Skip to main content
NCBI home page
Foods logoLink to Foods
. 2021 Jun 10;10(6):1347. doi: 10.3390/foods10061347

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

Ja-Myung Yu 1, Mingi Chu 1, Hyunbeen Park 1, Jooyeon Park 1, Kwang-Geun Lee 1,*
Editor: Beatriz Cancho-Grande1
PMCID: PMC8230519  PMID: 34200936

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

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).

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. Compounds Retention Time R.I R.I (Ref) Co-
Injection		 Mass Spectrum
a1 2-Methylpyrazine 25.99 1289 1288 o o
a2 2,3-Dimethylpyrazine 29.87 1370 1371 o o
a3 2,5-Dimethylpyrazine 28.73 1346 1348 o o
a4 2,6-Dimethylpyrazine 29.02 1352 1354 o o
a5 2-Ethylpyrazine 29.23 1356 1359 o o
a6 2,6-Diethylpyrazine 33.83 1456 1444 o
a7 2-Ethyl-3-methylpyrazine 32.54 1427 1422 o o
a8 2-Ethyl-5-methylpyrazine 31.64 1407 1415 o
a9 3-Ethyl-2,5-dimethylpyrazine 34.32 1467 1452 o o
a10 Isopropenyl pyrazine 40.82 1619 o
b1 2-Acetylfuran 36.74 1522 1527 o o
b2 2,2′-Bifuran 40.47 1610 1614 o
b3 2,2′-Methylenebisfuran 40.93 1622 1615 o
b4 2,2′-[Oxybis(methylene)]bisfuran 57.71 1997 1986 o
b5 Furfuryl alcohol 42.84 1669 1666 o o
b6 Furfuryl acetate 37.82 1547 1552 o o
b7 Furfuryl propionate 40.36 1607 1603 o o
b8 Furfural 34.81 1477 1482 o o
b9 5-Methylfurfural 39.63 1590 1596 o o
c1 1-Methylpyrrole 19.30 1147 1140 o o
c2 1-Furfurylpyrrole 49.66 1841 1833 o o
d1 Guaiacol (2-Methoxyphenol) 51.10 1874 1872 o o
d2 4-Ethylguaiacol 59.93 2052 2054 o o
d3 2-Methoxy-4-vinylphenol 64.80 2156 o o
Open in a new tab

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).

