Accepted Manuscript

Optimization of espresso machine parameters through the analysis of coffee

odorants by HS-SPME-GC/MS

Giovanni Caprioli, Manuela Cortese, Gloria Cristalli, Filippo Maggi, Luigi

Odello, Massimo Ricciutelli, Gianni Sagratini, Veronica Sirocchi, Giacomo

Tomassoni, Sauro Vittori

PII: S0308-8146(12)00996-X

DOI: http://dx.doi.org/10.1016/j.foodchem.2012.06.024

Reference: FOCH 12721

To appear in: Food Chemistry

Received Date: 28 February 2012

Revised Date: 11 May 2012

Accepted Date: 18 June 2012

Please cite this article as: Caprioli, G., Cortese, M., Cristalli, G., Maggi, F., Odello, L., Ricciutelli, M., Sagratini,

G., Sirocchi, V., Tomassoni, G., Vittori, S., Optimization of espresso machine parameters through the analysis of

coffee odorants by HS-SPME-GC/MS, Food Chemistry (2012), doi: http://dx.doi.org/10.1016/j.foodchem.

2012.06.024

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1

Optimization of espresso machine parameters through the analysis of 1

coffee odorants by HS-SPME-GC/MS 2

Giovanni Capriolia≠

, Manuela Cortesea≠

, Gloria Cristallia, Filippo Maggi

a, Luigi 3

Odellob, Massimo Ricciutelli

a, Gianni Sagratini

a, Veronica Sirocchi

a, Giacomo 4

Tomassonic, Sauro Vittori

a* 5

6

a School of Pharmacy, University of Camerino, Via Sant’Agostino 1, 62032 Camerino, 7

Italy 8

b Centro Studi Assaggiatori, Galleria V. Veneto 9, 25128 Brescia, Italy 9

c School of Science and Technology,

University of Camerino, Via Madonna delle 10

Carceri, 62032 Camerino, Italy 11

12

13

14

≠These two authors contributed equally to the research 15

16

17

18

19

*Corresponding author: Sauro Vittori, School of Pharmacy, University of Camerino, 20

via S.Agostino 1, 62032 Camerino, Italy. Phone: +390737402266. Fax: 21

+390737637345. Email: sauro.vittori@unicam.it 22

23

2

ABSTRACT 24

The aroma profile and the final quality of espresso coffee (EC) are influenced by such 25

technical conditions as the EC machine extraction temperature and the pressure used. 26

The effect of these two parameters on EC quality were studied in combination by 27

headspace Solid Phase Micro Extraction-Gas Chromatography-Mass Spectrometry 28

(SPME-GC-MS) and sensory profile. Moreover, 10 key odorants at the best EC 29

machine settings were examined to compare the two coffee cultivars (Arabica and 30

Robusta) and two EC machines [Aurelia Competizione (A) and Leva Arduino (B)]. The 31

data obtained provides important information about espresso making technique, 32

suggesting that the usual espresso machine temperature and pressure settings (i.e. 92 °C 33

and 9 bar) are very close to those needed to obtain the best quality espresso. This 34

confirms the traditional wisdom of coffee making, which judges 25 ml, the typical 35

volume of a certified Italian EC, to be ideal for very strong aroma intensity. 36

37

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Keywords: Espresso coffee, espresso coffee machine, extraction temperature and 47

pressure, HS-SPME-GC/MS, Arabica and Robusta coffee. 48

3

1. Introduction 49

The consumption of coffee, one of the most common hot drinks in the world, is 50

continually increasing (Maeztu, Anduez, Iban, & Pen, 2001; Illy, & Viani, 1995; Butler, 51

1999). According to the latest statistics from the International Coffee Organization 52

(ICO), about 1.4 billion cups of coffee a day are consumed worldwide. The two major 53

species of coffee, Coffea arabica and Coffea canephora Robusta, differ considerably in 54

price, quality and consumer acceptance. Arabica coffee is characterized by a more 55

acidic taste and intense aroma with richer body than Robusta, which is characterized by 56

its bitterness and a typical earthy and woody flavour (Illy, & Viani, 1995). While coffee 57

is prepared in a number of ways, one of the most common forms in southern Europe, 58

Central America and other areas is espresso coffee (EC), prepared from roasted and 59

ground coffee beans (Petracco, 2001). The preparation of EC is influenced by factors 60

related to coffee, water and technical conditions related to the machine (Andueza, 61

