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Decoding the Nonvolatile Sensometabolome of Orange Juice (Citrus sinensis) Anneke Glabasnia, Andreas Dunkel, and Thomas Hofmann J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b06142 • Publication Date (Web): 12 Feb 2018 Downloaded from http://pubs.acs.org on February 12, 2018
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Journal of Agricultural and Food Chemistry
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Decoding the Nonvolatile Sensometabolome of
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Orange Juice (Citrus sinensis)
3 4 5
Anneke Glabasnia+, Andreas Dunkel†, and Thomas Hofmann†,‡,*
6
+
7
Münster, Germany
8
†
9
Munich, Lise-Meitner Str. 34, D-85354 Freising, Germany
Institut für Lebensmittelchemie, Universität Münster, Corrensstrasse 45, D-48149
Chair of Food Chemistry and Molecular Sensory Science, Technical University of
10
‡
11
Lise-Meitner Str. 34, D-85354 Freising, Germany
Leibniz-Institute for Food Systems Biology at the Technical University of Munich,
12 13 14 15 16 17 18 19
*
To whom correspondence should be addressed
20
PHONE
+49-816171-2902
21
FAX
+49-816171-2949
22
E-MAIL
[email protected] 23 24 25
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ABSTRACT
27
Activity-guided fractionation in combination with the taste dilution analysis, followed
28
by LC-MS/MS and NMR experiments led to the identification of ten polymethoxylated
29
flavones (PMFs), six limonoid glucosides, and two limonoid aglycones as the key
30
bitterns of orange juice. Quantitative studies and calculation of dose-over-threshold
31
factors, followed by taste re-engineering demonstrated for the first time 25
32
sensometabolites to be sufficient to re-construct the typical taste profile of orange
33
juices and indicated that not a single compound can be considered a suitable marker
34
for juice bitterness. Intriguingly, the taste percept of orange juice seems to be created
35
by a rather complex interplay of limonin, limonoid glucosides, PMFs, organic acids,
36
and sugars. For the first time, sub-threshold concentrations of PMFs were shown to
37
enhance the perceived bitterness of limonoids. Moreover, the influence of sugars on
38
the perceived bitterness of limonoids and PMFs in orange juice relevant
39
concentration ranges was quantitatively elucidated.
40 41
Key Words: polymethoxylated flavones, 5,6,7,3´,4´-pentamethoxyflavone-3-O-β-D-
42
glucopyranoside, limonin, nomilin, limonoid glucosides, bitter taste, astringency, taste
43
dilution analysis, half-tongue test
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INTRODUCTION
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Due to its alluring aroma and the desirable taste profile, orange juice is one of the
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most popular fruit juices with a global production volume of 2.4 million metric tons per
49
year.1 Besides the aroma-active volatiles, the balanced harmony of sweetness and
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sourness accompanied by a velvety astringency and a faint bitterness are
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determining the acceptability of orange juice by the consumer.
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Screening of orange volatiles for odor-activity by means of gas chromatography-
53
olfactometry, quantitation of the most odor-active molecules by using stable isotope
54
dilution analyses, followed by aroma reconstitution experiments impressively
55
demonstrated that only about 25 out of more than 200 volatiles contribute to the
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typical aroma of a freshly squeezed orange juice.2-5 In contrast, the knowledge on the
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taste-active non-volatiles creating the attractive taste percept of orange juice in the
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brain is rather fragmentary. In particular, the knowledge on the molecules imparting
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the typical bitter taste of orange juice is rather contradictory.
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Limonin (1; Figure 1), a highly oxygenated triterpenoid of the family limonoids,
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has been suggested already in 1841 as a constituent of oranges,6 although its
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chemical structure as well as its contribution to the bitter taste of navel orange juice
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were reported almost one hundred years later.7,8 Sensory analysis revealed a bitter
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taste threshold of 6 ppm (in orange juice) for limonin and 2 ppm (in water) for nomilin
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(2), another member of the limonoid family.9-11 However, nomilin (2) is suggested to
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exhibit only a minor contribution to the bitter taste of orange juice.12 Although the
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bitterness of limonin (1) is reported to be suppressed by sucrose and citric acid,9,13
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systematic studies addressing the question as to how such mixture suppression
69
effects do quantitatively impact the perception of bitter compounds in orange juices
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are lacking.