Compounds 235 °C, 13 min
355 μm 500 μm 710 μm
Filtered Water Tap Water Bottled Water Filtered Water Tap Water Bottled Water Filtered Water Tap Water Bottled Water
E-1 C-1 E-2 C-2 E-3 C-3 E-4 C-4 E-5 C-5 E-6 C-6 E-7 C-7 E-8 C-8 E-9 C-9
2-Methylpyrazine 0.382 b 0.885 a 0.941 a 0.615 b 0.852 a 0.596 b 0.338 0.641 a 0.288 0.505 b 0.324 0.520 b 0.190 b 0.605 a 0.436 a 0.591 a 0.186 b 0.493 b
2,3-Dimethylpyrazine 0.048 b 0.070 b 0.068 ab 0.087 ab 0.056 a 0.098 a 0.040 0.097 a 0.031 0.075 b 0.041 0.056 c 0.019 b 0.056 a 0.032 a 0.046 b 0.022 b 0.039 b
2,5-Dimethylpyrazine 0.237 b 0.523 a 0.553 a 0.371 b 0.518 a 0.346 b 0.213 0.383 a 0.188 0.304 b 0.205 0.302 b 0.104 b 0.352 a 0.224 a 0.340 a 0.102 b 0.269 b
2,6-Dimethylpyrazine 0.215 b 0.438 a 0.451 a 0.292 b 0.435 a 0.298 b 0.184 0.302 0.164 0.245 0.182 0.253 0.090 b 0.279 a 0.187 a 0.266 ab 0.093 b 0.213 b
2-Ethylpyrazine 0.189 b 0.338 a 0.372 a 0.252 b 0.366 a 0.243 b 0.161 0.258 a 0.143 0.197 b 0.153 0.206 b 0.072 b 0.218 0.148 a 0.209 0.074 b 0.174
2,6-Diethylpyrazine 0.082 b 0.219 0.100 ab 0.213 0.125 a 0.209 0.057 0.199 a 0.052 0.163 b 0.061 0.172 a 0.032 0.114 a 0.035 0.104 a 0.044 0.079 b
2-Ethyl-
3-methylpyrazine		 0.247 b 0.402 0.489 a 0.312 0.482 a 0.316 0.211 0.312 a 0.196 0.245b 0.219 0.263 b 0.096 b 0.297 a 0.189 a 0.297 a 0.099 b 0.231 b
2-Ethyl-
5-methylpyrazine		 0.317 b 0.326 a 0.384 ab 0.256 b 0.393 a 0.278 ab 0.262 0.259 0.213 0.217 0.225 0.217 0.104 0.237 ab 0.141 0.257 a 0.140 0.205 b
3-Ethyl-
2,5-imethylpyrazine		 0.422 b 0.753 a 1.059 a 0.587 b 1.027 a 0.613 ab 0.336 a 0.588 0.297 b 0.491 0.347 a 0.522 0.139 b 0.500 a 0.398 a 0.462 a 0.145 b 0.399 b
Isopropenyl pyrazine 0.048 b 0.104 0.120 a 0.100 0.115 a 0.100 0.043 0.098 0.047 0.093 0.052 0.098 0.026 b 0.106 a 0.038 a 0.105 a 0.019 b 0.082 b
2-Acetylfuran 0.308 b 0.738 a 0.643 a 0.551 b 0.632 a 0.541 b 0.239 0.475 0.229 0.380 0.264 0.392 0.133 b 0.382 a 0.238 a 0.354 ab 0.147 b 0.260 b
