Influence of Water Quench Cooling on Degassing and Aroma Stability of Roasted Coffee

Abstract

Coffee roasting experiments with air cooling versus water quench cooling were carried out on laboratory scale with a fluidized-bed hot air roasting system (200 g batch size) and on production scale with a rotating bowl roaster (320 kg batch size). Two series of coffees with different water contents resulted, which were stored at 25 degrees C under normal atmospheric conditions. Carbon dioxide desorption was followed and stability of selected aroma compounds was tested with headspace solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) and stable isotope labeled compounds as internal standards. Degassing is faster in water-quenched coffees with higher moisture content, but pore size distribution in the different coffee samples did not correlate with degassing behavior. Bean firmness, which increases with increasing moisture content, might have an influence on degassing. Air- and water-quenched coffees exhibit similar stability of most aroma compounds despite different degassing behavior. However, evolution of dimethyl trisulfide was different in coffees with increased water content. This suggests higher thiol oxidation rates, a factor that is cited to be related to a faster loss of freshness attributes.

Full text

Influence of Water Quench Cooling on Degassing and Aroma
Stability of Roasted Coffee
JUERG BAGGENSTOSS,†LUIGI POISSON,‡REGINA LUETHI,†RAINER PERREN,†AND
FELIX ESCHER*,†
Institute of Food Science and Nutrition, Swiss Federal Institute of Technology (ETH),
Zurich, Switzerland and Nestle´ Product Technology Centre, Orbe, Switzerland
Coffee roasting experiments with air cooling versus water quench cooling were carried out on
laboratory scale with a fluidized-bed hot air roasting system (200 g batch size) and on production
scale with a rotating bowl roaster (320 kg batch size). Two series of coffees with different water
contents resulted, which were stored at 25 °C under normal atmospheric conditions. Carbon dioxide
desorption was followed and stability of selected aroma compounds was tested with headspace solid-
phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) and stable isotope
labeled compounds as internal standards. Degassing is faster in water-quenched coffees with higher
moisture content, but pore size distribution in the different coffee samples did not correlate with
degassing behavior. Bean firmness, which increases with increasing moisture content, might have
an influence on degassing. Air- and water-quenched coffees exhibit similar stability of most aroma
compounds despite different degassing behavior. However, evolution of dimethyl trisulfide was different
in coffees with increased water content. This suggests higher thiol oxidation rates, a factor that is
cited to be related to a faster loss of freshness attributes.
KEYWORDS: Coffee; roasting; water quenching; solid-phase microextraction; flavor stability; carbon
dioxide; coffee structure
INTRODUCTION
Once the desired degree of roast is achieved, fast quenching
of coffee beans is necessary in order to avoid overroasting and
to stop exothermic reactions within the beans. In industrial coffee
roasting, air and water quench cooling are applied. Air cooling
implies the use of large quantities of cold air for several minutes
(1) and is relatively slow. Therefore, exothermic reactions within
coffee beans may continue during the first 15 s of the cooling
process (2). Water quenching cools down coffee faster, and
temperature drops from 230 to 100 °C in less than1sare
reported (2). When bean temperature falls below 100 °C,
exothermic moisture condensation occurs and coffee beans can
take up moisture. Eggers (2) distinguishes three types of water
quenching. During spray quenching, coffee beans are cooled
rapidly by the evaporation enthalpy of water droplets on the
bean surface. In immersion quenching, coffee beans are im-
mersed in water and cooled by bulk boiling. In film quenching,
water is poured over coffee beans. Spray quenching is judged
as the most efficient method due to high evaporation rates and
intensive recurring contact of water with surface of coffee beans.
Generally cold water quenching is more efficient than hot water
quenching, which is slower but results in a better water uptake
into coffee beans (2).
Water quenching is generally associated with loss of coffee
quality, although explicit experimental data are lacking. Illy and
Viani (1) mention possible oxidation reactions on the surface
of coffee beans as well as the opening of pores, which allows
stripping of volatile substances off the bean. These authors relate
water quench cooling to cell wall cracking and more pronounced
structure collapse leading to faster degassing and aroma loss.
However, Geiger et al. (3) showed that carbon dioxide loss rates
during roasting greatly exceed degassing rates during storage.
