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Article

Analysis of Volatile Compounds in Coffee Prepared by Various Brewing and Roasting Methods

Department of Food Science and Biotechnology, Dongguk University-Seoul, 32, Dongguk-ro, Ilsandong-gu, Goyang-si 10326, Korea
*
Author to whom correspondence should be addressed.
Academic Editor: Beatriz Cancho-Grande
Foods 2021, 10(6), 1347; https://doi.org/10.3390/foods10061347
Received: 15 May 2021 / Revised: 5 June 2021 / Accepted: 9 June 2021 / Published: 10 June 2021

Abstract

Volatile compounds of coffee brewed under various roasting conditions and by different brewing methods were analyzed. Green coffee beans (Coffea arabica) were roasted at 235 °C for 13 min, 240 °C for 15 min, and 245 °C for 17 min. Roasted coffee beans were ground into particles of three different sizes (710, 500, and 355 μm) and brewed by an espresso coffee machine and the cold brew method. Three types of water (filtered, tap, and bottled) were used for coffee extraction. SPME-GC-MS results indicated that increasing the roasting temperature and time increased the levels of 2,2′-methylene-bis-furan, guaiacol, and 4-ethylguaiacol (p < 0.05) and decreased the levels of furfural (p < 0.05). Grind size was inversely proportional to the measured signal of volatiles by GC-MS (p < 0.05). The measured GC/MS intensities of 2-methylpyrazine, 2,5-dimethylpyrazine, and 2-methoxy-4-vinylphenol were significantly higher in coffee brewed with filtered water (p < 0.05) than tap and bottled water. 2-Methylpyrazine, 1-methylpyrrole, and 2-acetylfuran were the most abundant components in the cold brew. Overall, roasting conditions and extraction methods were determined to be significant factors for volatile compounds in coffee. This is the first study showing the analysis of volatile compounds in coffee according to various types of water and extraction methods, such as espresso and cold brew coffee.
Keywords: volatiles; coffee; brewing; roasting; grinding volatiles; coffee; brewing; roasting; grinding

1. Introduction

Coffee is one of the most consumed beverages around the world. The conventional processing of coffee includes roasting, grinding, and extraction. During roasting, a wide range of chemical reactions occur, including the Maillard reaction, caramelization, and Strecker degradation [1]. These reactions affect not only the color of coffee but also the flavor and aroma [2,3,4,5]. Volatile flavor compounds in coffee are produced from a variety of compounds in green coffee beans, such as reducing sugars, amino acids, lipid, chlorogenic acid, and trigonelline [6].
The chemicals in coffee can be affected by various factors such as roasting conditions, ground particle size, and brewing methods. The grinding process is one of the most important steps for brewing coffee [7,8,9]. The particle size of ground coffee beans plays a significant role in controlling the degree of extraction via the particle’s extraction kinetics [10,11]
Water (including its ionic composition) is an essential ingredient that can highlight the specificities of a coffee or leave it dull and flat [12]. The levels of ions and species in the water change the rate at which water is passed through the ground coffee [12]. It is well known that water treatment is required to remove possible off-flavors deriving from the disinfection (chlorination) process and to prevent limescale deposition associated with the water hardness [13].
Many brewing techniques may be used to prepare coffee [14,15]. Espresso is made by forcing hot water at high pressure (maximum of 19 bar) through finely ground coffee. This original Italian-style coffee is increasingly popular in many other countries [16,17]. Differences in flavor characteristics of espresso coffees as a result of the coffee variety and extraction temperature have been observed [18].
Cold brew coffee, which has been gaining popularity recently, is known to be smoother and sweeter than conventionally brewed coffee because the flavors and bioactive compounds in coffee are different from the hot water extraction method, such as expresso. Angeloni et al. reported that significant differences were found in the physicochemical parameters and sensory evaluation. Cold brew coffee was recognized as being less bitter with more contents of caffeine and chlorogenic compounds than expresso since higher temperature gives rise to an increase of solid compounds such as caffeine [19]. According to market research, the global cold brew market size was valued at USD 339.7 million in 2018 and is expected to reach USD 1.63 billion by 2025 [20]. Further, aside from its more appealing and less-acidic taste, cold brew coffee can contain up to two-fold more caffeine than hot brew coffee [21]. In addition, crude polysaccharides isolated from cold brew coffee serve as an inducer of the systemic immune system through the intestinal immune system [22]. Despite the growing popularity of cold brew coffee, there is little published on the chemistry or associated benefits or health risks of cold brew coffee.
Despite the growing popularity of cold brew coffee, there is little published on the chemistry or associated benefits or health risks of cold brew coffee. In this study, the volatile compounds produced in coffee under various roasting and brewing conditions, such as the grind size, types of water, and extraction methods, were analyzed and compared.

2. Materials and Methods

2.1. Chemical Reagents and Materials

Green coffee beans (Coffea arabica, from Brazil) were bought from a commercial market in Seoul, Korea. The origin of green coffee beans was verified by the importers and experts. 2-Methylpyrazine, 2,5-dimethylpyrazine, 2,6-dimethylpyrazine, 2-ethyl-3-methylpyrazine, 2-ethyl-5-methylpyrazine, 3-ethyl-2,5-dimethylpyrazine, furfural, furfuryl acetate, furfuryl propionate, furfuryl alcohol, 1-furfurylpyrrole, 4-ethylguaiacol, 2-methoxy-4-vinylphenol, 5-methylfurfural, quinoxaline, and C7–C30 alkane standard were obtained from Sigma–Aldrich Chemical Co. (St. Louis, MO, USA). 2-Ethylpyrazine, 2,3-dimethylpyrazine, 1-methylpyrrole, guaiacol, and 2-acetylfuran were bought from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS, 50-μm film thickness) solid-phase microextraction (SPME) fiber was purchased from Supelco, Inc. (Bellefonte, PA, USA).

