Comparison of the Key Aroma Compounds in Fresh, Raw Ginger (Zingiber officinale Roscoe) from China and Roasted Ginger by Application of Aroma Extract Dilution Analysis

Article abstract of DOI:10.1021/acs.jafc.0c06731

By application of a comparative aroma extract dilution analysis on the volatile fractions isolated by solvent extraction and solvent-assisted flavor evaporation (SAFE) from fresh raw Chinese ginger (Zingiber officinale Roscoe) and roasted ginger, 21 or 33 odorants, respectively, with flavor dilution (FD) factors in the range of 32-4096 were identified. In raw ginger, the highest FD factors were found for (E)-isoeugenol, 1,8-cineol, vanillin, geranial, and linalool. After roasting, in particular, the FD factors of 3-(methylthio)propanal (cooked potato-like), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like), 3-hydroxy-4,5-dimethyl-2(5H)-furanone (seasoning-like), and geraniol were substantially increased. The application of static headspace/olfactometry (SHO) on ground raw ginger revealed a high FD factor for highly volatile acetaldehyde which clearly decreased after roasting. By contrast, the SHO application revealed high FD factors for malty smelling methylpropanal and 3-methylbutanal, which both were exclusively detected in roasted ginger. Thirteen odorants, namely, decanoic acid, (Z)-2-decenal, (Z)-4-decenal, (E)-4,5-epoxy-(E)-2-decenal, (E)-4,5-epoxy-(E)-2-undecenal, fenchol, (Z)-3-hexenal, 3-hydroxy-4,5-dimethyl-2(5H)-furanone, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, 3-methyl-2-buten-1-thiol, 2-methylpropanal, (E)-2-nonenal, and 1-nonen-3-one, were identified in ginger for the first time. Chiral analysis showed a much higher percent by weight portion for the (R)-enantiomer in citronellal, citronellol, and linalool, which was not much changed during pan-frying.

Full text of DOI:10.1021/acs.jafc.0c06731

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Article
Comparison of the Key Aroma Compounds in Fresh, Raw Ginger (
Zingiber officinale Roscoe) from China and Roasted Ginger by
Application of Aroma Extract Dilution Analysis
Tanja Schaller and Peter Schieberle*
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ABSTRACT: By application of a comparative aroma extract dilution analysis on the volatile fractions isolated by solvent extraction
and solvent-assisted flavor evaporation (SAFE) from fresh raw Chinese ginger (Zingiber officinale Roscoe) and roasted ginger, 21 or
33 odorants, respectively, with flavor dilution (FD) factors in the range of 32−4096 were identified. In raw ginger, the highest FD
factors were found for (E)-isoeugenol, 1,8-cineol, vanillin, geranial, and linalool. After roasting, in particular, the FD factors of 3-
(methylthio)propanal (cooked potato-like), 4-hydroxy-2,5-dimethyl-3(2H)-furanone (caramel-like), 3-hydroxy-4,5-dimethyl-2(5H)-
furanone (seasoning-like), and geraniol were substantially increased. The application of static headspace/olfactometry (SHO) on
ground raw ginger revealed a high FD factor for highly volatile acetaldehyde which clearly decreased after roasting. By contrast, the
SHO application revealed high FD factors for malty smelling methylpropanal and 3-methylbutanal, which both were exclusively
detected in roasted ginger. Thirteen odorants, namely, decanoic acid, (Z)-2-decenal, (Z)-4-decenal, (E)-4,5-epoxy-(E)-2-decenal,
(E)-4,5-epoxy-(E)-2-undecenal, fenchol, (Z)-3-hexenal, 3-hydroxy-4,5-dimethyl-2(5H)-furanone, 4-hydroxy-2,5-dimethyl-3(2H)-
furanone, 3-methyl-2-buten-1-thiol, 2-methylpropanal, (E)-2-nonenal, and 1-nonen-3-one, were identified in ginger for the first time.
Chiral analysis showed a much higher percent by weight portion for the (R)-enantiomer in citronellal, citronellol, and linalool, which
was not much changed during pan-frying.
KEYWORDS: comparative aroma extract dilution analysis, (Z)-2-decenal, (Z)-2-decenol, isogeranial, chiral analysis, pan-frying
have been isolated from raw ginger either by distillation, steam
distillation, or extraction with liquid carbon dioxide, and more
than 200 volatiles have been reported.2−16
INTRODUCTION
■
The ginger rhizome (Zingiber officinale Roscoe) is used as a
spice since more than 5000 years. Its production is
continuously increasing, and today, about 3.3 million tons
are harvested overall the world with India, China, and Nigeria
being the main producers. For example, the harvest in India
increased from about 390,000 tons in 2008 to about 893,000
tons in 2018.1 Besides several health benefits, its typical hot,
sharp taste and the unique aroma often assigned as citrus-like
are for sure the main drivers for its acceptance by consumers
worldwide.
More than 120 years ago, by application of sophisticated
chemical reactions, Bertram and Walbaum2 were among the
first to study the chemical components of ginger and were able
to identify camphene and phellandrene in a steam distillate.
Much later, in a comprehensive approach, Lawrence3
identified 114 volatiles in ginger, among which 70 compounds,
such as citronellol and germacrene B, were described for the
first time in ginger. Erler et al.4 suggested citronellol, geranial,
neral, citronellyl acetate, geranyl acetate, and neryl acetate as
important volatiles in essential oils of ginger from India and
Australia. At the same time, Miyazawa and Kameoka5
suggested α-zingiberene, 1,8-cineol, nerol, geraniol, geranial,
α-terpineol, and β-bisabolene as contributors to the odor of
Japanese ginger.
However, ginger is not only used in the fresh, unprocessed
state but is also thermally treated, for example, in the
production of gingerbread or ginger jam and in the Indian
kitchen, a short heating process together with other spices in
hot oils is quite common, but only one study tried to get the
first insight into the changes in essential ginger oils induced by
hot water treatment.17 The authors reported on differences
between ginger volatiles obtained by steam distillation and a
ginger extract obtained by extraction with liquid carbon
dioxide and found a decrease in various monoterpenes,
hydrocarbons, aldehydes, and linalool in the steam distillation
extracts, while other compounds were increased.