Furthermore, near the end of roasting, volatile organic com-
pounds and many individual aroma compounds are emitted at
considerably higher rates than after quenching (4,5).
In studies on degassing of roasted coffee, Radtke (6) found
that between 40% and 50% of the entrapped carbon dioxide is
released during fine grinding, whereas loss of carbon dioxide
during coarse grinding is low. Radtke concluded that the main
part of carbon dioxide is entrapped in the fine pores of the coffee
bean tissue and not in large cavities from where entrapped CO2
is probably lost already during the roasting process through
relatively large fissures. When internal carbon dioxide pressure
is calculated on the basis of the amount of entrapped CO2in
coffee beans, values up to 8 bar are found (4,6). Shimoni and
Labuza (7) suggested that the major part of carbon dioxide in
coffee powder is in a sorption/desorption equilibrium and only
a minor part is entrapped in collapsed structure. However,
neither the mechanisms of CO2entrapping and sorption nor the
* Corresponding author: fax +41 44 632 11 23; e-mail felix.escher@
ilw.agrl.ethz.ch.
†Swiss Federal Institute of Technology.
‡Nestle´ Product Technology Centre, Orbe, Switzerland.
10.1021/jf070338d CCC: $37.00 © xxxx American Chemical Society
PAGE EST: 6.6Published on Web 07/06/2007

driving forces and mechanisms of degassing within coffee beans
are fully understood.
Spadone and Liardon (8) studied staling of coffee aroma
during storage of roasted coffee with high and low water content
under high and low oxygen storage conditions. Concentrations
of hexanal, several branched aldehydes, ketones, and alkylfurans
in coffee cooled by air and by water quenching changed in a
similar way during storage. As the two cooling methods resulted
in different moisture content, the authors concluded that lipid
oxidation and at least some chemical reactions involved in coffee
aging were independent of water content in roasted coffee.
Clinton (9) examined consumer and expert evaluations of stored
roast and ground coffee and found that higher water content
and higher oxygen conditions in packaging lead to faster product
deterioration. Hinman (10) showed that the reaction of roast
and ground coffee with oxygen can be considerably accelerated
by effect of temperature, moisture, and coffee density.
Nicoli et al. (11) examined the correlation between volatiles
and carbon dioxide release in roasted coffee beans and ground
coffee at different temperatures. They concluded that, during
storage, evolution of carbon dioxide is always related to an equal
behavior of volatile compounds. However, the authors measured
headspace concentrations and results were given as total peak
area without the use of internal standards. Therefore, volatile
substances that were bound to the coffee matrix were not taken
into account.
The aim of this study was to determine whether water quench
cooling with and without increase of water content implies
significant effects on aroma stability, degassing, oxidative
stability, and roast coffee structure. In addition, the transferability
of the results from laboratory roasting trials to industrial roasting
processes was to be verified.
MATERIALS AND METHODS
A. Roasting Process and Process Characterization: 1. Raw
Material. Wet-processed Coffea arabica Linn. variety from Colombia
was obtained from a Swiss roasting company for laboratory trials. For
industrial trials, a commercial 100% Arabica blend of the same company
was roasted.
2. Color Measurement. Roast degree was determined from the
lightness value (L*) of the L*a*b* color space. Coffee was ground
and gently pressed to form an even surface, and color was measured
with a colorimeter CR-310 (Minolta, Japan).
3. Moisture Content. Roasted coffee was ground in a disk grinder
(Buehler-Miag 4000, Bu¨hler Ltd., Milano, Italy), and weight loss of 5
g of ground coffee at 103 °C during 5 h was determined gravimetrically.
4. Laboratory Roasting Trials. Batches (200 g) of green coffee beans
were roasted with a fluidized-bed hot-air laboratory roaster (G. W. Barth
AG, Freiberg/Neckar, Germany) by a low-temperature long-time process
[LTLT, 228 °C, 12 min (3)]. The roaster has been described in detail
by Schenker (12) and Geiger et al. (3). Bean color of roasted coffee
was L*)21-22 (passive cooling) and L*)22-23 (all others). Four
different cooling methods were applied. Air-cooled coffee was cooled
with an ambient air stream of 1.4 m3min-1during 4 min as described
by Schenker (12), resulting in a water content of 1.9 g/100 g on a wet
basis (wb). In spray cooling, 8.7 g of water was sprayed through a
hollow cone nozzle into the cooling chamber during 20 s. This cooling
method was slightly faster than air cooling, but final water content
was nearly as low as in air-cooled coffee (2.1 g/100 g wb). The second
water cooling method (film cooling) consisted of pouring 35 g of water
directly on the coffee during the first 12 s of the cooling process.