Maeztu, Pascual, Ibanez, Paz, & Cid, 2003). The most common espresso machine is 62

made with a pump which provides continuous flow of water; water pressure is usually 63

around 9 bars, whilst water temperature ranges usually between 91 and 96 °C. Espresso 64

coffee is one of the strongest tasting forms of coffee regularly consumed, with a 65

distinctive flavour and cream, a layer of emulsified oils in the form of a colloidal foam 66

floating over the liquid (Petracco, 2001; Odello, & Odello, 2006). In spite of its 67

worldwide popularity, the conditions for optimal espresso preparation have not yet been 68

defined in detail. In fact, as noted by Petracco (Petracco, 1989), and Illy and Viani (Illy, 69

& Viani, 1995), there is a lack of standardization in the weight of roasted ground coffee 70

used, the beverage volume, and the extraction conditions (water pressure and 71

temperature), parameters likely to influence the EC aroma (Nunes, Coimbra, Duarte, & 72

Delgadillo, 1997). The composition of the volatile fraction of roasted and brewed coffee 73

4

has been studied for years, and several hundred compounds have been reported as 74

constituents of coffee aroma (Sanz, Maeztu, Zapelena, Bello, & Cid, 2003; Holscher, & 75

Steinhart, 1992; Ramos, Valero, & Iba, 1998; Sanz, Ansorena, Bello, & Cid, 2001; 76

Maeztu, Sanz, Andueza, Paz, Bello, & Cid, 2001). The main constituents are aldehydes, 77

ketones, furans, pyrazines, pyridines, phenolic compounds, indoles, lactone, ester and 78

benzothiazine. Most of these compounds have been known as products from the 79

Maillard reaction between amino acids and sugars, the Strecker degradation 80

(degradation of sugar, minor lipid degradation) and the interaction between intermediate 81

decomposition products. Some volatile compounds formation, which cannot be 82

explained by the previous reactions, arise from the less volatile constituents of coffee 83

beans, such as caffeic acid, quinic acid and chlorogenic acid that produce a great 84

amount of volatiles (Moon, & Shibamoto 2010; Illy, & Viani, 1995). Although the 85

volatile fraction in coffee is very complex, only some compounds (called key odorants) 86

are responsible for coffee flavour (Maeztu, Anduez, Iban, & Pen, 2001; Grosch, 1998; 87

Semmelroch, & Grosch, 1995; Rocha, Maeztu, Barros, Cid, & Coimbra, 2003) and they 88

increase sharply during roasting process. 89

Various methods of extraction have been used to study the aroma fraction of coffee 90

brews. As an alternative to injection of an organic solvent extract, the vapour phase 91

surrounding the brew (headspace) can be directly analyzed. This alternative gives the 92

most accurate composition of flavours. However, under the usual operating conditions 93

the headspace gas sample is considerably diluted by the carrier gas. This problem can be 94

solved by injecting the headspace gas directly into the interior of a capillary column 95

(on-column injection) (Shimoda, & Shibamoto, 1990), using a purge-and-trap system 96

(Semmelroch, & Grosh, 1995) with an adsorbent (Pollien, Krebs, & Chaintreau, 1997). 97

In 1990 a technique for headspace sampling was developed and it has been 98

5

progressively used for volatile coffee analysis (Liardon, & Ott, 1984; Yang, & Peppard, 99

1994; Falque´, Darriet , Fernandez, & Dubourdieu, 1995; Barcarolo, Tutta, & Casson, 100

1996; Bicchi, Binello, Pellegrino, & Vanni 1993; Bicchi, Panero, Pellegrino, & Vanni 101

1997). Head-Space Solid Phase Microextraction (HS-SPME) has specific advantages: it 102

is economic, faster and, perhaps most importantly, requires little manipulation of 103

samples, if the composition of headspace does not fully correspond to the real 104

concentration of volatiles in the sample (Elmore, Erbahadir, & Mottram, 1997; Jelen, 105

Wlazly, Wasowicz, & Kaminski, 1998; Penton, 1997; Stevenson, & Chen, 1996; 106

Xiaogen, & Peppard, 1994). Gas chromatography-mass spectrometry and gas 107

chromatography-olfactometry have been widely used to determine the potency and 108

sensory attributes of each key odorant (Maeztu, Sanz, Andueza, Paz, Bello, & Cid, 109

2001; Semmelroch, & Grosch, 1995; Pollien, Krebs, & Chaintreau, 1997). 110

In recent years, sensory analysis has gained growing importance for the determination 111

of food quality, with its ability to predict and, to a great extent, identify consumer 112

preferences for a specific food or beverage, depending on taste, flavour and texture. 113