2.2. Sample Preparation

Green coffee beans were roasted under three different conditions (235 °C for 13 min, 240 °C for 15 min, and 245 °C for 17 min) in a coffee bean roaster (CBR-101A, Gene Café, Korea). Each roasted bean was ground in a grinder (Hanil, Seoul, Korea) for 30 s twice. These samples passed through testing sieves (710, 500, and 355 μm) for 2 min twice in a sieve shaker. Two different extraction methods (expresso and cold brew) were then applied. For espresso coffee, roasted ground coffee (12.5 g) was brewed with 100 mL of water using an espresso coffee machine (BCC-480ES, Bean Cruise, Korea). Cold brew coffee was made based on the New York Times’ cooking website. A sample of 12.5 g was placed in 100 mL of water at room temperature for 4 h, and then at 4 °C for 8 h [21]. Three types of water (filtered, tap, and bottled water) were used for coffee extraction.
In total, fifty-four coffee samples were prepared, and the sample names were abbreviated from E-1 to E-27 (extraction method), and from C-1 to C-27 (cold brew). According to the roasting conditions, sample numbers were divided into 1 to 9 (235 °C for 13 min), 10–18 (240 °C for 15 min), and 19–27 (245 °C for 17 min). The sample number was also assigned to the particle size of grounded coffee (355 μm: 1–3, 10–12, 19–21; 500 μm: 4–6, 13–15, 22–24; 710 μm: 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).

2.3. Analysis of Volatile Compounds

Volatile compounds in coffee were extracted using SPME. A 10 mL of sample was added to the GC vial. The internal standard quinoxaline (10 μL), the alkane standard (20 μL), and a magnetic stirring bar were added. After stirring the samples at 70 °C for 10 min, to reach equilibrium, the SPME fiber was injected into the vial at 70 °C for 40 min for adsorption of volatile compounds. Afterward, the fiber was inserted into the gas chromatograph. Gas chromatography–mass spectrometry (GC-MS) analysis was performed using a DB-WAX column (length × inside diameter × phase thickness: 60 m × 250 μm × 0.25 μm) by modifying an existing method [6]. Helium was used as the carrier gas, with a flow rate of 1.0 mL/min and splitless mode (splitless time: 1 min). The oven temperature was maintained at 44 °C for 5 min, increased to 170 °C at 3 °C/min and held for 10 min, and then raised to 240 °C at 8 °C/min and held for 5 min. Volatile compounds were identified by their retention index (RI), co-injection, and by comparison of their mass spectra with those published in the Wiley mass spectrum database. The peak area ratio (peak area of each peak/peak area of internal standard) of each compound was calculated from the peak area of the internal standard. All analyses were conducted in three replications, and average value and standard deviations were calculated.

2.4. Statistical Analysis

Statistical analysis was performed with IBM SPSS Statistics 23 (IBM, Armonk, NY, USA), and graphs were constructed with GraphPad Prism 5.0 software (GraphPad Software, Inc., San Diego, CA, USA). All data were analyzed by one-way ANOVA and Duncan’s multiple range test for investigating significant differences (p < 0.05). Principal component analysis (PCA) was performed on the mean values of peak area ratio using XLSTAT (version 2018; Addinsoft, Paris, France).