2,2′-Bifuran 0.086 b 0.088 0.267 a 0.092 0.222 a 0.092 0.069 ab 0.077 a 0.062 b 0.060 b 0.077 a 0.067 ab 0.045 b 0.056 ab 0.127 a 0.062 a 0.061 b 0.045 b
2,2′-Methylenebisfuran 0.078 b 0.086 0.225 a 0.086 0.193 a 0.092 0.072 b 0.052 0.071 b 0.043 0.081a 0.046 0.047 b 0.041 a 0.103 a 0.041 a 0.054 b 0.031 b
2,2′-[Oxybis
(methylene)]bisfuran		 0.124 b 0.235 0.591 a 0.238 0.612 a 0.259 0.117 a 0.811 b 0.087 b 0.964 ab 0.112ab 1.063 a 0.081 c 0.247 0.244 a 0.226 0.154 b 0.215
Furfuryl alcohol 1.656 b 2.758 a 2.710 a 1.967 b 2.514 a 1.813 b 1.405 1.729 ab 1.335 1.495 b 1.355 1.788 a 0.728 b 1.479 1.379 a 1.553 0.768 b 1.349
Furfuryl acetate 0.891 b 1.916 2.194 a 1.545 2.321 a 1.532 0.706 ab 1.190 a 0.636 b 0.832 b 0.731a 0.937 b 0.289 b 0.749 0.715 a 0.775 0.332 b 0.627
Furfuryl propionate 0.058 b 0.103 0.091 a 0.083 0.103 a 0.084 0.043 ab 0.055 0.039 b 0.045 0.050a 0.045 0.013 b 0.037 a 0.033 a 0.036 a 0.010 b 0.028b
Furfural 2.604 b 4.906 a 5.492 a 3.746 b 4.828 a 3.665 b 2.015 4.096 a 1.850 3.236 b 1.990 3.381 ab 1.196 b 3.500 2.338 a 3.577 1.124 b 2.823
5-Methylfurfural 2.716 b 6.897 a 7.229 a 5.392 b 6.608 a 5.268 b 2.236 4.682 a 2.051 3.700 b 2.210 4.190 ab 1.176 b 3.503 2.429 a 3.559 1.168 b 3.106
1-Methylpyrrole 0.076 b 0.167 0.114 a 0.134 0.147 a 0.128 0.055 0.156 a 0.056 0.113 b 0.064 0.121 b 0.024 b 0.133 ab 0.075 a 0.140 a 0.025 b 0.103 b
1-Furfurylpyrrole 1.548 a 0.761 a 1.204 b 0.612 b 1.087 b 0.643 ab 1.062 0.565 1.210 0.471 1.258 0.495 0.599 0.395 0.490 0.401 0.510 0.357
Guaiacol 0.108 b 0.352 a 0.184 a 0.248 b 0.165 a 0.249 b 0.090 0.257 0.083 0.232 0.089 0.229 0.055 0.055 0.064 0.051 0.056 0.045
4-Ethylguaiacol 0.337 b 0.366 0.976 a 0.336 0.944 a 0.357 0.334 0.242 b 0.321 0.242 b 0.344 0.340 a 0.190 b 0.444 0.404 a 0.456 0.168 b 0.429
2-Methoxy-
4-vinylphenol		 3.612 a 1.367 2.937 b 1.388 2.851 b 1.412 3.253 b 1.520 b 3.348 b 1.576 b 3.735a 1.727 a 1.647 a 1.311 b 1.469 b 1.431 a 1.286 c 1.477 a
Open in a new tab

