# Analysis of Volatile Compounds in Coffee Prepared by Various Brewing and Roasting Methods
## 3 Results
### Principal Component Analysis (PCA) for Volatile Compounds in a Coffee Model System  
It is difficult to determine the difference in the mean of the volatile compounds produced by roasting and extraction methods because of the complex dynamics of the system. PCA is an effective mathematical method to reduce the dimensions of multivariate data [30]. The character with a number on the PCA biplot represents the volatile compounds defined in Table 1. PCA can identify where coffee samples made under different conditions, such as roasting conditions, grind coffee bean size, brewing methods, and types of water, are located on a volatile compounds data map [31]. Figure 1: Levels of volatile compounds in coffee according to (**a**) roasting time and temperature (E-4 ()) and C-4 (): roasted at 235 \({}^{\circ}\)C for 13 min; E-13 () and C-13 (): roasted at 240 \({}^{\circ}\)C for 15 min; E-22 () and C-22 (): roasted at 245 \({}^{\circ}\)C for 17 min); (**b**) grind size (E-10 (): and C-10 (): 355 \(\upmu\)m; E-13 () and C-13 (): 500 \(\upmu\)m; E-16 (): 710 \(\upmu\)m); (**c**) types of water (E-13 ()) and C-13 (): filtered water; E-14 () and C-14 (): tap water; E-15 () and C-15 (): bottled water). Different letters (a, b, c) represented significant differences. Figure 2a shows the PCA results explaining the relative location of volatile compounds in espresso coffee. The PCA biplot explains approximately 79.75% of the variability. PC1 explains 52.81% of the total variability as the primary axis, and PC2 explains 26.95% of the entire variability as the vertical axis. One of the drawbacks of a PCA biplot is that it only explains a certain percentage of the total variability; that is, not all of the dataset is reflected in the PCA biplot [32]. Espresso coffees E-1 to E-9 (roasted at 235 \({}^{\circ}\)C for 13 min) were located in the second quadrant. The first group consisted of E-2 and E-3, which contained high amounts of 2-ethyl-5-methylpyrazine, 2,2\({}^{\prime}\)-bifuran, furfural, 5-methylfurfural, and 2-methoxy-4-vinylphenol (a8, b2, b8, b9, d3). E-10 to E-27 (coffee roasted at 240 \({}^{\circ}\)C for 15 min and 245 \({}^{\circ}\)C for 17 min) were located in the negative PC2. The second group consisted of E-19, E-20, and E-21, associated with high levels of 2,2\({}^{\prime}\)-methylene-bis-furan, 2,2\({}^{\prime}\)-[oxybis(methylene)]bis-furan, 1-methylpyrrole, guaiacol, and 4-ethylguaiacol (b3, b4, c1 d1, d2). The 355-\(\upmu\)m (E-1 to E-3, E-10 to E-12, and E-19 to E-21) and 710-\(\upmu\)m grind sizes (E-7 to E-9, E-16 to E-18, E-25 to E-27) were located in positive PC1 and negative PC1, respectively. Figure 2b shows the PCA results and describes the relative location of volatile compounds in cold brew coffee. The PCA biplot explains about 78.82% of the variability. Most of the variability, 51.63%, was attributed to PC1, with PC2 (the vertical axis) accounting for just 27.19% of the total variability. Cold brew coffees C-2 to C-9, which were roasted at 235 \({}^{\circ}\)C for 13 min, were located in the fourth quadrant. The first group included C-2 and C-4, with high levels of 2,6-diethylpyrazine, furfural, 5-methylfurfural, and 2-methoxy-4-vinylphenol (a6, b8, b9, d3). The second group consisted of C-14 to C-27, which contained high levels of 2,2\({}^{\prime}\)-methylene-bis-furan, 2,2\({}^{\prime}\)-[oxybis(methylene)]bis-furan, guaiacol, 4-ethylguaiacol, and 1-methylpyrrole (b3, b4, c1, d1, d2). Figure 2 shows the PCA results and explains the relative location of volatile compounds in espresso and cold brew coffee roasted at 240 \({}^{\circ}\)C for 15 min. Together, PCI (60.62%) and PC2 (15.53%) describe 76.14% of the total variability. Espresso coffees (E-10 to E-18) had negative PC2 scores. Cold brew coffees (C-10 to C-18) had negative PC2 scores. Espresso coffee contained high levels of 2,3-dimethylpyrazine, 2,2\({}^{\prime}\)-methylene-bis-furan, 2,2\({}^{\prime}\)-[oxybis(methylene)]bis-furan, 1-furfurylpyrrole, guaiacol, 4-ethylguaiacol, and 2-methoxy-4-vinylphenol (a2, b3, b4, c2, d1, d2, d3).