It is meanwhile accepted that only volatile constituents of a
food, which are detectable by the human odorant receptors in
the nose, are finally able to generate the perception of an
aroma profile in the brain. As common for receptors, the
Received: October 24, 2020
Revised: November 21, 2020
Accepted: November 24, 2020
In the meantime, numerous studies have been reported
particularly on the composition of ginger essential oils which
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acid, borneol, 1,8-cineol, citral, (R)- citronellal, (R)-/(S)-citronellal,
(S)-citronellal, (R)-citronellol, (R)-/(S)-citronellol, decanoic acid,
(E)-2-decenal, (E)-2-dodecenal, fenchyl alcohol, geraniol, 2-heptanol,
hexanal, (Z)-3-hexenal, 3-hydroxy-4,5-dimethyl-2(5H)-furanone, iso-
borneol, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, (R)-linalool, (R)-
and (S)-linalool, 3-methylbutanal, methylpropanal, 3-(methylthio)-
propanal, β-myrcene, nonanal, (E)-2-nonenal, octanal, (E)-2-octenal,
and 2-undecanone (Sigma-Aldrich Chemie, Taufkirchen, Germany);
acetaldehyde, (S)-citronellol, and vanillin were from Merck
(Darmstadt, Germany), and isoeugenol was bought from Alfa Aesar
(Karlsruhe, Germany).
The following reference odorants were synthesized according to the
literature cited: (E)-4,5-epoxy-(E)-2-decenal and (E)-4,5-epoxy-(E)-
2-undecenal,23 3-methyl-2-buten-1-thiole,24 and 1-nonene-3-one.25
Syntheses of Reference Odorants. (Z)-2-Decenal. The
compound was obtained by hydrogenation of dec-2-yn-1-ol using
Lindlar’s catalyst and oxidation of (Z)-2-decenol formed with Dess-
Martin periodinane as described below.
(Z)-2-Decenol. A mixture of dec-2-yn-1-ol (6.5 mmol) and
Lindlar’s catalyst (200 mg) in ethanol (30 mL) was hydrogenated
in a laboratory autoclave (Roth, Karlsruhe, Germany) at 3 × 105 mPa
at room temperature (RT) until no dec-2-yn-1-ol was detectable by gc
analysis. The mixture was filtered through a paper filter followed by
filtration over a membrane (0.45 μm). The clear filtrate was filled up
to 50 mL with ethanol, and the yield was determined by HRGC-FID
with methyl octanoate as the internal standard. The solvent was
removed in a Christ rotational vacuum concentrator (Osterode am
Harz, Germany).
activation is dependent on the concentration of a given aroma
compound,18,19 but, while numerous investigations have
already been undertaken to identify the volatile compounds
in ginger, studies aimed at characterizing the main odor-active
compounds among the bulk of volatiles are rather scarce. The
first application of a technique combining analytical chemistry
with human aroma perception, the gas chromatography/
olfactometry (GC/O) method, was applied by MacLeod and
Pieris on ginger essential oils,20 and they suggested bornyl
acetate, β-eudesmol, geranial, neral, trans-β-sesquiphellandrol,
and β-zingiberene as important odorants in ginger aroma.
An aroma extract dilution analysis (AEDA)18 further
develops a GC/O approach by determination of flavor dilution
(FD) factors, and this was for the first time applied by
Nishimura21 on a special fraction of oxygenated ginger
compounds isolated by column chromatography. The highest
FD factors were found for linalool and geraniol followed by
geranial, citronellyl acetate, borneol, isoborneol, neral, and 1,8-
cineol.21
In a more recent study,22 the AEDA was applied on an
extract prepared from fresh ginger juice, and among the 44
odor-active compounds detected, eucalyptol, linalool, geranial,
and bornyl acetate showed by far the highest FD factors.
Although the composition of volatiles in either fresh ginger
or the essential oil or juice prepared thereof has been analyzed
in numerous investigations, so far, only a very few
investigations have used gentle isolation techniques for volatile
isolation. Furthermore, although ginger is often consumed as a
heat-treated material, data on changes in the key aroma
compounds induced by heating are scarcely available. The aim
of the present study was, therefore, to compare the changes in
the aroma compounds of raw Chinese ginger during heat
processing by application of the comparative aroma extract
dilution analysis (cAEDA) on extracts obtained by extraction/
solvent-assisted flavor evaporation (SAFE). Such data are the
basis for the entire Sensomics concept,18,19 that is, odorant
quantitation and aroma recombination to elucidate the total
set of aroma compounds generating the aroma profiles of raw
and heated ginger. Furthermore, the data will allow
conclusions on the presence of precursors of aroma
compounds in processed ginger.
Yield: 6.4 mmol (98%); RI (FFAP): 1793; RI (DB-5): 1269; odor
quality (HRGC-O): mushroom-like; MS (EI), m/z (%) 39 (12), 41
(57), 42 (17), 43 (58), 44 (12), 54 (44), 55 (59), 56 (26), 57 (100),
67 (38), 68 (33), 69 (25), 70 (20), 71 (14), 81 (32), 82 (27), 83
(17), 95 (20), 96 (16), 109 (10), 110 (12); MS (CI, isobutane), m/z
(%) 111 (31), 125 (26), 139 (100, M+ +1 − H2O), 140 (11).
1H NMR (400 MHz, CDCl3): δ [ppm] 0.88 (t, 3H, J = 6.8 Hz, C−
H-10), 1.20−1.41 (m, 11H, C−O−H, C−H-5, C−H-6, C−H-7, C−
H-8, C−H-9), 2.07 (q, 2H, J = 7.0 Hz, 14.0 Hz, C−H-4), 4.20 (d, 2H,
J = 6.1 Hz, C−H-1), 5.57 (m, 2H, C−H-2, C−H-3).
13C NMR (100 MHz, CDCl3): δ [ppm] 14.10 (C-10), 22.66 (C-
9), 27.44 (C-4), 29.15 (C-7), 29.19 (C-6), 29.62 (C-5), 31.83 (C-8),
58.64 (C-1), 128.29 (C-2), 133.33 (C-3).