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).

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 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).

Compounds 240 °C, 15 min
355 μm 500 μm 710 μm
Filtered Water Tap Water Bottled Water Filtered Water Tap Water Bottled Water Filtered Water Tap Water Bottled Water
E-10 C-10 E-11 C-11 E-12 C-12 E-13 C-13 E-14 C-14 E-15 C-15 E-16 C-16 E-17 C-17 E-18 C-18
2-Methylpyrazine 0.712 a 0.727 0.445 b 0.718 0.476 b 0.645 0.601 0.688 a 0.48 0.496 b 0.427 0.477 b 0.245 b 0.514 0.566 a 0.504 0.221 b 0.476
2,3-Dimethylpyrazine 0.072 0.057 0.085 0.076 0.074 0.064 0.114 a 0.081 a 0.072 b 0.042 b 0.072 b 0.044 b 0.023 b 0.046 0.054 a 0.043 0.028 b 0.045
2,5-Dimethylpyrazine 0.391 a 0.375 0.258 b 0.366 0.281 b 0.345 0.313 a 0.363 a 0.250 b 0.242 b 0.258 ab 0.237 b 0.119 b 0.254 0.279 a 0.249 0.128 b 0.241
2,6-Dimethylpyrazine 0.393 a 0.349 0.272 b 0.350 0.283 b 0.327 0.324 a 0.382 a 0.284 a 0.277 b 0.240 b 0.268 b 0.109 b 0.267 0.273 a 0.263 0.119 b 0.256
2-Ethylpyrazine 0.337 a 0.274 0.224 b 0.257 0.226 b 0.242 0.271 a 0.308 a 0.224 b 0.212 b 0.196 b 0.205 b 0.084 b 0.223 0.227 a 0.220 0.089 b 0.209
2,6-Diethylpyrazine 0.106 ab 0.101 0.040 b 0.118 0.140 a 0.104 0.224 a 0.126 a 0.128 b 0.082 b 0.121 b 0.081 b 0.071 0.088 0.056 0.091 0.071 0.084
2-Ethyl-
3-methylpyrazine		 0.419 a 0.332 0.309 b 0.347 0.341 b 0.320 0.386 a 0.386 a 0.302 b 0.267 b 0.310 b 0.265 b 0.144 b 0.262 0.270 a 0.259 0.139 b 0.257
2-Ethyl-
5-methylpyrazine		 0.327 a 0.280 0.225 b 0.263 0.247 b 0.266 0.261 a 0.307 a 0.208 b 0.193 b 0.192 b 0.192 b 0.078 b 0.227 0.186 a 0.208 0.099 b 0.204
3-Ethyl-
2,5-imethylpyrazine		 0.675 a 0.660 0.436 b 0.807 0.516 b 0.674 0.593 a 0.602 a 0.418 b 0.490 b 0.397 b 0.414 b 0.181c 0.526 0.379 a 0.519 0.244 b 0.478
Isopropenyl pyrazine 0.137 b 0.143 N.D c 0.095 0.213 a 0.117 0.164 a 0.112 a 0.111 b 0.078 b 0.097 b 0.080 b 0.076 0.106 0.065 0.095 0.065 0.094
2-Acetylfuran 0.577 a 0.655 0.404 b 0.626 0.441 b 0.587 0.557 a 0.553 a 0.365 b 0.361 b 0.382 b 0.338 b 0.171 b 0.389 0.349 a 0.376 0.170 b 0.375
