Sweet, caramel \\ \end{tabular}  &
\begin{tabular}{c} 534-22-5 \\ 98-02-2 \\ 98-01-1 \\ 1438-91-1 \\ 13493-97-5 \\ 1192-62-7 \\ 1062-17-6 \\ 1062-02-0 \\ 1062-17-6 \\ 1062-02-0 \\ 1062-17-6 \\ 1