Dess-Martin Periodinane. Dess-Martin periodinane was synthe-
sized as described.26,27
(Z)-2-Decenal. (Z)-2-Decenol (6.4 mmol) was dissolved in
anhydrous dichloromethane (15 mL) added to a suspension of
Dess-Martin periodinane (7.8 mmol) in anhydrous dichloromethane
(20 mL) under an argon atmosphere and was stirred for 3 h at RT.
The mixture was diluted with sodium thiosulfate (1 mol/L; saturated
with sodium hydrogen carbonate; 75 mL) and stirred vigorously for
10 min to obtain a clear organic phase. The water phase was removed,
and the organic phase was washed with sodium thiosulfate (1 M;
saturated with sodium hydrogen carbonate; 50 mL), followed by
saturated sodium hydrogen carbonate solution (50 mL), and finally
distilled water (50 mL). After drying over anhydrous sodium sulfate,
the solution was filled up to 50 mL with dichloromethane, and the
yield was determined by HRGC-FID with methyl octanoate as the
internal standard.
MATERIALS AND METHODS
Materials. Fresh, raw ginger from China was purchased in a local
supermarket. The ginger was peeled, cut into pieces of about 2 cm,
and immediately frozen with liquid nitrogen. For extraction, the
■
̈
frozen pieces were powdered in a Moulinex blender (Koln, Germany).
Heating was done on ginger slices of about 2 mm (160 g) for 13 min
at 200 °C in a Teflon-coated pan with constant turning. The roasted
slices were then frozen in liquid nitrogen and ground using the
Moulinex blender.
Chemicals. The following chemicals were obtained from the
suppliers given in parentheses: aluminum isopropoxide, citral, dec-2-
yn-1-ol, Lindlar’s catalyst, methyl octanoate, nerol, pyridine, and
sodium hydride (Sigma-Aldrich Chemie); deuterochloroform (Euro-
iso-Top, Giv-sur-Yvette, France) and liquid nitrogen (Linde, Munich,
Germany); acetic acid, acetone, acetonitrile, dichloromethane, diethyl
ether, ethanol, formic acid, hydrochloric acid, methanol, n-pentane,
silica gel 60 0.063−0.2 mm, sodium carbonate, sodium hydrogen
carbonate, sodium sulfate, and sodium thiosulfate (Merck); molecular
sieve 4 Å (J.T. Baker, Deventer, The Netherlands) and argon and
̈
hydrogen (Westfalen, Munster, Germany). Dichloromethane, diethyl
ether, and n-pentane were freshly distilled prior to use.
Reference Odorants. The following reference odorants were
obtained from the commercial sources given in parentheses: acetic
Yield: 2.1 mmol (33%); RI (FFAP): 1600; RI (DB-5): 1253; odor
quality (HRGC/O): fatty, green; MS (EI), m/z (%) 39 (21), 40 (20),
41 (51), 42 (13), 43 (41), 44 (13), 53 (10), 55 (42), 56 (14), 57
(22), 67 (18), 68 (11), 69 (25), 70 (100), 81 (18), 83 (99), 84 (17),
97 (11), 110 (22); MS (CI, isobutane), m/z (%) 137 (41, M+ +1 −
H2O), 155 (100, M+ + 1), 156 (12).
1H NMR (400 MHz, CDCl3): δ [ppm] 0.89 (t, 3H, J = 7.0 Hz, C−
H-10), 1.22−1.40 (m, 8H, C−H-6, C−H-7, C−H-8, C−H-9), 1.51
(m, 2H, C−H-5), 2.61 (dt, 2H, J = 8.0 Hz, 8.2 Hz, C−H-4), 5.96 (m,
1H, C−H-2), 6.64 (td, 1H, J = 8.2 Hz, 11.2 Hz, C−H-3), 10.08 (d,
1H, J = 8.2 Hz, C−H-1).
B
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13C NMR (100 MHz, CDCl3): δ [ppm] 14.06 (C-10), 22.61 (C-
9), 28.01 (C-4), 29.01 (C-7), 29.06 (C-5), 29.20 (C-6), 31.71 (C-8),
130.18 (C-2), 153.58 (C-3), 191.01 (C-1).
Geranial and neral are commercially available as citral, a mixture of
both. To obtain the pure aroma compounds, both aldehydes were
prepared by oxidation of their corresponding alcohols.
(14), 69 (36), 77 (19), 79 (31), 81 (100), 82 (40), 91 (29), 93 (24),
94 (64), 95 (32), 107 (13), 109 (65), 110 (16), 119 (15), 123 (17);
MS (CI, isobutane), m/z (%) 135 (64, M+ + 1 − H2O), 151 (15),
153 (100, M+ + 1), 154 (10).
Purification of the target compound for NMR experiments was
done by HPLC. Nucleosil C18 AB (250 mm × 10 mm, 5 μm,
Geranial. A solution of geraniol (3.3 mmol) in dry dichloro-
methane (15 mL) was added to a suspension of Dess-Martin
periodinane (4.0 mmol) in dry dichloromethane (15 mL) under an
argon atmosphere and stirred for 1 h at RT. The mixture was diluted
with diethyl ether (50 mL) followed by sodium thiosulfate (1 M;
saturated with sodium hydrogen carbonate; 100 mL) and vigorously
stirred for another 10 min at RT to obtain a clear organic phase. The
organic phase was washed successively with aqueous sodium
thiosulfate (1 mol/L; saturated with sodium hydrogen carbonate;
50 mL), saturated sodium hydrogen carbonate solution (100 mL),
and distilled water (100 mL) and then dried over anhydrous sodium
sulfate and concentrated in vacuo. Isolation of the target compound
was done by column chromatography on a water-cooled column (35
cm × 1.6 cm) at 10−12 °C filled with silica gel 60 (30 g; 0.04−0.063
mm; water content 7%). After flushing the column with n-pentane
and n-pentane/diethyl ether (75 mL; 80:20 by vol.), elution was
performed with n-pentane/diethyl ether (70:30; by vol.) in fractions
of 20 mL. Each fraction was analyzed by GC/O to localize geranial;
these fractions were pooled, and the solvent was removed by means of
a rotary evaporator. The yield was determined by HRGC-FID with
methyl octanoate as the internal standard, and the purity was checked
by GC/O.