2,2’-Bifuran 0.214 a 0.107 N.D c 0.096 0.086 b 0.109 0.197 a 0.137 0.116 b 0.065 0.112 b 0.063 0.083 b 0.068 0.177 a 0.069 0.081 b 0.051
2,2’-Methylenebisfuran 0.841 a 0.248 0.486 b 0.224 0.529 b 0.242 0.516 a 0.304 0.356 b 0.262 0.379 b 0.249 0.223 b 0.245 0.699 a 0.247 0.235 b 0.219
2,2’-[Oxybis
(methylene)]bisfuran		 2.142 a 0.905 1.454 b 0.993 1.601 b 0.988 1.243 a 1.075 1.185 ab 0.939 0.996 b 0.951 0.322 b 0.914 1.274 a 0.920 0.341 b 0.997
Furfuryl alcohol 2.653 2.279 2.333 2.807 2.742 2.888 2.327 2.308 a 2.363 1.923 b 1.985 1.898 b 0.849 b 2.193 1.957 a 2.085 0.825 b 2.114
Furfuryl acetate 4.393 a 2.891 2.364 b 2.677 2.450 b 2.916 2.748 a 3.004 2.075 b 2.628 1.890 b 2.673 0.736 b 2.804 2.567 a 2.405 0.688 b 2.553
Furfuryl propionate 0.354 a 0.197 0.202 b 0.217 0.198 b 0.221 0.257 a 0.276 a 0.184 b 0.204 b 0.174 b 0.209 b 0.058 b 0.205 a 0.251 a 0.199 ab 0.060 b 0.190 b
Furfural 1.044 a 1.736 0.685 b 1.58 0.702 b 1.485 1.139 a 0.708 a 0.877 b 0.527 b 0.900 b 0.511 b 0.700 0.584 0.700 0.575 0.712 0.542
5-Methylfurfural 2.313 a 2.978 1.811 b 3.360 1.905 b 2.743 2.610 a 1.514 a 2.183 b 1.214 b 2.060 b 1.204 b 1.213 b 1.333 1.441 a 1.291 1.297 b 0.944
1-Methylpyrrole 0.213 a 0.245 N.D b 0.198 N.D b 0.238 0.190 a 0.377 a 0.085 b 0.252 b 0.091 b 0.251 b N.D b 0.273 0.214 a 0.267 N.D b 0.229
1-Furfurylpyrrole 1.570 0.913 1.368 0.964 1.306 0.978 1.506 a 0.727 1.005 b 0.605 0.987 b 0.638 0.669 b 0.795 a 1.154 a 0.705 b 0.685 b 0.760 a
Guaiacol 0.732 0.385 0.570 0.395 0.519 0.389 0.397 a 0.409 a 0.339 ab 0.318 b 0.274 b 0.330 b 0.079 b 0.340 0.412 a 0.337 0.064 b 0.352
4-Ethylguaiacol 2.622 a 1.127 2.062 b 1.189 2.241 b 1.159 1.644 a 1.357 a 1.381 b 1.203 b 1.422 b 1.217 ab 0.467 b 1.155 1.421 a 1.162 0.472 b 1.219
2-Methoxy-
4-vinylphenol		 3.971 a 1.150 4.232 a 1.079 4.741 b 1.112 3.998 a 1.379 a 3.752 ab 1.192 b 3.431 b 1.119 b 1.529 b 1.591 1.951 a 1.514 1.472 b 1.499
Open in a new tab

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 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].

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).