3. Results

3.1. Analysis of Volatile Compounds in Coffee

The selected 24 volatile compounds are shown in Table 1. These selected volatile compounds are the main volatiles identified in coffee samples. In addition, these 24 volatiles were analyzed the most in our study and represent the aroma’s functional groups (chemical types) in coffee. Identification of the volatile compounds was based on their RI, co-injection, and comparison of their mass spectrum with those in the Wiley mass spectrum database. All values are represented as the peak area ratio (peak area of each peak/peak area of internal standard). Table 2 shows the peak area ratio of the volatile compounds in E-1 to E-9 (espresso roasted at 235 °C for 13 min) and C-1 to C-9 (cold brew roasted at 235 °C for 13 min). Among the espresso coffees, the finest grinds (355 μm) produced significantly higher measured GC/MS intensities of all 24 volatile compounds, except for 2,2′-[oxybis (methylene)]bis-furan and 2-methoxy-4-vinylphenol, compared with the medium (500 μm) and large (750 μm) grind sizes (p < 0.05). For espresso coffees extracted with different types of water, bottled water and tap water led to high measured GC/MS intensities of all 24 volatile compounds. Comparing cold brew coffees, fine grinds (355 μm) resulted in significantly high measured GC/MS intensities of volatile compounds, respectively (p < 0.05).
For cold brew coffees (C-1 to C-9), filtered water led to significantly higher measured GC/MS intensities of volatile compounds (p < 0.05). The water used in this study has different amounts of carbonates and bicarbonates. The levels of carbonates and bicarbonate were much lower in the filtered water compared to the bottled and tap water. A study by Gardner (1958) showed that the carbonates and bicarbonates of sodium ions slowed down the brewing time. Therefore, cold brew, which is extracted by soaking coffee beans in water, appears to have been more affected by the extraction of volatile compounds than espresso coffee [12].
Table 3 shows the volatile compounds in both types of coffee roasted at 240 °C for 15 min. Among the espresso coffees (E-10 to E-18), the smaller particle of ground coffee (355 μm) corresponded to significantly higher measured GC/MS intensities of volatile compounds (p < 0.05). Among the espresso coffees extracted with different types of water (E-10, E-11, and E-12), filtered water resulted in significantly higher measured GC/MS intensities of 18 volatile compounds (p < 0.05). Comparing E-13, E-14, and E-15, the measured GC/MS intensities of 22 volatile compounds, excluding 2-methylpyrazine and furfuryl alcohol, were notably high when filtered water was used (p < 0.05). Comparing E-16, E-17, and E-18, tap water led to significantly higher measured GC/MS intensities of 21 volatile compounds (p < 0.05). Comparing cold brew samples, fine grinds (355 μm) occasioned notably high levels of volatile compounds, respectively (p < 0.05). Among C-10 to C-12, the measured GC/MS intensities of all 24 volatile compounds were similarly detected. Among C-13 to C-15 and C-16 to C-18, filtered water gave rise to remarkably high measured GC/MS intensities of 19 and 2 volatile compounds, respectively (p < 0.05). For extraction with filtered water, more volatiles could be expected to be detected as grind size decreases and roasting temperature increases. However, in this study, espresso prepared from filtered water and at a relatively low roasting temperature did not generate a high concentration of volatiles. Similarly, cold brew prepared from fine grinds and filtered water did not have a high concentration of volatiles. The reason for this result should be examined in future studies.
Table 4 lists the volatile compounds in both types of coffee roasted at 245 °C for 17 min (E-19 to E-27 and C-19 to C-27). Comparing expresso coffee samples, fine grinds (355 μm) led to significantly high measured GC/MS intensities of volatile compounds, respectively (p < 0.05). Among E-19 to E-21, filtered water led to slightly but not significantly high measured GC/MS intensities of all 24 volatile compounds (p > 0.05), except for isopropenyl pyrazine, 2,6-diethylpyrazine, and 2-methoxy-4-vinylphenol. Among E-22 to E-24 and E-25 to E-27, filtered water resulted in significantly high measured GC/MS intensities of 6 and 18 volatile compounds, respectively (p < 0.05). Comparing cold brew samples, there were no or highly significant differences in grind sizes 500 and 710 μm. Filtered water occasioned significantly high measured GC/MS intensities of 1-furfurylpyrrole and 2-methoxy-4-vinylphenol among cold brews C-19 to C-21 (p < 0.05), furfuryl alcohol among cold brews C-22 to C-24 (p < 0.05), and 12 volatile compounds among cold brews C-25 to C-27 (p < 0.05). Comparing volatile compounds of middle size (500 μm) of ground coffee, the levels of 2-methylpyrazine, 2,5-Dimethylpyrazine, 2,6-Dimethylpyrazine, 2-ethylpyrazine, 2-ethyl-5-methylpyrazine, 3-ethyl-2,5-dimethylpyrazine, 2-acetylfuran, 2,2′-Bifuran, 5-Methylfurfural, and 1-methylpyrrole in cold brew coffee were higher than espresso coffee. These flavor compounds have a sweet, nutty, and fruity odor [23,24,25,26].
In Figure 1, among the many volatile compounds analyzed, significantly increased or decreased compounds were selected to show their levels. As shown in Figure 1a, increasing the roasting temperature and time increased the levels of 2,2′-methylene-bis-furan, guaiacol, and 4-ethylguaiacol (p < 0.05), and decreased the levels of furfural and 5-methylfurfural (p < 0.05). In Moon and Shibamoto’s (2009) study, intensifying the roasting conditions of green coffee beans yielded relatively lower furanone derivatives and furfural. This result is possibly associated with the interconversion of furan, furfural, furfuryl alcohol, and 2-methylfuran. In particular, the high activation energy of furfural reduction to furan is shown to be thermodynamically favored [27]. In addition, the formation of aroma compounds, such as pyrroles, generated by the Maillard reaction between reducing sugars and amino acids, were formed more in coffee beans roasted under higher roasting conditions than mild roasting conditions [3].
As shown in Figure 1b, the smaller the grind size, the higher the measured GC/MS intensities of 2-ethyl-5-methylpyrazine, 4-ethylguaiacol, and 2-acetylfuran (p < 0.05). As the grind size decreases, the solid–liquid interfacial area increases, in turn, increasing the levels of volatile compounds extracted [11]. Other studies reported that coffee prepared with rough coffee grounds had the lowest aromatic profile [16,28]. Trigonelline, chlorogenic acids, and lipids increased inversely with grind size. The caffeine content also increased significantly as grind size decreased [18,29]. These results suggest that different aromatic profiles of coffee can be obtained when different grind sizes are used. As the size of the particles decreases, the packing effect becomes higher, and the perfusion of the water in coffee samples becomes difficult. Therefore, the particle size is usually optimized according to the type of extraction methods, such as the machine used for the preparation of the coffee. In a future study, the optimization of coffee machines could be carried out.
In Figure 1c, high measured GC/MS intensities of 2-methylpyrazine, 2,5-dimethylpyrazine, and 2-methoxy-4-vinylphenol were observed in coffee brewed with filtered water than bottled water. Water quality can have a direct impact on the quality of espresso coffee. Brewed coffees often differed in flavor and appearance, depending on whether the water was distilled, soft, or hard. Beverages made with solutions containing carbonates were the least desirable, having a flat and dull characteristic [12].