̈
Macherey-Nagel, Duren, Germany) was used as the stationary phase
in a BIO-TEK Instruments HPLC-system (Eching, Germany)
equipped with two pumps (type 522), a gradient-mixture system
(M 800) and a rheodyne-injection system (7725 i). The mobile phase
was an isocratic 1:1 mixture of acetonitrile (with 0.1% formic acid)
and water (with 0.1% formic acid) at a flow rate of 3 mL/min. The
injection volume was 100 μL. The effluent of the column was
monitored at 210 nm using a UV detector (type 535). The effluent
from ten runs containing the target compound was pooled and, after
removal of the solvent, taken up in CDCl3.
1H NMR (400 MHz, CDCl3): δ [ppm] 1.64 (br s, 3H, C−H-9),
1.70 (br s, 6H, C−H-8, C−H-10), 2.76 (pt, 2H, J = 7.2 Hz, C−H-5),
3.03 (m, 2H, J = 2.6 Hz, C−H-2), 5.10 (m, 1H, J = 7.2 Hz, C−H-6),
5.30 (m, 1H, J = 7.2 Hz, C−H-4), 9.61 (t, 1H, J = 2.6 Hz, CHO).
13C NMR (100 MHz, CDCl3): δ [ppm] 16.91 (C-10), 17.74 (C-
9), 25.69 (C-8), 27.38 (C-5), 54.33 (C-2), 122.30 (C-6), 126.20 (C-
3), 129.81 (C-4), 132.26 (C-7), 200.57 (C-1). The data were in good
agreement with previously published spectra.30
Isolation of the Volatiles. A powder of either raw or roasted
ginger (50 g), respectively, was extracted with diethyl ether (250 mL)
for 3 h at RT with vigorous stirring. The mixture was filtered, and the
residue was extracted twice with diethyl ether (total volume: 200
mL). The combined organic layers were dried over anhydrous sodium
sulfate, and the volatiles were isolated by SAFE distillation.31 The
distillate was concentrated to 200 μL using a Vigreux column (60 cm
× 1 cm) and a microdistillation apparatus at 40 °C.32
RI (FFAP): 1722; RI (DB-5): 1266; odor quality (HRGC/O):
citrus-like; Neral was synthesized by oxidation of nerol with Dess-
Martin periodinane following the same method as described above for
geranial.
RI (FFAP): 1668; RI (DB-5): 1240; odor quality (HRGC/O):
citrus-like, soapy; Isogeranial was synthesized from citral following the
methods of Bedoukian28 and Fehr et al.29 with some modifications as
described below.
High-Resolution Gas Chromatography/Olfactometry
(HRGC/O). This was performed by means of a Fisons Instruments
gas chromatograph type 8000 (Mainz, Germany) using the following
J&W Scientific fused silica capillary columns: DB-FFAP (30 m × 0.32
mm i.d., 0.25 μm film thickness) and DB-5 (30 m × 0.32 mm i.d., 0.5
μm film thickness) (Agilent Technologies, Waldbronn, Germany).
The samples (0.5 μL) were applied by the cold-on column technique
at 40 °C using helium as a carrier gas at a flow rate of 1.9 mL/min.
For DB-FFAP, an initial temperature of 40 °C was held for 2 min,
then raised at 6 °C/min to 230 °C, and held for 5 min. For DB-5, an
initial temperature of 40 °C was held for 2 min, then raised at 4 °C/
min to 160 °C, held for 0 min, and then raised at 20 °C/min to 240
°C and held for 5 min. For GC/O, the effluent was split 1:1 (by
volume) at the end of the capillary column using a Y-shaped glass
splitter (Chrompack, Frankfurt, Germany) and two deactivated fused
silica capillaries (50 cm × 0.25 mm i.d.). One part was directed to a
flame ionization detector (FID) held at 230 °C and the other part to a
heated (190 °C) sniffing port. The retention indices (RI) were
calculated from the retention times of n-alkanes by linear
interpolation.
Aroma Extract Dilution Analysis (AEDA); Comparative
AEDA. To eliminate potential gaps in detecting odor-active regions,
which may be caused by potential anosmia, the concentrated, SAFE
distillate was analyzed by three experienced panelists using GC/O on
the DB-FFAP capillary. Then, the extract was diluted stepwise 1:1 (by
vol) with diethyl ether, and each dilution was analyzed by GC/O.
Again, this was done by three experienced panelists, and the results
were averaged. By definition, the FD factor obtained for each single
odorant in the AEDA is equal to the highest dilution in which the
odorant could still be perceived at the sniffing port.33 By using a
distillate prepared from the same amount of raw and processed ginger,
the same final distillate volume, and the same injection volume at the
gas chromatograph, a comparative AEDA can be performed. This way,
the FD factors of a given compound in both extracts can be compared
in a semi-quantitative way.33
Isogeranyl Acetate. Citral (16.4 mmol) was added to a mixture of
acetic acid anhydride (32.7 mmol) and pyridine (0.8 mmol) under an
argon atmosphere. The mixture was stirred for 4 h at 135 °C and,
after cooling to RT distilled water (10 mL), was added with ice-
cooling. After addition of diethyl ether (75 mL), the solution was
washed with an aqueous saturated sodium hydrogen carbonate
solution (150 mL) followed by brine (75 mL) and dried over
anhydrous sodium sulfate. After filtration, the solvent was removed,
and the residue was purified by column chromatography as described
above. Six fractions were obtained using the following n-pentane/
diethyl ether gradient: fraction 1 (100 mL; n-pentane), fraction 2
(100 mL; 95/5, v/v), and fraction 3 (100 mL; 85/15, v/v). Both
esters, isogeranyl acetate and isoneryl acetate, were eluted in fraction
3, and the yields were determined by HRGC-FID using methyl
octanoate as the internal standard.
Isogeranyl acetate; yield 1.6 mmol (20%); RI (FFAP): 1930; RI
(DB-5): 1441; odor quality (HRGC/O): citrus-like, sweet; MS (EI),
m/z (%) 39 (11), 41 (44), 43 (100), 53 (13), 55 (17), 59 (35), 67
(24), 69 (52), 77 (12), 79 (20), 80 (58), 81 (34), 82 (25), 83 (11),
84 (25), 91 (24), 92 (11), 93 (16), 94 (36), 95 (34), 96 (16), 97
(10), 107 (16), 109 (75), 119 (43), 121 (11), 134 (28), 137 (18);
MS (CI, isobutane), m/z (%) 135 (100), 136 (11), 139 (4), 153 (3),
195 (3, M+ + 1).