Analytical and sensory data have to be compared and matched to understand which 114

analytical values correspond to the most appreciated beverage. This work sought to 115

discover the most effective espresso coffee machine settings for obtaining the best 116

quality espresso coffee. For this purpose, we studied different characteristics of espresso 117

coffee (pH, total solids, lipids and protein, plus presence and level of 12 different aroma 118

constituents), and their correlations with different settings of EC machines. Moreover, 119

analytical results with the outcome of a sensory analysis run by experienced panelists 120

have been compared. 121

More in detail, we used the HS-SPME-GC/MS technique to study the presence and the 122

quantitative trend of 12 volatiles, that is, 10 of the “key odorants,” so-called for their 123

6

importance in affecting coffee aroma (Maeztu, Anduez, Iban, & Pen, 2001), 6 of which 124

give a positive contribution, and 4 of which are defined as negative “key odorants” 125

(Maeztu, Sanz, Andueza, Paz, Bello, & Cid, 2001); the remaining 2 compounds do not 126

affect coffee aroma, but show the highest concentration among aroma constituents in 127

EC samples. 128

The typology of contribution and description of odour associated with each studied 129

volatile are reported in Table 1 (Maeztu, Anduez, Iban, & Pen, 2001; Maeztu, Sanz, 130

Andueza, Paz, Bello, & Cid, 2001; Semmelroch, & Grosch, 1995). 131

In addition, the effect of water temperature and water pressure on coffee extraction were 132

investigated, and different EC machine settings were compared for their effects on 133

quality of the ECs obtained, studying physical-chemical characteristics and volatiles, as 134

described above. These experiments were carried out using an Aurelia Competizione 135

(A) EC machine, which can be used at pre-fixed, settable, and constant temperature and 136

pressure (Fig. 1). 137

Furthermore, the above analysis was applied to time portions of espresso coffee, 138

prepared by taking a portion of EC produced in the first 10 seconds, and then portions 139

every 5 seconds, to study the kinetics of extraction during espresso making and its 140

dependence on pressure and temperature and on the coffee blends used. For this reason, 141

two different espresso machines working with different pressure and temperature curves 142

and two different blends, Arabica and Robusta, were used. The extraction kinetics of the 143

Aurelia Competizione (A) EC machine was compared with that of the Leva Victoria 144

Arduino (B). In this case, the settings of machine A were 92°C and 9 bar; espresso 145

machine B is unsettable. Additionally, Arabica and Robusta ECs prepared with the two 146

EC machines were compared from a chemical point of view (pH, total solids, lipids and 147

7

protein) for a further evaluation of differences between samples (Petracco, 2001; 148

Odello, & Odello, 2006). 149

Finally, sensory analysis sessions were organized to identify which analytical values, 150

and hence which EC prepared in which conditions, were most preferred by panelists. 151

2. Experimental 152

2.1. Coffee types and espresso machines 153

Two types of coffee, Arabica (pure Coffea Arabica from Colombia, 2% moisture 154

content) and Robusta (95:5 blend of Coffea canephora and Coffea Arabica, 2% 155

moisture content) and two espresso coffee machines, the Aurelia Competizione (A) and 156

the Leva Victoria Arduino (B), which work with specific and different curves of 157

pressure and temperature (Fig. 1), were provided by a local factory (Nuova Simonelli, 158

Belforte del Chienti, Italy). The difference in settings is related to the method of 159

construction of the two EC machines. The first one is equipped with a pump and a heat 160

exchanger, while in the second one pressure is provided by the elastic force of a spring 161

and water is heated by passing through a boiler. 162

2.2. EC sample preparation 163

Ground coffee was obtained using a coffee grinder set so that the product obtains a 164

volume of 25 ml in 25 seconds of extraction by the coffee machine. Roasted coffee 165

beans were milled just before each preparation, and 7.5 g of the finely ground coffee 166

was used for each cup. For the espresso machine “A”, operating conditions were set 167

combining three water pressures (7, 9, 11 bar) and three temperatures (88, 92, 98 °C), 168

obtaining an array of 9 conditions, whilst keeping the extraction time constant (25 169

seconds). After in-depth studies of array results on the quality of EC, 9 bar of pressure 170

and a temperature of 92 °C were chosen as settings for the EC machine A for the 171

analysis of time portions of the EC sample (0-10, 11-15, 16-20, 21-25, 26-30, 31-35, 172