Compounds 245 °C, 17 Min
355 μm 500 μm 710 μm
Filtered Water Tap Water Bottled Water Filtered Water Tap Water Bottled Water Filtered Water Tap Water Bottled Water
E-19 C-19 E-20 C-20 E-21 C-21 E-22 C-22 E-23 C-23 E-24 C-24 E-25 C-25 E-26 C-26 E-27 C-27
2-Methylpyrazine 0.807 0.353 0.635 0.320 0.653 0.319 0.397 a 0.589 0.350 b 0.529 0.369 ab 0.489 0.388 0.519 a 0.643 0.451 b 0.188 0.451 b
2,3-Dimethylpyrazine 0.101 0.051 0.092 0.050 0.102 0.044 0.089 0.066 0.097 0.070 0.084 0.062 0.055 0.102 a 0.057 0.090 a 0.052 0.066 b
2,5-Dimethylpyrazine 0.336 0.162 0.276 0.147 0.286 0.149 0.190 0.255 0.166 0.224 0.175 0.218 0.167 a 0.238 a 0.094 b 0.212 b 0.090 b 0.198 b
2,6-Dimethylpyrazine 0.396 0.203 0.322 0.193 0.333 0.190 0.229 0.305 0.207 0.287 0.216 0.271 0.198 a 0.274 a 0.105 b 0.254 ab 0.108 b 0.246 b
2-Ethylpyrazine 0.254 0.162 0.222 0.153 0.224 0.151 0.170 0.219 0.158 0.208 0.155 0.199 0.140 a 0.210 0.079 b 0.197 0.082 b 0.179
2,6-Diethylpyrazine 0.148 a 0.057 0.124 ab 0.053 0.110 b 0.054 0.070 0.062 0.040 0.056 0.059 0.057 0.021 b 0.086 a 0.029 a 0.070 ab 0.031 a 0.064 b
2-Ethyl-
3-methylpyrazine		 0.353 0.195 0.324 0.184 0.303 0.188 0.212 0.251 0.203 0.244 0.226 0.225 0.175 a 0.241 0.094 b 0.229 0.101 b 0.223
2-Ethyl-
5-methylpyrazine		 0.199 0.112 0.167 0.113 0.168 0.116 0.111 0.132 0.107 0.128 0.118 0.128 0.095 a 0.181 0.051 b 0.166 0.061 b 0.156
3-Ethyl-
2,5-imethylpyrazine		 0.514 a 0.325 0.387 ab 0.320 0.243 b 0.285 0.334 a 0.388 0.240 b 0.394 0.326 a 0.363 0.193 a 0.388 0.138 b 0.389 0.140 b 0.363
Isopropenyl pyrazine 0.322 a 0.087 0.243 ab 0.093 0.219 b 0.095 N.D 0.115 0.24 0.093 N.D 0.102 0.102 a 0.096 a 0.020 b 0.073 b 0.024 b 0.065 b
2-Acetylfuran 0.788 0.302 0.638 0.310 0.651 0.287 0.415 0.485 0.368 0.435 0.381 0.417 0.254 a 0.426 a 0.158 b 0.368 b 0.161 b 0.326c
2,2’-Bifuran 0.058 0.055 0.055 0.064 0.050 0.052 0.043 0.110 0.036 0.083 0.046 0.073 0.097 a 0.069 a 0.047 b 0.047 b 0.036 b 0.040 b
2,2’-Methylenebisfuran 0.904 0.297 0.816 0.319 0.814 0.287 0.676 0.359 0.627 0.378 0.679 0.359 0.625 a 0.326 0.558 b 0.320 0.498 b 0.294
2,2’-[Oxybis
(methylene)]bisfuran		 1.376 0.932 1.364 0.974 1.365 0.933 1.508 0.867 1.430 0.944 1.718 0.952 1.172 a 1.534 0.955 b 1.735 0.959 b 1.661
Furfuryl alcohol 1.973 0.775 1.646 0.857 1.872 0.888 1.254 ab 1.198 a 1.105 b 1.032 b 1.318 a 0.970 b 0.987 a 1.717 0.734 b 1.575 0.772 b 1.616
Furfuryl acetate 2.977 1.540 2.633 1.536 2.386 1.492 2.096 a 1.785 1.632 b 1.760 2.016 a 1.695 1.589 a 2.280 1.017 b 2.247 1.067 b 2.061
Furfuryl propionate 0.242 0.158 0.228 0.153 0.192 0.142 0.204 a 0.154 0.160 b 0.164 0.202 a 0.161 0.160 a 0.196 a 0.114 b 0.184 ab 0.108 b 0.167 b
Furfural 0.434 0.112 0.377 0.109 0.379 0.099 0.212 0.173 0.165 0.152 0.174 0.150 0.140 0.321 a 0.138 0.252 b 0.145 0.237 b
5-Methylfurfural 0.448 0.234 0.402 0.198 0.416 0.210 0.182 0.347 0.179 0.312 0.214 0.327 0.255 a 0.793 0.205 b 0.748 0.194 b 0.747
1-Methylpyrrole N.D c 0.297 0.459 a 0.300 0.363 b 0.306 0.272 a 0.461 0.214 b 0.434 0.244 ab 0.416 0.272 a 0.377 0.126 b 0.343 0.110 b 0.308
1-Furfurylpyrrole 0.915 0.544 b 0.823 0.626 ab 0.797 0.727 a 1.293 0.600 1.123 0.649 1.378 0.591 0.811 0.640 0.819 0.555 0.834 0.555
Guaiacol 2.180 0.590 1.984 0.614 1.988 0.610 1.322 0.936 1.173 0.846 1.337 0.830 0.623 a 0.567 a 0.337c 0.510 ab 0.439 b 0.486 b
4-Ethylguaiacol 4.936 2.002 4.545 2.037 4.498 1.945 4.220 a 2.442 3.665 b 2.424 4.255 a 2.451 2.026 a 1.944 1.381c 1.774 1.583 b 1.835
2-Methoxy-
4-vinylphenol		 1.736 a 0.504 a 1.562 b 0.504 a 1.689 ab 0.413 b 1.898 ab 0.503 1.596 b 0.510 2.018s 0.485 0.948c 1.192 a 1.027 b 0.923 b 1.232 a 0.877 b
Open in a new tab