Isogeranial. The mixture of both esters was dissolved in anhydrous
methanol (10 mL) and added to a mixture of aluminum isopropoxide
(3.7 mmol) in anhydrous methanol (10 mL) under an argon
atmosphere. After stirring for 17 h at RT, acetic acid (33%; 20 mL)
and n-pentane (150 mL) were added, and the organic phase was
washed with a saturated sodium hydrogen carbonate solution (300
mL) followed by brine (300 mL) and dried over anhydrous sodium
sulfate.
Yield: 0.6 mmol (75%); RI (FFAP): 1560; RI (DB-5): 1183; odor
quality (HRGC-O): lemon-like, balm-like, green; MS (EI), m/z (%)
39 (20), 41 (79), 43 (40), 53 (24), 55 (34), 65 (10), 67 (74), 68
Column Chromatography. The SAFE distillate was separated
into a fraction of acidic volatiles (AF) and a fraction of neutral/basic
volatiles (NBF) as recently reported.34 To avoid overlapping peaks
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during gas chromatography, column chromatography was applied on
the NBF fraction using a pentane/diethyl ether gradient.34
High-Resolution Gas Chromatography-Mass Spectrometry
(HRGC-MS). Mass spectra were generated by means of a Finnigan
MAT 95S sector field mass spectrometer (Bremen, Germany) in the
electron impact mode (MS-EI) at 70 eV using the capillaries and
temperature programs described above in a Hewlett-Packard gas
chromatograph 5890 series II (Waldbronn, Germany). Mass spectra
in the chemical ionization mode (MS-CI) were recorded at 115 eV
using isobutane as the reactant gas.
Two-Dimensional-High Resolution Gas Chromatography/
Olfactometry-Mass Spectrometry (2D-HRGC/O-MS). To obtain
unequivocal mass spectra, in some cases, two dimensional-HRGC/O-
MS was performed using the Fisons Moving Column Stream
Switching (MCSS) system as recently reported.35
Chiral Analysis. The percent by weight portion of the
enantiomers (R)- and (S)-citronellal, (R)- and (S)-citronellol, and
(R)- and (S)-linalool was determined by 2D-HRGC/O-MS using the
MCSS system.35 A Thermo Quest gas chromatograph-type TRACE
2000 (Egelsbach, Germany) equipped with a CTC Analytics Combi
PAL autosampler (Zwingen, Switzerland) and a J&W Scientific DB-
FFAP column (30 m × 0.25 mm i.d., 0.25 μm film thickness)
(Agilent, Waldbronn, Germany) was connected via a heated transfer
line (uncoated and deactivated fused silica, 0.32 mm i.d.) with a
Varian CP 3800 gas chromatograph (Varian, Germany) equipped
with a BGB-174 column (30 m × 0.25 mm, 0.25 μm film thickness)
(BGB-Analytik, Boeckten, Switzerland). Detection was performed
using a Varian Saturn ion trap mass spectrometer running in the MS-
CI mode with methanol as the reactant gas. The elution order was
assigned by analyzing a mixture of enantiomers first and then at least
one pure enantiomer.
Static Headspace Olfactometry (SHO). For the characterization
of highly volatile odorants which might get lost during sample work-
up, a static headspace dilution analysis was performed. Powdered
ginger (8 g) was placed in a glass vessel (volume 120 mL) and sealed
with a septum. After equilibration for 30 min at 37 °C, decreasing
headspace volumes (5, 2.5, 1.25, 0.6, 0.3, and 0.15 mL) were
withdrawn from different samples using an SGE gastight syringe
(Darmstadt, Germany) and injected by means of the CTC-Analytics
CombiPal onto a J&W Scientific DB-5 capillary column (30 m × 0.25
mm i.d., 0.5 μm film thickness) (Agilent Technologies) installed in a
Thermo Finnigan Trace GC Ultragas chromatograph (Hamburg,
Germany). The volatiles were trapped with liquid nitrogen on an
uncoated fused-silica capillary (0.53 mm i.d.) by means of a cold-trap
915 (SGE Analytic Science, Darmstadt, Germany) and were finally
transferred to the capillary column by immediately heating the cold-
trap at 15 °C/s to 250 °C. The volatiles were separated using the
following temperature program: 0 °C for 2 min, then increased by 6
°C/min to 60 °C, and then ramped at 40 °C/min 240 °C. By using
two Y-shaped glass splitters, it was possible to detect the volatiles in
parallel by FID (250 °C), the ion trap mass spectrometer, and a
sniffing port (190 °C).
Figure 1. Flavor dilution (FD) chromatogram of a distillate obtained
by extraction/SAFE distillation of fresh, raw ginger (compounds with
FD factors ≥32 are displayed).
evaporation, it displayed the typical aroma of raw ginger as
agreed by a sensory panel. To characterize the odor-active
volatiles in the entire distillate, the aroma extract dilution
analysis (AEDA) was applied. The analysis revealed 21 odor-
active regions with FD (flavor dilution) factors between 32 and
4096 (Figure 1). The highest FD factor of 4096 was found for
a clove-like, smoky smelling region (33), followed by a
eucalyptus-like (3) and a vanilla-like (35) smelling region with
a FD factor of 2048. Also high FD factors were assigned to
elution volumes eliciting citrus-like (23), olibanum-like (34),
and citrus, bergamot-like (14) odor attributes.
To get an idea on the chemical compounds behind the odors
perceived, the retention indices and odor qualities were
compared to the data present in an in-house database
containing more than 1000 odorants found in numerous
food aromas over the past 40 years in the research group. The
suggested chemical structures were finally confirmed by
analyzing reference compounds on the basis of the following
criteria: retention indices on two capillary columns of different
polarity, mass spectra recorded in MS-EI and MS-CI as well as
odor quality, and odor intensity at the sniffing port compared
to the height of the FID signal.