3.2. 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 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 °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′-bifuran, furfural, 5-methylfurfural, and 2-methoxy-4-vinylphenol (a8, b2, b8, b9, d3). E-10 to E-27 (coffee roasted at 240 °C for 15 min and 245 °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′-methylene-bis-furan, 2,2′-[oxybis(methylene)]bis-furan, 1-methylpyrrole, guaiacol, and 4-ethylguaiacol (b3, b4, c1 d1, d2). The 355-μm (E-1 to E-3, E-10 to E-12, and E-19 to E-21) and 710-μm 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 °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′-methylene-bis-furan, 2,2′-[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 °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 positive 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′-methylene-bis-furan, 2,2′-[oxybis(methylene)]bis-furan, 1-furfurylpyrrole, guaiacol, 4-ethylguaiacol, and 2-methoxy-4-vinylphenol (a2, b3, b4, c2, d1, d2, d3). Conversely, cold brew contained high levels of 2-methylpyrazine, 3-ethyl-2,5-dimethylpyrazine, furfural, 5-methylfurfural, and 1-methylpyrrole (a1, a9, b8, b9, c1).

4. Discussion

In this study, the higher the roasting temperature and the longer the roasting time, the higher the concentrations of 2,2′-methylene-bis-furan, guaiacol, and 4-ethylguaiacol 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.

Author Contributions

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.

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.