8

36-40 sec.), in order to compare these results with those obtained in the same analysis 173

performed using espresso machine B. As mentioned above, the conditions of espresso 174

machine “B” (temperature and pressure) are unsettable, as it has specific and peculiar 175

curves of pressure and temperature, reported in Fig. 1. 176

2.3. Total solids, lipids, proteins and pH 177

Total solids and lipids were determined according to the procedure described by Maeztu 178

et al (Maeztu, Anduez, Iban, & Pen, 2001). Total solids were determined by oven 179

drying 5 ml of EC to a constant weight (24 h, 100°C). The total amount of lipids was 180

determined by liquid-liquid extraction using cyclohexane. Twenty milliliters of EC were 181

extracted with 20 ml of cyclohexane three times in a separating funnel. The organic 182

fraction was washed with distilled water (5 ml) three times. Total lipids were quantified 183

by weight after evaporation of the solvent. The pH of EC samples was measured using a 184

pH meter (Jenway 3510, Staffordshire, UK). Protein determination was carried out 185

using the Coomassie blue staining procedure as described by Fry (Fry, 1988), using 186

bovine serum albumin (Sigma, St. Louis, MO) as the protein standard. 187

2.4. Headspace solid-phase microextraction 188

The selection of fibre and the extraction conditions used, were selected according to the 189

method described by Rocha et al. (Rocha, Maeztu, Barros, Cid, & Coimbra, 2003) and 190

Maetzu et al. (Maeztu, Sanz, Andueza, Paz, Bello, & Cid, 2001). For each HS-SPME 191

analysis, 6.5 ml of coffee sample were placed in a vial (20 ml). The vial was tightly 192

capped with a PTFE-silicon septum and heated at 60 °C for 20 min on a heating 193

platform with agitation at 1300 rpm. This temperature (60°C) allowed better estimation 194

of the aroma profile as perceived by the human nose (Holscher, & Steinhart, 1992). 195

After this time, a PDMS fibre (Sigma Aldrich, Milan, Italy; 100 µm thickness) was 196

extended through the needle to place the stationary phase in contact with the headspace 197

9

of the sample. This fibre seems to be most suitable also in term of reproducibility of 198

results (Rocha, Maeztu, Barros, Cid, & Coimbra, 2003). After 30 min, the fibre was 199

removed from the vial and inserted into the injection port of the gas-chromatographic 200

system. A desorption time of 5 min was sufficient to desorb most of the analytes from 201

the fibre. The extracts, after desorption, were directly transferred to the analytical 202

column. Fibres were cleaned before each microextraction process to prevent 203

contamination. Cleaning was performed by inserting the fibre in the auxiliary injection 204

port at 230°C for 30 min. 205

2.5. GC-MS analysis 206

A gas chromatograph/mass selective detector (GC/MSD) (Agilent, Santa Clara, CA, 207

USA, Agilent 6890N with Agilent 5973N) was used. Separation was performed on a 208

DB - Wax column (60 m, 0.25 mm i.d., 0.25 µm film thickness) (J&W Scientific, 209

Folsom, CA, USA). An AgilentChem workstation was used with the GC/MS system. 210

The flow rate (He) was 1 ml min-1

under splitless mode. The injector temperature was 211

200°C. The column temperature program was: from 35°C (10 min) to 172°C at 5°C 212

min-1

, from 172°C to 200°C at 15°C min-1

, then 200°C for 5 min. Data were acquired in 213

the electron impact (EI) mode, using the single ion monitoring (SIM) mode. The SIM 214

ions and time conditions for each compound are reported in Table 1. A mixture of 215

aliphatic hydrocarbons (C8-C30) (Sigma, Milan, Italy) diluted in hexane was absorbed 216

onto the SPME fibre and injected under the above temperature program to calculate the 217

retention indices (as Kovats indices) of each extracted compound. The peak assignment 218

was based on computer matching with the NIST 08, taking into account the coherence 219

of the retention indices (Kovats indices) of the analyzed compounds with those reported 220

by the NIST 08 library,

in accordance with the standard of the International 221

Organization of the Flavor Industry (IOFI, http://www.iofi.org/) statement. The total 222

10

content of the volatile of each headspace analysis was defined by integrating the peak 223

areas of the 12 compounds monitored. The values were the mean of three replicates of 224

each sample. Data were analyzed by using MSD ChemStation software (Agilent, 225

Version G1701DA D.01.00). 226

2.6 Sensory profile: panel test 227

The aroma sensory profile was made using a selected panel of eight trained judges. 228

Judges were recruited among members of the IIAC (International Institute of Coffee 229

Tasters) and were trained over two 8-h sessions by the Centro Studi Assaggiatori 230

(Brescia, Italy) with a final examination. The first session offered a general background 231

about the origin of coffee, the washing and roasting process, the working of espresso 232

coffee machines and their effects on espresso taste. The second session started with 233

definition of descriptive terms about EC flavour. Then, a visual, olfactive and 234

gustative/tactile evaluation of ten EC were carried out on the basis of a hedonistic card 235

prepared by IIAC. Afterwards, sensory flavour profile evaluation of the EC samples 236

was carried out in three sessions. During each session, three ECs were analyzed. The 237

accuracy of results was assessed by the additional repetition of one of the three ECs. 238