Significant higher end water content of coffee resulted from this method
(4.2 g/100 g wb). In addition to the air and the two water quenching
techniques, slow cooling was applied, whereas coffee beans were cooled
for 45 min in a wide steel container at ambient temperature without
air stream. Due to the slow decrease in temperature, the degree of roast
of slowly cooled coffee was somewhat higher than in the other coffees,
whereas water content was slightly lower (1.7 g/100 g wb).
5. Industrial Roasting Trials. Coffee was roasted on a RZ 3500Y
rotating bowl roaster (Probat, Emmerich, Germany) with batch size of
320 kg. Inlet air temperature was 307 (2°C, and coffee was roasted
to a final bulk temperature of 222 (1°C, resulting in roasting times
of about 275 s and a bean color of L*)29-30. Due to safety
considerations water cooling had to be applied on all batches, but the
amount of water was varied (38, 35, 25, 15, or 5 L), resulting in end
water contents of 5.3, 4.8, 3.8, 3.2, and 2.4 g/100 g on a wet basis.
6. Firmness. Firmness of coffee beans was determined by a shearing
test in a Kramer cell using a force deformation testing equipment (Z010/
TH2S, Zwick, Ulm, Germany). A single layer of roasted coffee beans
(n)50-60) was placed in the cell and the maximum force was
measured at a deformation rate of 100 mm/min. After roasting and
quenching, coffee beans were stored at room temperature for 24 h,
and then at -80 °C until firmness measurements took place.
7. Gas Desorption Measurement. Batches (80 g) of coffee beans or
finely ground coffee powder (Ditting KFA 1403 disk mill, level 2;
Ditting, Bachenbu¨lach, Switzerland) were placed in 500 mL septum
flasks immediately after roasting and quenching. Headspace pressure
was measured periodically. The flasks were vented after each measure-
ment. Results were adjusted to 100 g dry mass.
8. Mercury Intrusion Porosimetry. Porosimetry was carried out using
mercury porosimeters Pascal 140 and 440 (Thermo Electron Corp.,
Waltham, MA) as described by Schenker et al. (13).
B. Aroma Analysis: 1. Chemicals. Isotopically labeled standards
were obtained from Dr. Ehrenstorfer GmbH, Augsburg, Germany ([2H6]-
dimethyl sulfide, [2H5]pyridine, and [2H2]-3-methylbutanal), Witega
Laboratorien, Berlin, Germany ([2H3]-4-vinylguaiacol), and Toronto
Research Chemicals, North York, Canada ([2H6]-2-ethyl-3,5-dimeth-
ylpyrazine). The following substances were synthesized at Nestle´
Research Center (Lausanne, Switzerland): [2H2]hexanal (14), [2H6]-
dimethyl trisulfide (15), [13C4]-2,3-butanedione, and [13C2]-2,3-pen-
tanedione (16).
2. SPME-GC-MS Analysis and Quantification of Coffee Aroma
Compounds. Samples of coffees roasted at laboratory scale were taken
directly after roasting and then after 1, 8, 15, 23, 35, 56, and 133 days
of storage. Coffee samples from industrial roasting trials were taken
after roasting and quenching (12 h equilibration time) and then after 8,
15, 23, 35, 56, and 126 days of storage. Coffee beans were stored in
open containers in the dark at 25 °C.
Three aldehydes (2-methylbutanal, 3-methylbutanal, and hexanal),
two ketones (2,3-butanedione and 2,3-pentanedione), two sulfides
(dimethyl sulfide and dimethyl trisulfide), one pyridine (pyridine), six
alkylpyrazines (2-ethyl-3-methylpyrazine, 2-ethyl-5-methylpyrazine,
2-ethyl-6-methylpyrazine, 2,3,5-trimethylpyrazine, 2-ethyl-3,5-dimeth-
ylpyrazine, and 2-ethyl-3,6-dimethylpyrazine), and one phenolic com-
pound (4-vinylguaiacol) were analyzed (Table 1).
