# Analysis of Volatile Compounds in Coffee Prepared by Various Brewing and Roasting Methods
## 4 Discussion  
In this study, the higher the roasting temperature and the longer the roasting time, the higher the concentrations of 2,2\({}^{\prime}\)-methylene-bis-furan, guaiaicol, and 4-ethylguaiaicol and the lower the concentration of furfural. Volatile compounds from green coffee beans were reduced as the intensity of the roasting conditions increased. On the contrary, aroma chemicals produced by the Maillard reaction were increased under higher roasting conditions than mild roasting conditions. The smaller the grind size, the higher the concentration of volatile compounds. As the grind size decreases, the solid-liquid interfacial area increases, in turn, increasing the amount of volatiles that can be extracted. Types of water with different ions can have an impact on the flavor and aroma of coffee. 2-Methylpyrazine, 1-methylpyrrole, and 2-acetylfuran were the most abundant components in the cold brew coffee. Overall, the roasting conditions and extraction methods were determined to be significant factors for volatile compounds in coffee.  
Conceptualization, K.-G.L. and J.-M.Y.; methodology, J.-M.Y. and H.P.; software, H.P.; validation, M.C. and H.P.; formal analysis, J.-M.Y. and J.P.; investigation, J.-M.Y.; resources, M.C.; data curation, J.-M.Y.; writing--original draft preparation, J.-M.Y.; writing--review and editing, J.-M.Y.; visualization, J.P.; supervision, K.-G.L.; project administration, K.-G.L.; funding acquisition, K.-G.L. All authors have read and agreed to the published version of the manuscript.  
Figure 2: Principal component analysis (PCA) bipid of volatile compounds (**a**) in espresso coffee, (**b**) cold brew coffee. The relative location of volatile compounds in espresso and cold brew coffee roasted at 240 \({}^{\circ}\)C for 15 min. (a1: 2-methylpyrazine, a2: 2,3-dimethylpyrazine, a3: 2,5-dimethylpyrazine, a4: 2,6-dimethylpyrazine, a7: 2-ethyl-3-methylpyrazine, a8: 2-ethyl-5-methylpyrazine, a9: 3-ethyl-2,5-dimethylpyrazine, a10: isopropenyl pyrazine, b1: 2-acetylfuran, b2: 2,2\({}^{\prime}\)-bifuran, b3: 2,2\({}^{\prime}\)-methylenebisfuran, b4: 2,2\({}^{\prime}\)-{oxybis(methylene)}bisfuran, b5: furfuryl alcohol, b6: furfuryl acetate, b7: furfuryl propionate, b8: furfuryl, b9: 5-methylfurfurral, c1: 1-methylpyrrole, c2: 1-furfurylpyrrole, d1: guaiaicol, d2: 4-ethylguaiaicol, d3: 2-methoxy-4-vinylphenol.). E and C mean extraction method of espresso and cold brew, respectively. According to the roasting conditions, sample numbers were divided into 1 to 9 (235 \({}^{\circ}\)C for 13 min), 10–18 (240 \({}^{\circ}\)C for 15 min), and 19–27 (245 \({}^{\circ}\)C for 17 min). The sample number was also assigned to the particle size of grounded coffee (355 \(\upmu\)m: 1–3, 10–12, 19–21; 500 \(\upmu\)m: 4–6, 13–15, 22–24; 710 \(\upmu\)m: 7–9, 16–18, 25–27). The sample number assigned according to the size of the coffee bean was divided into three equal parts according to the type of water (filtered water: 1, 4, 7, 10, 13, 16, 19, 22, 25; tap water: 2, 5, 8, 11, 14, 17, 20, 23, 26; bottled water: 3, 6, 9, 12, 15, 18, 21, 24, 27).  
**Funding:** This research was supported by the Basic Science Research Program through the National Research Foundation of Korea [NRF, 2021R1A2B5B01002296].  
**Acknowledgments:** The authors would like to thank the support of Dongguk University.  
**Conflicts of Interest:** The authors declare that there is no conflict of interest.