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Figure 1. Levels of volatile compounds in coffee according to (a) roasting time and temperature (E-4 ( Foods 10 01347 i001) and C-4 ( Foods 10 01347 i002): roasted at 235 °C for 13 min; E-13 ( Foods 10 01347 i003) and C-13 ( Foods 10 01347 i004): roasted at 240 °C for 15 min; E-22 ( Foods 10 01347 i005) and C-22 ( Foods 10 01347 i006): roasted at 245 °C for 17 min); (b) grind size (E-10 ( Foods 10 01347 i007) and C-10 ( Foods 10 01347 i008): 355 μm; E-13 ( Foods 10 01347 i009) and C-13 ( Foods 10 01347 i010): 500 μm; E-16 ( Foods 10 01347 i011) and C-16 ( Foods 10 01347 i012): 710 μm); (c) types of water (E-13 ( Foods 10 01347 i013) and C-13 ( Foods 10 01347 i014): filtered water; E-14 ( Foods 10 01347 i015) and C-14 ( Foods 10 01347 i016): tap water; E-15 ( Foods 10 01347 i017) and C-15 ( Foods 10 01347 i018): bottled water). Different letters (a, b, c) represented significant differences.
Figure 1. Levels of volatile compounds in coffee according to (a) roasting time and temperature (E-4 ( Foods 10 01347 i001) and C-4 ( Foods 10 01347 i002): roasted at 235 °C for 13 min; E-13 ( Foods 10 01347 i003) and C-13 ( Foods 10 01347 i004): roasted at 240 °C for 15 min; E-22 ( Foods 10 01347 i005) and C-22 ( Foods 10 01347 i006): roasted at 245 °C for 17 min); (b) grind size (E-10 ( Foods 10 01347 i007) and C-10 ( Foods 10 01347 i008): 355 μm; E-13 ( Foods 10 01347 i009) and C-13 ( Foods 10 01347 i010): 500 μm; E-16 ( Foods 10 01347 i011) and C-16 ( Foods 10 01347 i012): 710 μm); (c) types of water (E-13 ( Foods 10 01347 i013) and C-13 ( Foods 10 01347 i014): filtered water; E-14 ( Foods 10 01347 i015) and C-14 ( Foods 10 01347 i016): tap water; E-15 ( Foods 10 01347 i017) and C-15 ( Foods 10 01347 i018): bottled water). Different letters (a, b, c) represented significant differences.
Foods 10 01347 g001
Figure 2. Principal component analysis (PCA) biplot 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 °C for 15 min. (a1: 2-methylpyrazine, a2: 2,3-dimethylpyrazine, a3: 2,5-dimethylpyrazine, a4: 2,6-dimethylpyrazine, a5: 2-ethylpyrazine, a6: 2,6-diethylpyrazine, 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′-bifuran, b3: 2,2′-methylenebisfuran, b4: 2,2′-[oxybis(methylene)]bisfuran, b5: furfuryl alcohol, b6: furfuryl acetate, b7: furfuryl propionate, b8: furfural, b9: 5-methylfurfural, c1: 1-methylpyrrole, c2: 1-furfurylpyrrole, d1: guaiacol, d2: 4-ethylguaiacol, d3: 2-methoxy-4-vinylphenol.). E and C mean extraction method of expresso and cold brew, respectively. According to the roasting conditions, sample numbers were divided into 1 to 9 (235 °C for 13 min), 10–18 (240 °C for 15 min), and 19–27 (245 °C for 17 min). The sample number was also assigned to the particle size of grounded coffee (355 μm: 1–3, 10–12, 19–21; 500 μm: 4–6, 13–15, 22–24; 710 μm: 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).
Figure 2. Principal component analysis (PCA) biplot 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 °C for 15 min. (a1: 2-methylpyrazine, a2: 2,3-dimethylpyrazine, a3: 2,5-dimethylpyrazine, a4: 2,6-dimethylpyrazine, a5: 2-ethylpyrazine, a6: 2,6-diethylpyrazine, 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′-bifuran, b3: 2,2′-methylenebisfuran, b4: 2,2′-[oxybis(methylene)]bisfuran, b5: furfuryl alcohol, b6: furfuryl acetate, b7: furfuryl propionate, b8: furfural, b9: 5-methylfurfural, c1: 1-methylpyrrole, c2: 1-furfurylpyrrole, d1: guaiacol, d2: 4-ethylguaiacol, d3: 2-methoxy-4-vinylphenol.). E and C mean extraction method of expresso and cold brew, respectively. According to the roasting conditions, sample numbers were divided into 1 to 9 (235 °C for 13 min), 10–18 (240 °C for 15 min), and 19–27 (245 °C for 17 min). The sample number was also assigned to the particle size of grounded coffee (355 μm: 1–3, 10–12, 19–21; 500 μm: 4–6, 13–15, 22–24; 710 μm: 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).
Foods 10 01347 g002
Table 1. Volatile compounds in coffee (E-1: espresso brewing method, roasted at 235 °C for 13 min, particle size of ground coffee bean 355 μm, and purified water).
Table 1. Volatile compounds in coffee (E-1: espresso brewing method, roasted at 235 °C for 13 min, particle size of ground coffee bean 355 μm, and purified water).
No.CompoundsRetention TimeR.IR.I (Ref)Co-
Injection
Mass Spectrum
a12-Methylpyrazine25.9912891288oo
a22,3-Dimethylpyrazine29.8713701371oo
a32,5-Dimethylpyrazine28.7313461348oo
a42,6-Dimethylpyrazine29.0213521354oo
a52-Ethylpyrazine29.2313561359oo
a62,6-Diethylpyrazine33.8314561444 o
a72-Ethyl-3-methylpyrazine32.5414271422oo
a82-Ethyl-5-methylpyrazine31.6414071415 o