Ground coffee (5 g for the first group of compounds, 3,6-11, and
14; 1 g for the second group of compounds, 1,2,4,5,12,13, and 15)
Table 1.
Analytes and Standards Used in GC-MS Analyses
analyte (A) selected ion
(
m/z
)ofA internal
standard (IS) selected ion
(
m/z
)ofIS
2-methylbutanal (1)86[
2
H
2
]-288
3-methylbutanal (2)71[
2
H
2
]-273
hexanal (3)56[
2
H
2
]-358
2,3-butanedione (4)43[
13
C
4
]-445
2,3-pentanedione (5) 100 [
13
C
2
]-5102
2-ethyl-3-methylpyrazine (6) 121 [
2
H
6
]-10 141
2-ethyl-5-methylpyrazine (7) 121 [
2
H
6
]-10 141
2-ethyl-6-methylpyrazine (8) 121 [
2
H
6
]-10 141
2,3,5-trimethylpyrazine (9) 122 [
2
H
6
]-10 141
2-ethyl-3,5-dimethylpyrazine (10) 135 [
2
H
6
]-10 141
2-ethyl-3,6-dimethylpyrazine (11) 135 [
2
H
6
]-10 141
pyridine (12)79[
2
H
5
]-12 84
4-vinylguaiacol (13) 150 [
2
H
3
]-13 153
dimethyl sulfide (14)62[
2
H
6
]-14 68
dimethyl trisulfide (15) 126 [
2
H
6
]-15 132
BBaggenstoss et al.

was weighed in a 100 mL flask and extracted with 100 mL boiling
water for 10 min under constant stirring. During extraction, the flasks
were kept closed to avoid evaporation and loss of volatile compounds.
After cooling, the coffee solution was spiked with definite amounts of
the isotopically labeled internal standards [2H2]-3,[
2H6]-10, and [2H6]-
14 (for the first group) and [2H2]-2,[
13C4]-4, [13C2]-5,[
2H5]-12,[
2H3]-
13, and [2H6]-15 (for the second group). The coffee solution was
subsequently stirred for 10 min, and 7 mL was transferred to a 20 mL
headspace vial.
Coffee aroma compounds were sampled with solid-phase microex-
traction (SPME) at 40 °C for 10 min by use of a Supelco 50/30 µm
StableFlex divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/
PDMS) fiber (Supelco, Buchs, Switzerland). Injection was done at 240
°C in the splitless mode with a splitless time of 240 s. Separation was
carried out on a 60 m ×0.25 mm ×0.25 µm medium polar ZB-1701
column (Phenomenex, Aschaffenburg, Germany) on a Fisons 8000Se-
ries gas chromatograph (GC) (Thermo Electron, Allschwil, Switzerland)
with the following temperature programs: 40 °C (6 min), 4 °C/min,
135 °C (0 min), 40 °C/min, 240 °C (5 min) for compounds 3,6-11,
and 14;40°C (4 min), 4 °C/min, 140 °C (0 min), 40 °C/min, 240 °C
(5 min) for compounds 1,2,4,5,12,13, and 15. Helium 5.6 was used
as carrier gas at a constant column head pressure of 135 kPa. Detection
of aroma compounds was done on a quadrupole mass spectrometer
(MS) SSQ710 (Finnigan MAT, San Jose, CA) with single-ion monitor-
ing (SIM) in the EI mode with an ionization potential of 70 eV. All
SPME-GC-MS measurements were run in triplicate.
RESULTS AND DISCUSSION
Gas Desorption and Physical Structure. Initial gas desorp-
tion of whole coffee beans is markedly higher in coffees with
higher water content, as shown for the coffees obtained from
the laboratory roasting trials (Figure 1). However, after 1 week
of storage, no marked difference in degassing rate is found
anymore. In addition, Figure 1 shows that coffees with same
roast degree and comparable water content exhibit similar
degassing rates throughout storage, unaffected by cooling
methods. To evaluate the influence of water content on
degassing and at the same time exclude any influence of cooling
method, one batch of air-cooled coffee was divided in two parts.