Each EC was prepared immediately before taste and served in a white porcelain coffee 239

cup. Each judge was served one cup at time. The order of presentation was randomized 240

among judges and sessions. All evaluations were conducted in isolated sensory booths 241

illuminated with white light in the sensory lab under standardized conditions as 242

described by UNE 87-004-79 (Cipolla, 1999). Rinse drinking water was provided for 243

the judges between individual samples. 244

3. Results and discussion 245

3.1. Pressure and temperature curves from the two EC machines 246

11

Considering the pressure curve (Fig.1a), values increase up to a maximum of 9 bar for 247

EC machine A and 8 bar for EC machine B. After that, the pressure curve in machine A 248

is constant during the extraction time, since it is equipped with an electric pump, while 249

in machine “B” the pressure decreases until it reaches a final value of 2 bar. 250

Considering the temperature curve (Fig.1b), in the first seconds of extraction the values 251

increase up to a maximum of 93°C for machine A and 101°C for machine B. Due to the 252

differences in the method of construction, in machine A the temperature value remains 253

fairly constant, while in machine B the temperature drops about 10 degrees during 254

extraction (Fig.1). 255

3.2. Influence of water temperature and water pressure on aroma/flavour of ECs 256

Twelve volatile compounds were identified by HS-SPME analysis of the EC samples 257

(Table 1): 2 furans, 2 pyrazines, 6 aldehydes, 1 thiol and 1 phenolic derivative. Ten of 258

these are reported as key odorants for coffee aroma by a number of scientific papers 259

(methanethiol, acetaldehyde, propanal, 2-methylpropanal, 2-methylbutanal, 3-260

methylbutanal, hexanal, 2-ethyl-6-methylpyrazine, 2-ethyl-3,5-dimethylpirazine and 261

guaiacol), and can be divided into two classes, according to their type of contribution: 6 262

of them are considered positive odorants, the other 4, negative. To better understand the 263

trend of positive and negative odorants within all analyzed samples, the area of each 264

compound was normalized for each class of substance with respect to the lowest value 265

of the whole series. The aim of our research was to investigate the influence of EC 266

machine water temperature and pressure on the level and ratio of positive and negative 267

key odorants by using a semi-quantitative HS-SPME approach. Relative abundance was 268

calculated summing up the areas of the peaks related to all positive (or negative) 269

odorants analyzed in a specific condition of P and T, and normalizing the result of the 270

sum with respect to the lowest value (which, in Fig. 2, is the 11 bar and 88 °C 271

12

condition). As explained above, abundance shows a general indication of the positive 272

and negative odorants ratio among different conditions; the semi-quantitative approach 273

that has been used gives an indication of higher or lower level, without a real numerical 274

value of concentration ratio. The sum of positive and negative key odorants shows a 275

similar trend (Fig. 2). At 9 bar, the intensity of aroma is higher than at other pressure 276

conditions; regarding water temperature, the maximum of positive contributions and the 277

minimum of negative ones is evident at 92 °C. The global positive odorants (GOP) 278

trend, as given by panelists, was quite similar to that seen in Fig. 2, with an evident 279

parallelism between total positive odorants, as detected with analytical means, and 280

GOP. On the contrary, the trend of global negative odorants (GON), as perceived by the 281

panel test, is different and almost specular to GOP (Fig. 3) and analytical data. These 282

two coffee flavour parameters, reported in the hedonistic card prepared according to the 283

IIAC, indicate the olfactive intensity, respectively positive and negative, released from 284

the EC. They are indicators for comparing the sensory flavour profile to the analysis 285

performed in HS-SPME-GC/MS. GOP and GON were evaluated by panelists using a 286

scale between the the expressions “very bad” (0) and “very good” (10); the results are 287

the mean of panelist judgments. In conclusion, the trend of the GOP hedonistic index is 288

very close to that of the analytical data obtained for positive and negative key odorants, 289

while the GON hedonistic index is specular. From a sensorial point of view, we can 290

hypothesize that positive odorants cancel out negative ones; for this reason, panelists do 291

not taste the increase of negative ones observed in the chromatographic analysis. The 292

sensorial analyses were repeated on two days, and results proved to be mostly 293

superimposable, with only small differences. Finally, of the combinations of three water 294

temperatures and three pressures, the setting of 92°C and 9 bar, in EC machine A, 295

yielded the highest positive contribution and the lowest number of negative flavour 296