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.

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].

Figure 1.

Figure 1

Open in a new tab

Levels of volatile compounds in coffee according to (a) roasting time and temperature (E-4 (Inline graphic) and C-4 (Inline graphic): roasted at 235 °C for 13 min; E-13 (Inline graphic) and C-13 (Inline graphic): roasted at 240 °C for 15 min; E-22 (Inline graphic) and C-22 (Inline graphic): roasted at 245 °C for 17 min); (b) grind size (E-10 (Inline graphic) and C-10 (Inline graphic): 355 μm; E-13 (Inline graphic) and C-13 (Inline graphic): 500 μm; E-16 (Inline graphic) and C-16 (Inline graphic): 710 μm); (c) types of water (E-13 (Inline graphic) and C-13 (Inline graphic): filtered water; E-14 (Inline graphic) and C-14 (Inline graphic): tap water; E-15 (Inline graphic) and C-15 (Inline graphic): bottled water). Different letters (a, b, c) represented significant differences.

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 2.

Figure 2

Open in a new tab

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 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.

Acknowledgments

The authors would like to thank the support of Dongguk University.

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].

Conflicts of Interest

The authors declare that there is no conflict of interest.

Footnotes

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

  • 1.Simões J., Maricato É., Nunes F.M., Domingues M.R., Coimbra M.A. Thermal stability of spent coffee ground polysaccharides: Galactomannans and arabinogalactans. Carbohydr. Polym. 2014;101:256–264. doi: 10.1016/j.carbpol.2013.09.042. [DOI] [PubMed] [Google Scholar]
  • 2.Albouchi A., Murkovic M. Formation kinetics of furfuryl alcohol in a coffee model system. Food Chem. 2018;243:91–95. doi: 10.1016/j.foodchem.2017.09.112. [DOI] [PubMed] [Google Scholar]
  • 3.Moon J.-K., Shibamoto T. Role of Roasting Conditions in the Profile of Volatile Flavor Chemicals Formed from Coffee Beans. J. Agric. Food Chem. 2009;57:5823–5831. doi: 10.1021/jf901136e. [DOI] [PubMed] [Google Scholar]
  • 4.Herawati D., Giriwono P.E., Dewi F.N.A., Kashiwagi T., Andarwulan N. Critical roasting level determines bioactive content and antioxidant activity of Robusta coffee beans. Food Sci. Biotechnol. 2019;28:7–14. doi: 10.1007/s10068-018-0442-x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 5.Seok Y.-J., Lee K.-G. Analysis of furan in semi-solid and paste type foods. Food Sci. Biotechnol. 2020;29:293–301. doi: 10.1007/s10068-019-00654-0. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Lee S.J., Kim M.K., Lee K.-G. Effect of reversed coffee grinding and roasting process on physicochemical properties including volatile compound profiles. Innov. Food Sci. Emerg. Technol. 2017;44:97–102. doi: 10.1016/j.ifset.2017.07.009. [DOI] [Google Scholar]
  • 7.Severini C., Derossi A., Ricci I., Fiore A.G., Caporizzi R. How Much Caffeine in Coffee Cup? Effects of Processing Operations, Extraction Methods and Variables. Volume 69002. Intechopen; London, UK: 2017. pp. 45–85. [Google Scholar]
  • 8.Park S.-H., Jo A., Lee K.-G. Effect of various roasting, extraction and drinking conditions on furan and 5-hydroxymethylfurfural levels in coffee. Food Chem. 2021;358:129806. doi: 10.1016/j.foodchem.2021.129806. [DOI] [PubMed] [Google Scholar]
  • 9.Kwon J., Ahn H., Lee K.-G. Analysis of α-dicarbonyl compounds in coffee (Coffea arabica) prepared under various roasting and brewing methods. Food Chem. 2021;343:128525. doi: 10.1016/j.foodchem.2020.128525. [DOI] [PubMed] [Google Scholar]
  • 10.Cameron M.I., Morisco D., Hofstetter D., Uman E., Wilkinson J., Kennedy Z., Fontenot S.A., Lee W., Hendon C.H., Foster J.M. Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter. 2020;2:631–648. doi: 10.1016/j.matt.2019.12.019. [DOI] [Google Scholar]
  • 11.Severini C., Ricci I., Marone M., DeRossi A., De Pilli T. Changes in the Aromatic Profile of Espresso Coffee as a Function of the Grinding Grade and Extraction Time: A Study by the Electronic Nose System. J. Agric. Food Chem. 2015;63:2321–2327. doi: 10.1021/jf505691u. [DOI] [PubMed] [Google Scholar]
  • 12.Navarini L., Rivetti D. Water quality for Espresso coffee. Food Chem. 2010;122:424–428. doi: 10.1016/j.foodchem.2009.04.019. [DOI] [Google Scholar]
  • 13.Cammenga H., Zielasko B. Kinetics and development of boiler scale formation in commercial coffee brewing machines; Proceedings of the COLLOQUE Scientifique International sur le Café; Nairobi, Kenya. 20–25 July 1997. [Google Scholar]