One part was remoistened to a water content of 5 g/100 g
immediately after cooling. The second part was left untreated.
Degassing of the remoistened coffee is very similar to degassing
Figure 1.
Degassing of whole coffee beans: passively cooled (
9
, 1.7 g
of H
2
O/100 g wb), air-cooled (
b
,1.9gofH
2
O/100 g wb), spray-cooled
(
2
,2.1gofH
2
O/100 g wb), and film-cooled coffee (
1
,4.2gofH
2
O/100
g wb) [bar/100 g dm].
Figure 2.
Degassing of air-cooled coffee (
9
) and remoistened air-cooled
coffee (
b
) [bar/100 g dm].
Figure 3.
Degassing of ground coffee: passively cooled (
9
,1.7gof
H
2
O/100 g wb), air-cooled (
b
,1.9gofH
2
O/100 g wb), spray-cooled (
2
,
2.1gofH
2
O/100 g wb), and film-cooled (
1
,4.2gofH
2
O/100 g wb)
[bar/100 g dm].
Figure 4.
Cumulated intruded mercury volume in coffees from laboratory
roasting trials.
Figure 5.
Firmness of coffee beans roasted in laboratory and industrial
trials.
Water Quench Cooling of Roasted Coffee C

of film-cooled coffee (Figure 2), which gives rise to the
assumption that fast degassing of coffees with high water content
is due to remaining moisture only and is unaffected by the
specific cooling method.
In a further experiment, coffee was ground immediately after
roasting, and degassing of coffee powder was measured.
Degassing was noticeably decreased in ground coffee with
higher water content (Figure 3). Apparently, CO2loss during
grinding is higher in coffee beans with increased water content,
and less CO2is available for degassing during storage.
It is known from research on carbon dioxide diffusion kinetics
of roast and ground coffee (7,17) that diffusion is very complex
and likely to be a combination of various mechanisms including
Knudsen and transition-region diffusion, pressure-driven viscous
flow, surface diffusion, and interactions between carbon dioxide
molecules and coffee matrix. Results of these studies suggest
indeed that at least two mechanisms control the degassing
process. In the early stage of degassing, carbon dioxide
desorption is fast and large differences in degassing behavior
can be seen between coffees with different water contents.
For water-quenched coffee beans, the assumption was made
that fast degassing is linked to opening of pores during the
cooling step (1). Schenker et al. (13) showed that mercury
intrusion porosimetry is a suitable method to determine the
internal pore structure of roasted coffee beans and found
significant differences in average pore size between fast- and
slow-roasted coffees. The assumption that water-quenched
coffees have higher porosity than air-quenched coffees was not
corroborated by porosimetry measurements of the laboratory
roasting trials, where no correlation between internal pore
structure and degassing behavior was found (Figure 4). It must
be noted, though, that mercury intrusion porosimetry gives
insight into the intracellular pore system but does not provide
information about surface porosity (12). The origin and structure
of the micropore system within coffee cell walls still remains
unclear, and the question of how gas and oil are transferred
from the bean core to the outside is not yet answered
satisfactorily.
Even if carbon dioxide was transported through the intra-
cellular micropore system, transfer through the outer cell barrier
Figure 6.
Alteration of selected coffee aroma compounds in passively cooled coffee (
9
,1.7gofH
2
O/100 g wb), air-cooled coffee (
b
,1.9gofH
2
O/100
g wb), spray-cooled coffee (
2
,2.1gofH
2
O/100 g wb), and film-cooled coffee (
1
,4.2gofH
2
O/100 g wb).
DBaggenstoss et al.

(epidermis) might be limiting. Therefore it is hypothesized that
in a first stage of gas desorption, when carbon dioxide pressure
is equal throughout the coffee bean, transport through epidermis
is limiting. In a second stage, when CO2pressure becomes lower
and a pressure gradient from the bean core to the outer cells is
further built up, carbon dioxide transfer from the bean core to
the outer cell barrier will be an additional limiting factor. As
pore size distribution is similar in all investigated coffees, no
major differences are expected in degassing during this second
stage. Differences in gas desorption between coffees with
different water contents are found particularly in the first stage;
therefore the assumption is made that due to higher water content
the outer cell walls are more permeable to carbon dioxide.