This way, 19 aroma compounds with high FD factors could
be identified in the distillate of raw ginger. The highest FD
factors were assigned to (E)-isoeugenol (33), 1,8-cineol (3),
and vanillin (35) (Figure 2). Further odorants with high FD
factors were identified as geranial (23) and linalool (14)
(Figure 2). Altogether six compounds eliciting citrus- or rose-
like odor notes were identified, namely, citronellal (11),
citronellol (24), geranial (23), geraniol (26), linalool (14),
neral (21), and octanal (4) indicating the importance of these
odor qualities for the overall aroma profile of ginger (Table 1).
The determination of the percent by weight distribution in
the enantiomers in some chiral compounds showed that
citronellal and citronellol mainly consisted of the (R)-
enantiomer (both 97%), while for linalool, 71% (R)-
enantiomer and 29% (S)-enantiomer were determined
(Table 2).
Nuclear Magnetic Resonance Spectroscopy (NMR). The
nuclear magnetic resonance spectra (1H, 13C, COSY, HMBC, and
HSQC) were acquired by means of a 400 MHz Bruker AVANCE III
NMR spectrometer (Rheinstetten, Germany). The samples were
dissolved in deuterochloroform (CDCl3) with 0.03% tetramethylsi-
lane (TMS) as the internal standard. The transmitter frequencies used
were 400 MHz (1H) and 100 MHz (13C). The data were processed
using TopSpin-NMR (version 3.0, Bruker, Rheinstetten, Germany).
The spectra were analyzed by means of MestRe-C (Mestrelab
Research, Santiago de Compostela, Spain).
RESULTS AND DISCUSSION
■
Identification of Odor-Active Compounds in Fresh,
Raw Ginger. Raw ginger was peeled, frozen, and ground in
liquid nitrogen, and the volatile fraction was extracted with
diethyl ether followed by SAFE distillation. When a drop of the
distillate was added on a strip of filter paper, after solvent
The important role of the comparison of the signal intensity
with the odor intensity at the sniffing port was obvious during
the identification of the soapy, pungent smelling compound 6.
D
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Figure 2. Structures of the most odor-active odorants identified in fresh, raw ginger (numbering refers to Table 1).
While all criteria, the retention indices, mass spectra, and odor
quality, pointed at nonanal, the signal height found during
analysis of the highest dilution of the ginger extract in which its
odorant could still be perceived was clearly lower than the
signal height of nonanal at its odor threshold in air. This result
revealed that despite nearly identical analytical data, the
unidentified compound 6 co-eluted with nonanal, which itself,
however, did not cause the odor detected at the respective
retention index, but, unfortunately, the structure of compound
6 remained open.
Highly volatile compounds might get lost during sample
work-up or might co-elute with the solvent during AEDA.
Therefore, a solventless static headspace dilution analysis
(SHO) was carried out. Three compounds showed FD factors
greater than 1, among which myrcene and 1,8-cineol were
already identified during AEDA of the distillate (Table 3).
Also, the green and grassy smelling hexanal and (Z)-3-hexenal
were detected in the AEDA but showed FD factors lower than
32 [hexanal: FD 1 (Z)-3-hexenal: FD 16; data not shown in
Table 1]. However, the pungent smelling acetaldehyde
showing a high FD factor of 8 was not detected during
AEDA, probably because it co-eluted with the solvent peak.
Altogether five compounds, namely fenchol, (Z)-3-hexenal,
3-methyl-2-buten-1-thiol, (E)-2-nonenal, and vanillin, were
identified in raw ginger for the first time.
Odor-Active Compounds in Roasted Ginger. To
systematically investigate the influence of a heat treatment
on the odor-active compounds, raw ginger of the same batch
was pan-roasted, and the volatiles were extracted with the
solvent followed by SAFE distillation. The SAFE distillate
clearly represented the characteristic odor of the roasted ginger
material when sniffed from a strip of paper, but in addition to
the unique citrus-like and eucalyptus-like odor notes, a
caramel-like, a cooked potato-like, a seasoning-like, and a
roasty odor additionally appeared after roasting.
The distillate subsequently underwent a comparative AEDA,
and already a first look at the results obtained indicated a clear
difference between the FD chromatogram of the roasted ginger
(Figure 3) compared to the FD chromatogram of the raw
ginger (Figure 1). In the roasted ginger, 33 odor-active regions
showing FD factors between 32 and 2048 were detected
(Figure 3). The identification experiments in combination with
Figure 3. Flavor dilution (FD) chromatogram of a distillate obtained
by extraction/SAFE distillation of roasted ginger (compounds with
FD factors ≥32 are displayed).
the AEDA results revealed 1,8-cineol (3, eucalyptus-like), 4-
hydroxy-2,5-dimethyl-3(2H)-furanone (29, caramel-like), (E)-
isoeugenol (33, clove-like, smoky), and vanillin (35, vanilla-
like) with the highest FD factor of 2048 (Figure 4). In
addition, 3-(methylthio)propanal (10, cooked potato-like), 2-
undecanone (17, fruity, sweaty), neral (21, citrus-like, soapy),
geranial (23, citrus-like), and 3-hydroxy-4,5-dimethyl-2(5H)-
furanone (31, seasoning-like) appeared with high FD factors
(Table 2). Unfortunately, as already in the raw ginger, the
olibanum-like smelling 34 appearing with a high FD factor of
1024 (Table 1) could not be identified.
The odor quality and the retention index on nonpolar DB-5
suggested compound 15 to be isocitral, which was previously
reported as an aroma compound in Japanese ginger.21 Because
no reference compound was commercially available, a synthesis
starting from citral, a commercially available mixture of neral
and geranial, was carried out resulting in isogeranial and
isoneral formation. Because isogeranial showed the same
retention indices on two stationary phases of different polarity
E
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Table 1. Most Odor-Active Compounds (FD ≥ 32 in at Least One of Both Samples) in Raw and Roasted Ginger
d
e
RI on
DB-FFAP
FD factor
a
b
c
f
g
no.
odorant
odor quality
hop-like
DB-5
raw
64
128
2048
256
32
32
1
128
2
8
128
64
1
512
2
32
128
8
64
1
64
16
1024
32
16
32
32
8
16
8
16
8
4096
1024
2048
roasted
fraction
earlier identified.