a93-Ethyl-2,5-dimethylpyrazine34.3214671452oo
a10Isopropenyl pyrazine40.821619 o
b12-Acetylfuran36.7415221527oo
b22,2′-Bifuran40.4716101614 o
b32,2′-Methylenebisfuran40.9316221615 o
b42,2′-[Oxybis(methylene)]bisfuran57.7119971986 o
b5Furfuryl alcohol42.8416691666oo
b6Furfuryl acetate37.8215471552oo
b7Furfuryl propionate40.3616071603oo
b8Furfural34.8114771482oo
b95-Methylfurfural39.6315901596oo
c11-Methylpyrrole19.3011471140oo
c21-Furfurylpyrrole49.6618411833oo
d1Guaiacol (2-Methoxyphenol)51.1018741872oo
d24-Ethylguaiacol59.9320522054oo
d32-Methoxy-4-vinylphenol64.80 2156oo
Identification for volatile compounds compared with Retention Index (RI) on DB-WAX column in NIST and VCF library, co-injection, and mass spectrum.
Table 2. Volatile compounds (PAR: peak area ratio) in coffee roasted at 235 °C for 13 min (model E-1 to E-9 for expresso, C-1 to C-9 for cold brew). The sample number was also assigned to the particle size of grounded coffee (355 μm: 1–3; 500 μm: 4–6; 710 μm: 7–9). 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; tap water: 2, 5, 8; bottled water: 3, 6, 9).
Table 2. Volatile compounds (PAR: peak area ratio) in coffee roasted at 235 °C for 13 min (model E-1 to E-9 for expresso, C-1 to C-9 for cold brew). The sample number was also assigned to the particle size of grounded coffee (355 μm: 1–3; 500 μm: 4–6; 710 μm: 7–9). 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; tap water: 2, 5, 8; bottled water: 3, 6, 9).
Compounds235 °C, 13 min
355 μm500 μm710 μm
Filtered WaterTap WaterBottled WaterFiltered WaterTap WaterBottled WaterFiltered WaterTap WaterBottled Water
E-1C-1E-2C-2E-3C-3E-4C-4E-5C-5E-6C-6E-7C-7E-8C-8E-9C-9
2-Methylpyrazine0.382 b0.885 a0.941 a0.615 b0.852 a0.596 b0.3380.641 a0.2880.505 b0.3240.520 b0.190 b0.605 a0.436 a0.591 a0.186 b0.493 b
2,3-Dimethylpyrazine0.048 b0.070 b0.068 ab0.087 ab0.056 a0.098 a0.0400.097 a0.0310.075 b0.0410.056 c0.019 b0.056 a0.032 a0.046 b0.022 b0.039 b
2,5-Dimethylpyrazine0.237 b0.523 a0.553 a0.371 b0.518 a0.346 b0.2130.383 a0.1880.304 b0.2050.302 b0.104 b0.352 a0.224 a0.340 a0.102 b0.269 b
2,6-Dimethylpyrazine0.215 b0.438 a0.451 a0.292 b0.435 a0.298 b0.1840.3020.1640.2450.1820.2530.090 b0.279 a0.187 a0.266 ab0.093 b0.213 b
2-Ethylpyrazine0.189 b0.338 a0.372 a0.252 b0.366 a0.243 b0.1610.258 a0.1430.197 b0.1530.206 b0.072 b0.2180.148 a0.2090.074 b0.174
2,6-Diethylpyrazine0.082 b0.2190.100 ab0.2130.125 a0.2090.0570.199 a0.0520.163 b0.0610.172 a0.0320.114 a0.0350.104 a0.0440.079 b
2-Ethyl-
3-methylpyrazine
0.247 b0.4020.489 a0.3120.482 a0.3160.2110.312 a0.1960.245b0.2190.263 b0.096 b0.297 a0.189 a0.297 a0.099 b0.231 b
2-Ethyl-
5-methylpyrazine
0.317 b0.326 a0.384 ab0.256 b0.393 a0.278 ab0.2620.2590.2130.2170.2250.2170.1040.237 ab0.1410.257 a0.1400.205 b
3-Ethyl-
2,5-imethylpyrazine
0.422 b0.753 a1.059 a0.587 b1.027 a0.613 ab0.336 a0.5880.297 b0.4910.347 a0.5220.139 b0.500 a0.398 a0.462 a0.145 b0.399 b
Isopropenyl pyrazine0.048 b0.1040.120 a0.1000.115 a0.1000.0430.0980.0470.0930.0520.0980.026 b0.106 a0.038 a0.105 a0.019 b0.082 b
2-Acetylfuran0.308 b0.738 a0.643 a0.551 b0.632 a0.541 b0.2390.4750.2290.3800.2640.3920.133 b0.382 a0.238 a0.354 ab0.147 b0.260 b
2,2′-Bifuran0.086 b0.0880.267 a0.0920.222 a0.0920.069 ab0.077 a0.062 b0.060 b0.077 a0.067 ab0.045 b0.056 ab0.127 a0.062 a0.061 b0.045 b
2,2′-Methylenebisfuran0.078 b0.0860.225 a0.0860.193 a0.0920.072 b0.0520.071 b0.0430.081a0.0460.047 b0.041 a0.103 a0.041 a0.054 b0.031 b
2,2′-[Oxybis
(methylene)]bisfuran
0.124 b0.2350.591 a0.2380.612 a0.2590.117 a0.811 b0.087 b0.964 ab0.112ab1.063 a0.081 c0.2470.244 a0.2260.154 b0.215
Furfuryl alcohol1.656 b2.758 a2.710 a1.967 b2.514 a1.813 b1.4051.729 ab1.3351.495 b1.3551.788 a0.728 b1.4791.379 a1.5530.768 b1.349
Furfuryl acetate0.891 b1.9162.194 a1.5452.321 a1.5320.706 ab1.190 a0.636 b0.832 b0.731a0.937 b0.289 b0.7490.715 a0.7750.332 b0.627
Furfuryl propionate0.058 b0.1030.091 a0.0830.103 a0.0840.043 ab0.0550.039 b0.0450.050a0.0450.013 b0.037 a0.033 a0.036 a0.010 b0.028b
Furfural2.604 b4.906 a5.492 a3.746 b4.828 a3.665 b2.0154.096 a1.8503.236 b1.9903.381 ab1.196 b3.5002.338 a3.5771.124 b2.823
5-Methylfurfural2.716 b6.897 a7.229 a5.392 b6.608 a5.268 b2.2364.682 a2.0513.700 b2.2104.190 ab1.176 b3.5032.429 a3.5591.168 b3.106
1-Methylpyrrole0.076 b0.1670.114 a0.1340.147 a0.1280.0550.156 a0.0560.113 b0.0640.121 b0.024 b0.133 ab0.075 a0.140 a0.025 b0.103 b
1-Furfurylpyrrole1.548 a0.761 a1.204 b0.612 b1.087 b0.643 ab1.0620.5651.2100.4711.2580.4950.5990.3950.4900.4010.5100.357
Guaiacol0.108 b0.352 a0.184 a0.248 b0.165 a0.249 b0.0900.2570.0830.2320.0890.2290.0550.0550.0640.0510.0560.045