Higher permeability in the outer cell wall may also be caused
by higher solubility of carbon dioxide in higher water contents.
The fact that tissue and cell wall structure of coffee beans is
influenced by water content, which in turn influences degassing
behavior, is also evident from firmness measurements (Figure
5). An almost linear relationship between water content and
maximum force upon shearing coffee beans in the Kramer cell
was found. Light-roasted coffee beans are less brittle than dark-
roasted beans, and the increase of maximum force with water
content is less pronounced in dark-roasted coffee. The detailed
relationship between bean firmness, porosity, and degassing
behavior would have to be explored further.
Aroma Stability. Odorants for the assessment of coffee shelf
life were chosen on the basis of studies that linked analytical
to sensory data (18,19). In addition to impact compounds
identified in the before-mentioned studies, dimethyl sulfide was
used as an additional freshness marker, hexanal was chosen as
a secondary product of lipid oxidation, and pyridine was selected
as a relatively stable marker substance.
Storage trials with roasted coffee beans obtained from
laboratory trials revealed substantial changes in aroma profile
(Figure 6 and Table 2). After 133 days of storage, concentra-
tions of volatile substances like 2-methylbutanal, 3-methylbu-
tanal, 2,3-butanedione, and 2,3-pentanedione decreased to 40-
50% of the initial value. Reduction of dimethyl sulfide was
especially distinct: after 133 days of storage, about 10% of the
initial concentration was left. 4-Vinylguaiacol, pyridine, and
pyrazines were less affected by long-term storage; reduction of
these compounds was in the region of 25%.
Table 2 shows retention of aroma compounds after 56 days
of storage, while Figure 6 shows aroma alteration during storage
of selected compounds. Despite different degassing behavior
of the examined coffees, no substantial differences in loss of
2-methylbutanal, 3-methylbutanal, 2,3-butanedione, and 2,3-
pentanedione was observed. Among the highly volatile com-
pounds, dimethyl sulfide probably exhibited a somewhat faster
reduction rate in film-cooled coffee. Evolution of 4-vinylguai-
acol, pyridine, and all examined pyrazines was comparable in
all coffees. Since degassing in film-cooled coffee was markedly
higher than in the other coffees, these findings suggest that loss
of aroma compounds is not directly linked to degassing behavior.
Therefore, in whole roasted coffee beans, aroma stripping due
to degassing is a negligible effect compared to chemical
degradation, which is primarily affected by temperature and
ambient oxygen content. Hexanal contents showed that during
the first weeks of storage no difference in lipid oxidation was
induced by different cooling methods or water content. After
133 days, however, coffee beans that were cooled slowly
exhibited noticeably higher amounts of hexanal. As hexanal is
a suitable marker substance for lipid oxidation reactions, this
is evidence of faster lipid oxidation in slowly cooled coffee.
However, it has to be noted that lipid oxidation should not be
a key problem in coffee storage, as after 133 days of storage,
roasted coffee beans already underwent major aroma alterations.
Arackal and Lehmann (20) indicate that after 6-8 weeks of
open storage of roasted coffee beans, significant aroma alteration
and staleness is perceived by consumers.
Large differences in alteration of dimethyl trisulfide were
found in coffees cooled with different methods (Figure 6).
Directly after roasting, around 100 µg of dimethyl trisulfide/kg
dry mass was present. During the first 21 days of storage,
dimethyl trisulfide content increased up to 430% (film cooling),
370% (passive cooling), 290% (air cooling), and 280% (spray
cooling) of the initial value. The observed buildup of dimethyl
trisulfide was most likely due to oxidation of methanethiol to
dimethyl disulfide and dimethyl trisulfide (21,22). Water-
quenched coffee with increased moisture content as well as
slowly cooled coffee possibly underwent faster degradation of
methanethiol. As loss of methanethiol may be linked to loss of
the characteristic freshness of coffee flavor (23), water-quenched
coffee with increased moisture content probably exhibits an
accelerated loss of freshness attributes.