h
1
2
3
4
5
6
7
8
9
3-methyl-2-buten-1-thiol
β-myrcene
1093
1151
1193
1280
1314
1380
1394
1419
1442
1451
1469
1521
1530
1540
1560
1570
1588
1600
1632
1654
1668
1685
1722
1757
1813
1845
1853
1998
2024
2100
2191
2244
2342
2495
2566
819
989
32
256
2048
256
32
3
geranium-like, hop-like
eucalyptus-like
citrus-like
1, 2
3, 4
3
5
3
7
7
13
7
1,8-cineol
octanal
2-heptanol
unknown
1-nonen-3-one
(E)-2-octenal
acetic acid
1034
1004
900
1110
1074
1058
612
i
coconut-like
soapy, pungent
mushroom-like
fatty, nutty
16
64
h
3
256
32
1024
64
256
32
256
32
3, 4
6, AF
5
3
3
5
39
pungent
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
3-(methylthio)propanal
(R)-/(S) citronellal
(E)-2-nonenal
(Z)-4-decenal
cooked potato
citrus, lemon balm-like
fatty, musty
fatty, citrus-like
citrusy, bergamot-like
lemon balm-like, green
earthy, mouldy
fruity, sweaty
fatty, green
905
1153
1158
1195
1100
1183
1125
1290
1253
1263
1164
1240
1170
1266
1245
1576
1261
1471
1380
1062
1483
1110
1373
1455
12
3
(R)-/(S)-linalool
isogeranial
4, 5
3, 4
4, 5
3, 4
3
3
4
4
5
4
5
2
5, 6
3
4
AF
3
AF
6, AF
5, 6
7
21
i
fenchol
32
2-undecanone
(Z)-2-decenal
(E)-2-decenal
1024
128
64
13
fatty, green
21
42
7
7
7
42
15
7
i
isoborneol
earthy, mouldy
citrusy, soapy
earthy, mouldy
citrus-like
rose-like
mushroom-like
rose-like
peach-like
metallic
caramel-like
metallic, sweaty
seasoning-like
soapy, sweaty
clove-like, smoky
olibanum-like
vanilla-like
32
neral
borneol
1024
64
1024
128
64
i
geranial
(R)-/(S) citronellol
j
germacrene B
geraniol
512
16
(E)-2-dodecenal
21
h
(E)-4,5-epoxy-(E)-2-decenal
4-hydroxy-2,5-dimethyl-3(2H)-furanone
(E)-4,5-epoxy-(E)-2-undecenal
3-hydroxy-4,5-dimethyl-2(5H)-furanone
32
2048
32
1024
64
2048
1024
2048
h
decanoic acid
(E)-isoeugenol
unknown
21
k
nd
4, 5, AF
5, 6, AF
vanillin
1401
a
b
Numbering according to the elution order on the DB-FFAP gc column. The compound was identified by comparing its mass spectra (MS-EI),
retention indices on capillaries DB-FFAP and DB-5, as well as the odor quality and the odor intensity perceived during sniffing with data of
c
d
e
reference compounds. Odor quality perceived at the sniffing port. Retention index. Flavor dilution factor determined by AEDA on capillary DB-
FFAP. Fraction containing the compound after chemical fractionation and column chromatography. The compound was first reported as a
volatile compound in ginger in the given reference. No unequivocal mass spectrum was obtained. Identification is based on the remaining criteria
f
g
h
b i
j
given in footnote . Stereochemistry was not determined. Odor qualities are given for the racemate. No reference compound was available.
Identification according to the criteria given in footnote b, reference data obtained from literature.42,43 nd, not determined.
k
enantiomer for all three compounds (Table 2) indicating that
no substantial rearrangement of the enantiomers was caused by
the thermal treatment.
Table 2. Comparison of the Percent by Weight Portion in
the Enantiomers of Chiral Odorants in Raw and Roasted
Chinese Ginger
A static headspace dilution analysis revealed the malty
smelling compounds 2-methylpropanal and 3-methylbutanal as
additional odorants (Table 3).
percent by weight portion (%) in
raw
roasted ginger
odorant
(R)
(S)
(R)
(S)
A comparison with available literature data on ginger
volatiles (Table 1) showed that the following thirteen odorants
were for the first time identified as aroma compounds in ginger
or heat-treated ginger, respectively: decanoic acid, (Z)-2-
decenal, (Z)-4-decenal, (E)-4,5-epoxy-(E)-2-decenal, (E)-4,5-
epoxy-(E)-2-undecenal, fenchol, (Z)-3-hexenal, 3-hydroxy-4,5-
dimethyl-2(5H)-furanone, 4-hydroxy-2,5-dimethyl-3(2H)-fur-
anone, 3-methyl-2-buten-1-thiol, 2-methylpropanal, (E)-2-
nonenal, and 1-nonen-3-one.
citronellal
citronellol
linalool
97
97
71
3
3
29
90
98
79
10
2
21
as well as the identical odor quality and mass spectra as 15, the
odorant was successfully identified as isogeranial.
As found in raw ginger, the determination of the percent by
weight distribution in the enantiomers of citronellal,
citronellol, as well as linalool showed an excess of the (R)-
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Table 3. Key Odorants in the Headspace of Raw and Roasted Ginger (FD > 1 in at Least One Sample)
d
FD factor
a
b
c
e
odorant
odor quality
RI (DB-5)
raw
roasted
lit.
acetaldehyde
methylpropanal
3-methylbutanal
hexanal and/or (Z)-3-hexenal
3-(methylthio)-propanal
pungent
malty
malty
green, grassy
cooked potato
<500
546
652
8
<1
<1
1
<1
16
32
1
4
2
7
7
11
f
797
2
f
>800
>800
>800
16
32
32
β-myrcene
1,8-cineol
geranium-like, hop-like
eucalyptus-like
4
4
a
The compound was identified by comparing its mass spectra (MS-EI), retention index, and odor quality perceived during sniffing with data of the
b
c
d
reference compounds. Odor quality perceived at the sniffing port. Retention index. The relative FD factor was calculated by dividing the highest
volume analyzed (5 mL) by the smallest volume in which the odorant was detected. The compound was first reported as the volatile compound in
ginger in the given reference. Signals were too weak for an unequivocal identification. Identification is thus based on the remaining criteria given in
footnote .
e
f
a
Figure 4. Structures of the most odor-active odorants identified in roasted ginger (numbering refers to Table 1).