4-Ethylguaiacol0.337 b0.3660.976 a0.3360.944 a0.3570.3340.242 b0.3210.242 b0.3440.340 a0.190 b0.4440.404 a0.4560.168 b0.429
2-Methoxy-
4-vinylphenol
3.612 a1.3672.937 b1.3882.851 b1.4123.253 b1.520 b3.348 b1.576 b3.735a1.727 a1.647 a1.311 b1.469 b1.431 a1.286 c1.477 a
All values represented as the average of three replicates are the peak area ratio (peak area of each peak/peak area of internal standard). Different letters of the alphabet paired with a cardinal number within a particle size and extraction method indicate significant differences in the type of water according to Duncan’s test between each sample (p < 0.05).
Table 3. Volatile compounds in coffee roasted at 240 °C for 15 min (model E-10 to E-18 for expresso, C-10 to C-18 for cold brew). The sample number was also assigned to the particle size of grounded coffee (355 μm: 10–12; 500 μm: 13–15; 710 μm: 16–18). 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: 10, 13, 16; tap water: 11, 14, 17; bottled water: 12, 15, 18).
Table 3. Volatile compounds in coffee roasted at 240 °C for 15 min (model E-10 to E-18 for expresso, C-10 to C-18 for cold brew). The sample number was also assigned to the particle size of grounded coffee (355 μm: 10–12; 500 μm: 13–15; 710 μm: 16–18). 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: 10, 13, 16; tap water: 11, 14, 17; bottled water: 12, 15, 18).
Compounds240 °C, 15 min
355 μm500 μm710 μm
Filtered WaterTap WaterBottled WaterFiltered WaterTap WaterBottled WaterFiltered WaterTap WaterBottled Water
E-10C-10E-11C-11E-12C-12E-13C-13E-14C-14E-15C-15E-16C-16E-17C-17E-18C-18
2-Methylpyrazine0.712 a0.7270.445 b0.7180.476 b0.6450.6010.688 a0.480.496 b0.4270.477 b0.245 b0.5140.566 a0.5040.221 b0.476
2,3-Dimethylpyrazine0.0720.0570.0850.0760.0740.0640.114 a0.081 a0.072 b0.042 b0.072 b0.044 b0.023 b0.0460.054 a0.0430.028 b0.045
2,5-Dimethylpyrazine0.391 a0.3750.258 b0.3660.281 b0.3450.313 a0.363 a0.250 b0.242 b0.258 ab0.237 b0.119 b0.2540.279 a0.2490.128 b0.241
2,6-Dimethylpyrazine0.393 a0.3490.272 b0.3500.283 b0.3270.324 a0.382 a0.284 a0.277 b0.240 b0.268 b0.109 b0.2670.273 a0.2630.119 b0.256
2-Ethylpyrazine0.337 a0.2740.224 b0.2570.226 b0.2420.271 a0.308 a0.224 b0.212 b0.196 b0.205 b0.084 b0.2230.227 a0.2200.089 b0.209
2,6-Diethylpyrazine0.106 ab0.1010.040 b0.1180.140 a0.1040.224 a0.126 a0.128 b0.082 b0.121 b0.081 b0.0710.0880.0560.0910.0710.084
2-Ethyl-
3-methylpyrazine
0.419 a0.3320.309 b0.3470.341 b0.3200.386 a0.386 a0.302 b0.267 b0.310 b0.265 b0.144 b0.2620.270 a0.2590.139 b0.257
2-Ethyl-
5-methylpyrazine
0.327 a0.2800.225 b0.2630.247 b0.2660.261 a0.307 a0.208 b0.193 b0.192 b0.192 b0.078 b0.2270.186 a0.2080.099 b0.204
3-Ethyl-
2,5-imethylpyrazine
0.675 a0.6600.436 b0.8070.516 b0.6740.593 a0.602 a0.418 b0.490 b0.397 b0.414 b0.181c0.5260.379 a0.5190.244 b0.478
Isopropenyl pyrazine0.137 b0.143N.D c0.0950.213 a0.1170.164 a0.112 a0.111 b0.078 b0.097 b0.080 b0.0760.1060.0650.0950.0650.094
2-Acetylfuran0.577 a0.6550.404 b0.6260.441 b0.5870.557 a0.553 a0.365 b0.361 b0.382 b0.338 b0.171 b0.3890.349 a0.3760.170 b0.375
2,2’-Bifuran0.214 a0.107N.D c0.0960.086 b0.1090.197 a0.1370.116 b0.0650.112 b0.0630.083 b0.0680.177 a0.0690.081 b0.051
2,2’-Methylenebisfuran0.841 a0.2480.486 b0.2240.529 b0.2420.516 a0.3040.356 b0.2620.379 b0.2490.223 b0.2450.699 a0.2470.235 b0.219
2,2’-[Oxybis
(methylene)]bisfuran
2.142 a0.9051.454 b0.9931.601 b0.9881.243 a1.0751.185 ab0.9390.996 b0.9510.322 b0.9141.274 a0.9200.341 b0.997
Furfuryl alcohol2.6532.2792.3332.8072.7422.8882.3272.308 a2.3631.923 b1.9851.898 b0.849 b2.1931.957 a2.0850.825 b2.114
Furfuryl acetate4.393 a2.8912.364 b2.6772.450 b2.9162.748 a3.0042.075 b2.6281.890 b2.6730.736 b2.8042.567 a2.4050.688 b2.553
Furfuryl propionate0.354 a0.1970.202 b0.2170.198 b0.2210.257 a0.276 a0.184 b0.204 b0.174 b0.209 b0.058 b0.205 a0.251 a0.199 ab0.060 b0.190 b
Furfural1.044 a1.7360.685 b1.580.702 b1.4851.139 a0.708 a0.877 b0.527 b0.900 b0.511 b0.7000.5840.7000.5750.7120.542
5-Methylfurfural2.313 a2.9781.811 b3.3601.905 b2.7432.610 a1.514 a2.183 b1.214 b2.060 b1.204 b1.213 b1.3331.441 a1.2911.297 b0.944
1-Methylpyrrole0.213 a0.245N.D b0.198N.D b0.2380.190 a0.377 a0.085 b0.252 b0.091 b0.251 bN.D b0.2730.214 a0.267N.D b0.229
1-Furfurylpyrrole1.5700.9131.3680.9641.3060.9781.506 a0.7271.005 b0.6050.987 b0.6380.669 b0.795 a1.154 a0.705 b0.685 b0.760 a
Guaiacol0.7320.3850.5700.3950.5190.3890.397 a0.409 a0.339 ab0.318 b0.274 b0.330 b0.079 b0.3400.412 a0.3370.064 b0.352
4-Ethylguaiacol2.622 a1.1272.062 b1.1892.241 b1.1591.644 a1.357 a1.381 b1.203 b1.422 b1.217 ab0.467 b1.1551.421 a1.1620.472 b1.219
2-Methoxy-
4-vinylphenol
3.971 a1.1504.232 a1.0794.741 b1.1123.998 a1.379 a3.752 ab1.192 b3.431 b1.119 b1.529 b1.5911.951 a1.5141.472 b1.499