  • 14.Dórea J.G., da Costa T.H.M. Is coffee a functional food? Br. J. Nutr. 2005;93:773–782. doi: 10.1079/BJN20051370. [DOI] [PubMed] [Google Scholar]
  • 15.Petracco M. COFFEE Recent Developments. Blackwell Science Ltd.; London, UK: 2001. Technology IV: Beverage Preparation: Brewing Trends for the New Millennium. [Google Scholar]
  • 16.Andueza S., De Peña M.-P., Cid C. Chemical and Sensorial Characteristics of Espresso Coffee as Affected by Grinding and Torrefacto Roast. J. Agric. Food Chem. 2003;51:7034–7039. doi: 10.1021/jf034628f. [DOI] [PubMed] [Google Scholar]
  • 17.Salamanca C.A., Fiol N., González C., Saez M., Villaescusa I. Extraction of espresso coffee by using gradient of temperature. Effect on physicochemical and sensorial characteristics of espresso. Food Chem. 2017;214:622–630. doi: 10.1016/j.foodchem.2016.07.120. [DOI] [PubMed] [Google Scholar]
  • 18.Andueza S., Maeztu L., Pascual L., Ibáñez C., De Peña M.-P., Cid C. Influence of extraction temperature on the final quality of espresso coffee. J. Sci. Food Agric. 2003;83:240–248. doi: 10.1002/jsfa.1304. [DOI] [Google Scholar]
  • 19.Angeloni G., Guerrini L., Masella P., Innocenti M., Bellumori M., Parenti A. Characterization and comparison of cold brew and cold drip coffee extraction methods. J. Sci. Food Agric. 2018;99:391–399. doi: 10.1002/jsfa.9200. [DOI] [PubMed] [Google Scholar]
  • 20.Perez M. Cold Brew might Save the Coffee Market. [(accessed on 2 February 2021)]; Available online: https://www.bloomberg.com/news/articles/2017-07-19/hot-sales-of-cold-brew-might-just-be-what-coffee-market-needs.
  • 21.Fuller M., Rao N.Z. The Effect of Time, Roasting Temperature, and Grind Size on Caffeine and Chlorogenic Acid Concentrations in Cold Brew Coffee. Sci. Rep. 2017;7:16206. doi: 10.1038/s41598-017-18247-4. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.Shin K.-S. The Chemical Characteristics and Immune-Modulating Activity of Polysaccharides Isolated from Cold-Brew Coffee. Prev. Nutr. Food Sci. 2017;22:100–106. doi: 10.3746/pnf.2017.22.2.100. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Burdock G.A. Fenaroli’s Handbook of Flavor Ingredients. CRC Press; Boca Raton, FL, USA: 2016. [Google Scholar]
  • 24.Flament I. Coffee Flavor Chemistry. John Wiley & Sons; Hoboken, NJ, USA: 2001. [Google Scholar]
  • 25.Mottram D.S. Flavours and Fragrances. Springer; Berlin/Heidelberg, Germany: 2007. The Maillard Reaction: Source of Flavour in Thermally Processed Foods; pp. 269–283. [Google Scholar]
  • 26.Sekiwa Y., Kubota K., Kobayashi A. Characteristic Flavor Components in the Brew of Cooked Clam (Meretrix lusoria) and the Effect of Storage on Flavor Formation. J. Agric. Food Chem. 1997;45:826–830. doi: 10.1021/jf960433e. [DOI] [Google Scholar]
  • 27.Vorotnikov V., Mpourmpakis G., Vlachos D.G. DFT Study of Furfural Conversion to Furan, Furfuryl Alcohol, and 2-Methylfuran on Pd(111) ACS Catal. 2012;2:2496–2504. doi: 10.1021/cs300395a. [DOI] [Google Scholar]
  • 28.Bhumiratana N., Adhikari K., Chambers E. Evolution of sensory aroma attributes from coffee beans to brewed coffee. LWT. 2011;44:2185–2192. doi: 10.1016/j.lwt.2011.07.001. [DOI] [Google Scholar]
  • 29.Derossi A., Ricci I., Caporizzi R., Fiore A., Severini C. How grinding level and brewing method (Espresso, American, Turkish) could affect the antioxidant activity and bioactive compounds in a coffee cup. J. Sci. Food Agric. 2018;98:3198–3207. doi: 10.1002/jsfa.8826. [DOI] [PubMed] [Google Scholar]
  • 30.Jeong D.-W., Heo S., Lee B., Lee H., Jeong K., Her J.-Y., Lee K.-G., Lee J.-H. Effects of the predominant bacteria from meju and doenjang on the production of volatile compounds during soybean fermentation. Int. J. Food Microbiol. 2017;262:8–13. doi: 10.1016/j.ijfoodmicro.2017.09.011. [DOI] [PubMed] [Google Scholar]
  • 31.Lee H.J., Cho I.H., Lee K.E., Kim Y.-S. The Compositions of Volatiles and Aroma-Active Compounds in Dried Omija Fruits (Schisandra chinensis Baillon) According to the Cultivation Areas. J. Agric. Food Chem. 2011;59:8338–8346. doi: 10.1021/jf200762h. [DOI] [PubMed] [Google Scholar]
  • 32.Kim M.K., Kim M.-Y., Lee K.-G. Categorization of fruits according to their content of polyphenols and vitamin C, antiradical activity, and quality parameters. J. Food Process. Preserv. 2017;42:e13421. doi: 10.1111/jfpp.13421. [DOI] [Google Scholar]

Articles from Foods are provided here courtesy of Multidisciplinary Digital Publishing Institute (MDPI)

RESOURCES