Changes Induced by Roasting. The AEDAs of the raw as
well as the roasted ginger were carried out according to the
guidelines for a comparative aroma extract dilution analysis
(cAEDA),33 that is, using exactly the same amount of sample
and the same analytical conditions. Therefore, the FD factors
in both samples can be directly compared on a semi-
quantitative basis. Numerous aroma compounds were
substantially increased during roasting (Table 1), but the
most pronounced increase was found for 3-(methylthio)-
propanal (10) and 4-hydroxy-2,5-dimethyl-3(2H)-furanone
(29) followed by 1-nonen-3-one (7) and 3-hydroxy-4,5-
dimethyl-2(5H)-furanone (31). In addition, the FD factors
of acetic acid (9), (Z)-4-decenal (13), isogeranial (15), (Z)-2-
decenal (18), isoborneol (20), neral (21), and geraniol (26)
were also higher in roasted compared to raw ginger. In
addition, the malty smelling aldehydes 3-methylbutanal and
methylpropanal were only detected in roasted ginger (Table
3).
These data suggested that these odorants are generated by
thermal degradation of odorless precursors in raw ginger. The
formation of 3-hydroxy-4,5-dimethyl-2(5H)-furanone and 4-
hydroxy-2,5-dimethyl-3(2H)-furanone as well as of 3-methyl-
butanal, methylpropanal, and 3-(methylthio)-propanal has
frequently been reported for many heat-treated foods, for
example, during processing of peanuts or meat, respec-
tively.36,37 It is well accepted in the literature that both
furanones are generated by degradation of monosaccharides,
while the aldehydes are well-known as Strecker aldehydes, in
this case formed from the amino acids leucine, valine, and
methionine.1-Nonen-3-one, a very potent odorant eliciting the
extremely low odor threshold 8 pg/L water has previously been
identified in several heat-processed foods, such as heated
raspberries.38
Several terpene alcohols have previously been reported as
glycosides in raw ginger,39 and it might be possible that the
increase in the concentrations of citronellol and geraniol may
be due to a release of the bound alcohols during pan-frying.
G
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(8) Connell, D. W. The chemistry of the essential oil and oleoresin
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Citronellal and linalool did not show large differences in
their FD factors between raw and roasted ginger, however, the
percent by weight portion in the enantiomers slightly differed
(Table 2). In citronellal, the (S)-enantiomer was increased,
while in linalool the increase in the (R)-enantiomer was higher
after roasting. The latter might be caused by the loss of both
linalool enantiomers during heating, but there is a specific new
generation of (R)-linalool from precursors as previously
described for beer or orange juice.40,41
In summary, the characterization of the key aroma
compounds by application of the AEDA revealed nearly the
same set of odorants in raw and roasted ginger, however with
clearly differing FD factors for several key odorants, that is,
their concentrations. The results are useful for the quality
assessment of fresh ginger, ginger oils, and stored, processed
ginger products, but to confirm the contribution of such
differences in the concentrations of key aroma compounds to
the differences in the overall aroma profiles of raw and roasted
ginger, the quantitation of the key aroma compounds and
aroma reconstitution experiments are underway.
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(15) Chen, C. C.; Ho, C. T. Gas chromatographic analysis of volatile
components of ginger oil (Zingiber officinale Roscoe) extracted with
liquid carbon dioxide. J. Agric. Food Chem. 1988, 36, 322−328.
(16) Toure, A.; Xiaoming, Z. Gas chromatographic analysis of
volatile components of guinean and chinese ginger oils (Zingiber
officinale) extracted by steam distillation. J. Agron. 2007, 6, 350−355.
(17) Chen, C.-C.; Ho, C.-T. Volatile Compounds in ginger oil
generated by thermal treatment. ACS Symp. Ser. 1989, 409, 366−375.
(18) Schieberle, P.; Hofmann, T. Mapping the combinatorial code of
food flavors by means of molecular sensory science approach. In
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AUTHOR INFORMATION
■
Corresponding Author
Peter Schieberle − Faculty of Chemistry, Technical University
of Munich, 85748 Garching, Germany; orcid.org/0000-
0003-4153-2727; Phone: +49 87197698160;
Email: peter.schieberle@ch.tum.de
Author
(19) Dunkel, A.; Steinhaus, M.; Kotthoff, M.; Nowak, B.;
Krautwurst, D.; Schieberle, P.; Hofmann, T. Nature’s Chemical
Signatures in Human Olfaction: A Foodborne Perspective for Future
Biotechnology. Angew. Chem., Int. Ed. 2014, 53, 7124−7143.
(20) MacLeod, A. J.; Pieris, N. M. Volatile aroma constituents of Sri
Lankan ginger. Phytochemistry 1984, 23, 353−359.
(21) Nishimura, O. Identification of the characteristic odorants in
fresh rhizomes of ginger (Zingiber officinale Roscoe) using aroma
extract dilution analysis and modified multidimensional gas
chromatography-mass spectroscopy. J. Agric. Food Chem. 1995, 43,
2941−2945.
Tanja Schaller − Deutsche Forschungsanstalt für
Lebensmittelchemie (Now Leibniz Institute for Food Systems
Biology at the TU Munich), D-85354 Freising, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jafc.0c06731
Notes
The authors declare no competing financial interest.
(22) Pang, X.; Cao, J.; Wang, D.; Qiu, J.; Kong, F. Identification of
Ginger (Zingiber officinale Roscoe) Volatiles and Localization of
Aroma-Active Constituents by GC-Olfactometry. J. Agric. Food Chem.
2017, 65, 4140−4145.
ACKNOWLEDGMENTS
■
We thank Dr. P. Steinhaus for a careful review of the
manuscript.
(23) Schieberle, P.; Grosch, W. Potent odorants of the wheat bread
crumb Differences to the crust and effect of a longer dough
fermentation. Z. Lebensm. Unters. Forsch. 1991, 192, 130−135.
(24) Holscher, W.; Vitzthum, O. G.; Steinhart, H. Prenyl alcohol -
source for odorants in roasted coffee. J. Agric. Food Chem. 1992, 40,
655−658.
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