13

notes from a chemical point of view, and at the same time, the best overall performance 297

from the sensorial point of view, with the highest GOP and the lowest GON indices. 298

3.3. Comparison of key odorants profile between Arabica and Robusta coffee samples 299

As reported above, a water pressure of 9 bar and temperature of 92 °C were chosen for 300

analysis of time portion EC samples (0-10, 11-15, 16-20, 21-25, 26-30, 31-35, 36-40 301

sec.). The kinetics of coffee extraction during espresso making was studied with the 302

same EC machine (Aurelia Competizione A), using two different blends, Arabica and 303

Robusta. As indicator of EC quality, the ratio between positive and negative key 304

odorants in each time portion was considered (Fig. 4). With the Arabica blend, this ratio 305

increases from the first fraction (0-10 sec.) to the fourth one (21-25 sec.), with a 306

particularly high level in the third (16-20 sec.) and fourth one (21-25 sec.). On the other 307

hand, for the Robusta blend this ratio is particularly high for the first and the third 308

fraction. When the total espresso volume reaches 25 ml (summing the first 4 fractions), 309

which is the typical volume of an Italian certified EC, for both blends the aroma 310

intensity is particularly high and the level of positive key odorants is, on average, quite 311

high. 312

3.4. Comparison between “Aurelia” and “Leva” EC machines 313

For this purpose, we compared physical and chemical properties of time portions of EC 314

samples (0-10 sec., 11-15, 16-20, 21-25, 26-30, 31-35, 36-40) prepared with the Aurelia 315

machine, set at 9 bar and 92 °C, with those of the same kind of portions of EC prepared 316

with the Leva Machine, using the Arabica blend. As reported above, the two EC 317

machines show different pressure and temperature curves (Fig. 1). 318

More in detail, the ratio between positive and negative key odorants in each time 319

portion sample was reported (Fig. 5). With the Leva machine, the ratio is particularly 320

favourable for the first fraction (0-10 sec.) and it steeply decreases in the following 321

14

ones. In the last four fractions (21-25 sec., 26-30 sec., 31-35 sec., 35-40 sec.), the ratio 322

between positive and negative key odorants is quite constant. By using the Leva EC 323

machine, the ratio between positive and negative odorants is particularly high, at least 324

for the very first fractions. A possible explanation for this different behavior may have 325

to do with the fact that the increase in pressure in the first seconds starts later in the 326

Leva machine (Fig.1a), giving more time to much hotter water (Fig. 1b) in the coffee 327

filter to extract volatiles with higher efficiency, especially the positive ones. This seems 328

to be confirmed by the steep decrease in positive odorants in the intermediate and last 329

fractions of EC prepared with the Leva EC machine (Fig. 5). 330

On the contrary, with the Aurelia EC machine, this ratio increases from the first fraction 331

(0-10 sec.) to the fourth one (21-25 sec.) when the total espresso volume reaches 25 ml, 332

typical of an Italian certified EC. This confirms that in this volume of EC, the aroma 333

intensity is highest and positive key odorants are extracted with higher efficiency than 334

negative ones. 335

3.5. Evaluation of chemical parameters of Arabica and Robusta blends with EC 336

machines A and B 337

A total chemical evaluation for Arabica and Robusta blends in time portions of ECs is 338

shown in Table 2. The concentration of total solids decreases in time in each fraction in 339

both Arabica and Robusta time portions. Robusta shows higher values of total solids, 340

and the decreasing trend is similar to that of Arabica. The concentration of proteins 341

decreases in time with a logarithmic trend, with Robusta showing a higher concentration 342

of proteins than Arabica, with a steeper decrease. Concerning lipids, in the first fraction 343

(0-10 sec.), the concentration is higher in Arabica than in Robusta. The last time 344

portions show very low levels of lipid concentrations, often lower than the limit of 345

detection. Regarding pH, there is no evident change in time portions; the pH of Arabica 346

15

coffee is lower than that of Robusta coffee (average values of 5,45 vs 5,85, 347

respectively). 348

Chemical evaluation for the Arabica blend prepared with Aurelia Competizione A and 349

Leva B EC machines in time portions of ECs is shown in Table 2. 350

Total solids are not influenced by the type of espresso machine. With machine B, the 351

concentration of proteins extracted is higher, whereas protein concentration trends are 352

very similar for the two espresso coffee machines. Extracted lipids are also higher with 353