Table 2.
Percent Retention of Aroma Compounds after 56 Days of
Open Storage (Laboratory Trials)
quenching method
a
passive
cooling,
1.7gof
H
2
O
air
cooling,
1.9gof
H
2
O
spray
cooling,
2.1gof
H
2
O
film
cooling,
4.2gof
H
2
O
dimethyl sulfide 32 33 34 22
dimethyl trisulfide 272 204 216 347
hexanal 74 57 56 79
2-methylbutanal 63 67 61 70
3-methylbutanal 57 65 64 68
4-vinylguaiacol 93 91 85 76
pyridine 84 82 88 88
2,3-butanedione 51 56 67 64
2,3-pentanedione 55 62 62 65
2-ethyl-5-methylpyrazine 85 80 81 76
2-ethyl-6-methylpyrazine 80 76 80 75
2-ethyl-3-methylpyrazine 78 78 77 73
2,3,5-trimethylpyrazine 79 77 89 76
2-ethyl-3,5-dimethylpyrazine 82 75 84 73
2-ethyl-3,6-dimethylpyrazine 77 74 73 70
a
Water content is expressed as grams of H
2
O per 100 g wet basis for each
method.
Table 3.
Percent Retention of Aroma Compounds after 56 Days of
Open Storage (Industrial Trials)
2.37 g of
H
2
O
a
3.17 g of
H
2
O
a
3.83 g of
H
2
O
a
4.82 g of
H
2
O
a
5.26 g of
H
2
O
a
dimethyl sulfide 35 31 39 28 37
dimethyl trisulfide 339 447 582 608 511
hexanal 97 104 141 131 132
2-methylbutanal 60 55 66 55 66
3-methylbutanal 54 55 61 57 61
4-vinylguaiacol 96 96 103 96 94
pyridine 106 88 84 102 107
2,3-butanedione 61 67 62 71 71
2,3-pentanedione 55 55 57 60 64
2-ethyl-5-methylpyrazine 70 66 67 70 75
2-ethyl-6-methylpyrazine 73 69 72 76 79
2-ethyl-3-methylpyrazine 76 75 80 82 80
2,3,5-trimethylpyrazine 82 80 81 80 97
2-ethyl-3,5-dimethylpyrazine 94 81 94 93 99
2-ethyl-3,6-dimethylpyrazine 108 95 107 93 107
a
Water content is expressed as grams of H
2
O per 100 g wet basis.
Water Quench Cooling of Roasted Coffee E

The series of industrial roasting trials showed very similar
behavior to the laboratory trials. Due to the lighter roast degree
and due to differing raw material, some initial contents of aroma
compound were different. Industrial roastings exhibited mark-
edly higher contents of 2-methylbutanal, 3-methylbutanal, 2,3-
butanedione, and 2,3-pentanedione, whereas concentrations of
pyridine, dimethyl sulfide, dimethyl trisulfide, and 2-ethyl-3,5-
dimethylpyrazine were lower. The other compounds were at
roughly equal concentrations in both roasting series. Similarly
to the laboratory roasting trials, no basic differences in aroma
alteration upon storage of the coffees with differing water
content were observed (Figure 7 and Table 3), although
degassing was noticeably higher in coffees with higher
water content (results not shown). Namely, dimethyl sulfide,
2-methylbutanal, 3-methylbutanal, 2,3-butanedione, 2,3-pen-
tanedione, 4-vinylguaiacol, pyridine, and all examined pyrazines
exhibited very similar alterations throughout storage in all
coffees. Again, the major variation between these coffees was
found in the storage alteration of dimethyl trisulfide, where
coffee beans with higher water content showed a higher increase
during storage. As mentioned before, this could be a sign of
faster oxidation of methanethiol and hence an indication of faster
decay of freshness attributes.
ACKNOWLEDGMENT
We thank Nestle´ PTC Orbe for providing labeled standards and
advice about the analytical methodology and Migros Betriebe
Birsfelden (MBB) for providing green coffee and for offering
access to industrial roasting equipment.
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Received for review February 6, 2007. Revised manuscript received
May 21, 2007. Accepted May 27, 2007. We thank G. W. Barth AG for
financial support.
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