All values represented as the average of three replicates are the peak area ratio (peak area of each peak/peak area of internal standard). Different letters of the alphabet paired with a cardinal number within a particle size and extraction method indicate significant differences in the type of water according to Duncan’s test between each sample (p < 0.05). N.D: not detected.
Table 4. Volatile compounds in coffee roasted at 245 °C for 17 min (model E-19 to E-27 for expresso, C-19 to C-27 for cold brew). The sample number was also assigned to the particle Scheme 355. μm: 19–21; 500 μm: 22–24; 710 μm: 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: 19, 22, 25; tap water: 20, 23, 26; bottled water: 21, 24, 27).
Table 4. Volatile compounds in coffee roasted at 245 °C for 17 min (model E-19 to E-27 for expresso, C-19 to C-27 for cold brew). The sample number was also assigned to the particle Scheme 355. μm: 19–21; 500 μm: 22–24; 710 μm: 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: 19, 22, 25; tap water: 20, 23, 26; bottled water: 21, 24, 27).
Compounds245 °C, 17 Min
355 μm500 μm710 μm
Filtered WaterTap WaterBottled WaterFiltered WaterTap WaterBottled WaterFiltered WaterTap WaterBottled Water
E-19C-19E-20C-20E-21C-21E-22C-22E-23C-23E-24C-24E-25C-25E-26C-26E-27C-27
2-Methylpyrazine0.8070.3530.6350.3200.6530.3190.397 a0.5890.350 b0.5290.369 ab0.4890.3880.519 a0.6430.451 b0.1880.451 b
2,3-Dimethylpyrazine0.1010.0510.0920.0500.1020.0440.0890.0660.0970.0700.0840.0620.0550.102 a0.0570.090 a0.0520.066 b
2,5-Dimethylpyrazine0.3360.1620.2760.1470.2860.1490.1900.2550.1660.2240.1750.2180.167 a0.238 a0.094 b0.212 b0.090 b0.198 b
2,6-Dimethylpyrazine0.3960.2030.3220.1930.3330.1900.2290.3050.2070.2870.2160.2710.198 a0.274 a0.105 b0.254 ab0.108 b0.246 b
2-Ethylpyrazine0.2540.1620.2220.1530.2240.1510.1700.2190.1580.2080.1550.1990.140 a0.2100.079 b0.1970.082 b0.179
2,6-Diethylpyrazine0.148 a0.0570.124 ab0.0530.110 b0.0540.0700.0620.0400.0560.0590.0570.021 b0.086 a0.029 a0.070 ab0.031 a0.064 b
2-Ethyl-
3-methylpyrazine
0.3530.1950.3240.1840.3030.1880.2120.2510.2030.2440.2260.2250.175 a0.2410.094 b0.2290.101 b0.223
2-Ethyl-
5-methylpyrazine
0.1990.1120.1670.1130.1680.1160.1110.1320.1070.1280.1180.1280.095 a0.1810.051 b0.1660.061 b0.156
3-Ethyl-
2,5-imethylpyrazine
0.514 a0.3250.387 ab0.3200.243 b0.2850.334 a0.3880.240 b0.3940.326 a0.3630.193 a0.3880.138 b0.3890.140 b0.363
Isopropenyl pyrazine0.322 a0.0870.243 ab0.0930.219 b0.095N.D0.1150.240.093N.D0.1020.102 a0.096 a0.020 b0.073 b0.024 b0.065 b
2-Acetylfuran0.7880.3020.6380.3100.6510.2870.4150.4850.3680.4350.3810.4170.254 a0.426 a0.158 b0.368 b0.161 b0.326c
2,2’-Bifuran0.0580.0550.0550.0640.0500.0520.0430.1100.0360.0830.0460.0730.097 a0.069 a0.047 b0.047 b0.036 b0.040 b
2,2’-Methylenebisfuran0.9040.2970.8160.3190.8140.2870.6760.3590.6270.3780.6790.3590.625 a0.3260.558 b0.3200.498 b0.294
2,2’-[Oxybis
(methylene)]bisfuran
1.3760.9321.3640.9741.3650.9331.5080.8671.4300.9441.7180.9521.172 a1.5340.955 b1.7350.959 b1.661
Furfuryl alcohol1.9730.7751.6460.8571.8720.8881.254 ab1.198 a1.105 b1.032 b1.318 a0.970 b0.987 a1.7170.734 b1.5750.772 b1.616
Furfuryl acetate2.9771.5402.6331.5362.3861.4922.096 a1.7851.632 b1.7602.016 a1.6951.589 a2.2801.017 b2.2471.067 b2.061
Furfuryl propionate0.2420.1580.2280.1530.1920.1420.204 a0.1540.160 b0.1640.202 a0.1610.160 a0.196 a0.114 b0.184 ab0.108 b0.167 b
Furfural0.4340.1120.3770.1090.3790.0990.2120.1730.1650.1520.1740.1500.1400.321 a0.1380.252 b0.1450.237 b
5-Methylfurfural0.4480.2340.4020.1980.4160.2100.1820.3470.1790.3120.2140.3270.255 a0.7930.205 b0.7480.194 b0.747
1-MethylpyrroleN.D c0.2970.459 a0.3000.363 b0.3060.272 a0.4610.214 b0.4340.244 ab0.4160.272 a0.3770.126 b0.3430.110 b0.308
1-Furfurylpyrrole0.9150.544 b0.8230.626 ab0.7970.727 a1.2930.6001.1230.6491.3780.5910.8110.6400.8190.5550.8340.555
Guaiacol2.1800.5901.9840.6141.9880.6101.3220.9361.1730.8461.3370.8300.623 a0.567 a0.337c0.510 ab0.439 b0.486 b
4-Ethylguaiacol4.9362.0024.5452.0374.4981.9454.220 a2.4423.665 b2.4244.255 a2.4512.026 a1.9441.381c1.7741.583 b1.835
2-Methoxy-
4-vinylphenol
1.736 a0.504 a1.562 b0.504 a1.689 ab0.413 b1.898 ab0.5031.596 b0.5102.018s0.4850.948c1.192 a1.027 b0.923 b1.232 a0.877 b
All values represented as the average of three replicates are the peak area ratio (peak area of each peak/peak area of internal standard). Different letters of the alphabet paired with a cardinal number within a particle size and extraction method indicate significant differences in the type of water according to Duncan’s test between each sample (p < 0.05). N.D: not detected.
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