EC machine B, with an irregular trend among fractions. Moreover, even when the 354

coffee mixture is the same (Arabica blend), there are some differences in pH trend (data 355

not shown), with EC from the Aurelia Competizione machine showing a lower pH than 356

that obtained from the Leva EC machine. The pH of samples obtained with the latter 357

ranged between 5.456 and 5.606. 358

3.6. Method repeatability 359

The repeatability can be expressed by coefficient of variation (CV) % obtained 360

performing the HS-SPME-GC/MS analyses in triplicate. 361

The coefficient of variation obtained combining three water pressures (7, 9, 11 bar) and 362

three water temperatures (88, 92, 98 °C) to evaluate EC aroma/flavour ranged from 3 to 363

18.2 %, while that obtained comparing the key odorants profiles in Arabica and Robusta 364

using EC machine A at 92°C and 9 bar ranged from 2.9 to 17.9 % for Arabica and from 365

0.8 to 17.1 % for Robusta, and that obtained using EC machine B ranged from 3 to 19.2 366

%. 367

The inter-day repeatability of the HS-SPME-GC/MS method was determined by 10-day 368

replicate analyses of volatiles, evaluated on the same aliquot of EC samples stored in 369

16

the refrigerator, by machine “A”, set at the same conditions; CV % ranged from 1 % to 370

6.9, accounting for a very high constant results in the stability of EC. 371

4. Conclusions 372

In conclusion, the analysis of key odorants and the sensory profile as determined by a 373

professional panel test indicates that the usual temperature and pressure settings of the 374

Aurelia Competizione (A) espresso machine (92 °C and 9 bar) are close to the ones 375

needed to obtain the best quality espresso coffee, as indicated also by the Italian 376

tradition of espresso making. The aroma intensity, in terms of balance of positive and 377

negative key odorants in Arabica and Robusta, is of particular importance in the first 25 378

ml of espresso coffee extracted; this is confirmed comparing the coffee prepared with 379

the two EC machines utilized in our work. As expected, chemical differences in 380

espresso prepared using the two blends (Arabica and Robusta) are evident, but not in 381

espresso prepared using the two machines. 382

Acknowledgments 383

The authors are grateful to Ms. Flavia Gigli (Centro Strumentazioni Complesse, 384

University of Camerino, Italy) for help with GC-MS analysis, Nuova Simonelli 385

(Belforte del Chienti, Macerata, Italy) for providing coffee samples and espresso coffee 386

machines and partial economic support, Sheila Beatty for editing the English usage in 387

the manuscript. 388

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471

472

473

21

Figure captions 474

475

Figure 1. 476

Curves of pressure (a) and temperature (b) on the coffee filter exhibited by Aurelia 477

Competizione and Leva “Victoria Arduino” EC machines. 478

479

Figure 2. 480

Influence of water temperature and water pressure on the profile of total positive and 481

negative key odorants from GC-MS analysis of Arabica samples. 482

483

Figure 3. 484

Hedonistic index of GOP and GON obtained from judges in the panel test. 485

486

Figure 4. 487

Ratio between positive and negative key odorants in each time portion sample for 488

Robusta and Arabica blends, using the Aurelia Competizione “A” EC machine. 489

490

491

492

493

Table 1.

Conditions used in GC/MS experiments. Retention index, description of odour, typology of

contribution, time windows and monitored ions are reported.

Compounds RIa Description of

odour

Typology of

contribution

Time

windows

(min)

SIM Ion

(m/z)

Methanethiol 636 freshness positive 0 - 5.13 47, 48

Acetaldehyde 649 fruity positive 5.13 - 6.20 29, 43

Propanal 732 fruity positive 6.20 - 6.60 28, 58

2-Methylpropanal 759 fermented negative 6.60 - 8.00 41,72

2-Methylbutanal 914 fermented negative

8.00 - 12.00 29, 41, 43

3-Methylbutanal 919 fruity positive

Hexanal 1083 fruity positive 12.00 - 21.00 41, 56

2-Ethyl-6-methyl

pyrazine 1393 earth, mould negative 21.00 - 30.80 121,122

2-Ethyl-3,5-dimethyl

pyrazine 1472 paper, burned negative 30.80 - 32.50 135, 136

2-Furanmethanol,

acetate 1538 no no

32.50 - 39.50 41, 81, 98

2-Furanmethanol 1662 no no

Guaiacol 1873 phenolic, spicy positive 39.50 - 44.27 109, 124

aRetention index on DB-Wax column, experimentally determined using homologous series of

C8-C30 alkanes.

Table 2. Chemical evaluation with the “Aurelia Competizione” (“A”) and “Leva” (“B”) EC machines (n=3, CV%≤11 ).

Time

portions

(sec.)

Total solids in

Arabica (A)

mg/ml

Total solids in

Robusta (A)

mg/ml

Proteins in

Arabica (A)

mg/ml

Proteins in

Robusta (A)

mg/ml

Lipids in

Arabica (A)

mg/ml

Lipids in

Robusta (A)

mg/ml

Total solids in

Arabica (B)

mg/ml

Proteins in

Arabica (B)

mg/ml

Lipids in

Arabica (B)

mg/ml

0-10 241 326 3.160 7.980 1.910 1.115 220 4.937 6.090

11-15 109 195 1.609 5.220 0.230 0.955 106 2.378 0.460

16-20 61 105 1.120 2.620 0.175 0.150 55 2.050 1.010

21-25 39 60 0.760 3.360 0.130 0.000 36 1. 960 2.785

26-30 26 44 0.530 2.175 0.029 0.000 21 1.740 0.620

31-35 18 34 0.490 1.860 0.000 0.000 20 1.108 3.835

36-40 14 26 0.380 2.260 0.000 0.000 19 1.209 2.615

0

2

4

6

8

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Aurelia EC

machine

Leva EC

machine

Pre

ssu

re (

ba

r)

a)

Time in seconds

80

82

84

86

88

90

92

94

96

98

100

Aurelia EC machine

Leva EC machine

Tem

per

atu

re

C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

b)

Time in seconds

Fig. 1.

Curves of pressure (a) and temperature (b) on the coffee filter exhibited by Aurelia

Competizione and Leva “Victoria Arduino” EC machines.

Fig. 2.

Influence of water temperature and water pressure on the profile of total positive and

negative key odorants from GC-MS analysis of Arabica samples.

0,000

0,500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

positive contribution

negative contribution

7 bar 88 ° C

7 bar 92 ° C

7 bar 98 ° C

9 bar 88 ° C

9 bar 92 ° C

9 bar 98 ° C

11bar 88 ° C

11 bar 92 ° C

11bar 98 ° C

Ab

un

dan

ce

Fig. 3.

Hedonistic index of GOP and GON obtained from judges in the panel

test.

0

1

2

3

4

5

6

7

8

GOP

GON

7 bar 88 ° C

7 bar 92 ° C

7 bar 98 ° C

9 bar 88 ° C

9 bar 92 ° C

9 bar 98 ° C

11bar 88 ° C

11 bar 92 ° C

11bar 98 ° C

Ab

un

dan

ce

0,500

0,700

0,900

1,100

1,300

1,500

1,700

Robusta

Arabica

0-10s 10-15s 15-20s 20-25s 25-30s 30-35s 35-40s

Ab

un

dan

ce

Fig. 4.

Ratio between positive and negative key odorants in each time portion

sample for Robusta and Arabica blends, using the Aurelia Competizione

“A” EC machine.

0

2

4

6

8

10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Aurelia EC machine

Leva EC machine

Pres

sure

(bar

)

a)

Time in seconds

80

82

84

86

88

90

92

94

96

98

100

Aurelia EC machine

Leva EC machine

Tem

pera

ture

°C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

b)

Time in seconds

Fig. 1.

Curves of pressure (a) and temperature (b) on the coffee filter exhibited by

Aurelia Competizione and Leva “Victoria Arduino” EC machines.

Fig. 2.

Influence of water temperature and water pressure on the profile of total

positive and negative key odorants from GC-MS analysis of Arabica samples.

0,000

0,500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

positive contribution

negative contribution

7 bar 88 °C

7 bar 92 ° C

7 bar 98 ° C

9 bar 88 °C

9 bar 92 °C

9 bar 98 °C

11bar88 °C

11 bar 92 °C

11bar 98 °C

Abu

ndance

Fig. 3.

Hedonistic index of GOP and GON obtained from judges in

the panel test.

0

1

2

3

4

5

6

7

8

GOP

GON

7 bar 88 °C

7 bar 92 °C

7 bar 98 °C

9 bar 88 °C

9 bar 92 °C

9 bar 98 °C

11bar 88 °C

11 bar 92 °C

11bar 98 °C

Abu

ndance

0,500

0,700

0,900

1,100

1,300

1,500

1,700

Robusta

Arabica

0-10s 10-15s 15-20s 20-25s 25-30s 30-35s 35-40s

Abu

ndan

ce

Fig. 4.

Ratio between positive and negative key odorants in each time

portion sample for Robusta and Arabica blends, using the

Aurelia Competizione “A” EC machine.

Analysis of espresso coffee aroma constituents by HS-SPME-GC/MS

Physico-chemical properties of espresso coffee at different P and T

Correlations of EC characteristics with different settings of EC machines and sensory

analysis