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Whole orange fruits usually do not contain the bitter-tasting limonin in supra-
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threshold concentrations but contain its precursors, namely limonoate A-ring lactone
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and limonin-17-O-β-D-glucopyranoside (3; Figure 1). After juice preparation from
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early to mid season harvested oranges, limonoate A-ring lactone is reported to be
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gradually converted into the bitter tasting limonin (1) by proton-catalyzed
76
lactonization and/or upon catalysis of limonoid D-ring lactone hydrolase, thus
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imparting the so-called “delayed” bitterness.14-16 Limonoid glucosides such as
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compound 3 are known to be generated from the corresponding aglycone during the
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late stages of fruit growth and maturation and are present in orange juices in an
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aglycone/glucoside ratio of about 1:150.17-19 However, data available on the sensory
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impact of these limonoid glucosides are rather contradictory, e.g. some researchers
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suggest limonin-17-O-β-D-glucopyranoside (3) to contribute to the bitter taste of
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orange juices,20 whereas other publications report this glucoside to be tasteless at
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all.21 Model studies on the degradation of compound 3 under acidic conditions as well
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as during storage of orange juice revealed the formation of limonin (1) besides its
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epimer C17-epilimonin.22 Quantitative data showed evidence for the use of the C17-
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epilimonin/limonin ratio as a suitable marker for the analytical determination of the
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thermal input applied during orange juice manufacturing.22
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Another family of phytochemicals hypothesized in literature as candidate taste
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molecules of orange products is the group of polymethoxylated flavones with the
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general structure displayed in Figure 2. As these polyphenols are mainly located in
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the peel and, in particular, in the flavedo of the fruit,23 their concentration in orange
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juice was reported to be dependent on the extraction method used and pressure
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applied during juicing.24 The highest levels of polymethoxylated flavones were found
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in young developing fruits,25 although a relation of some of these polyphenols to the
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maturation phase of the fruit was suggested.23 Sensory analysis of a chemically
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undefined, crude mixture of polymethoxylated flavones, isolated from orange juice
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concentrate, revealed a taste threshold of 15-46 mg/kg without giving any information
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about the perceived taste quality.26 Overall, these flavones were concluded not to
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contribute to the flavor of orange juice.
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Driven by the need to discover the key players imparting the typical taste of
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foods, the research area “sensomics” has made tremendous efforts in recent years in
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order to analytically decode the so-called sensometabolome, defined as the
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composition of the group of sensory active key molecules “synthesizing” the sensory
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percept of a given food in the brain.27,28 Aimed at identifying the essential molecules
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coding the typical taste signature of orange juice, the objectives of the present
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investigation were, therefore, to systematically identify and quantify the key
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sensometabolites in orange juice, to functionally validate their sensory contribution by
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means of taste re-engineering experiments, and to gain some first insight into their
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natural variation in commercial orange juices.
111 112 113 114
MATERIALS AND METHODS
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Chemicals. The following compounds were obtained commercially: limonin,
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nomilin, hesperidine (Sigma, Steinheim, Germany), scutellarein tetramethylether,
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sinensetin (Extrasynthèse, Lyon, France); formic acid and organic solvents were of
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HPLC
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Taufkirchen, Germany); de-ionized water used for chromatography was purified by
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means of a Milli-Q Gradient A10 system (Millipore, Billerica, USA). For sensory
122
analyses, bottled water (Evian, low mineralization 500 mg/L) was adjusted to pH 4.5
grade
(Merck,
Darmstadt,
Germany);
deuterated
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(Aldrich,
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with 0.1% aqueous formic acid. Juice prepared from Valencia oranges harvested on
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30th November 2003 (sample OJ1), 13th March 2006 (sample OJ2), 10th January
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2004 (sample OJ3), 24th February 2004 (sample OJ4), 30th March 2006 (sample
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OJ5), respectively, were provided by the industry (USA). Orange seeds and orange
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molasses, a concentrated thick viscous liquid and intensely bitter tasting by-product
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of citrus juice extraction, were provided by the industry (USA). Commercial orange
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juices (OJ6 - OJ15) were purchased at a local store in Germany. provided by the
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industry (USA)
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Sequential Solvent Extraction of Orange Juice (OJ1) and Orange
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Molasses. Aliquots (100 g) of orange juice OJ1 and molasses, respectively, were
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centrifuged for 10 min at 11269 × g (Varifuge 20 RS, Heraeus), the supernatant was
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diluted with water (pH 4.5) to a volume of 250 mL, and, then, sequentially extracted
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with pentane, followed by dichloromethane, and ethyl acetate (3 x 300 mL each). The
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corresponding organic layers were combined to give the pentane solubles (fraction I),
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the dichloromethane solubles (fraction II), and the ethyl acetate solubles (fraction III)
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after solvent separation in vacuum, suspension of the residues in water (10 mL), and
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freeze-drying. Freeze-drying of the remaining aqueous layer gave the water soluble
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fraction IV. After determination of the yields (Table 1), fractions I-IV isolated from
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orange juice OJ1 and orange molasses, respectively, were kept at -18° C until used
142
for sensory and chemical analysis.
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HPLC Separation of Fraction II Isolated from Orange Molasses. An aliquot
144
(500 mg) of fraction II isolated from orange molasses was dissolved in a mixture
145
(50/50, v/v; 10 mL) of methanol and aqueous formic acid (0.1% in water) using an
146
ultrasonic bath. After membrane filtration (0.45 µm, Satorius Hannover, Germany),
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aliquots (300 µL) were separated by semi-preparative HPLC on a 250 x 10 mm i.d., 5
148
µm, Microsorb–MV RP-18 column (Varian, Germany). Monitoring the effluent at 272
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nm, 19 fractions, namely II-1 to II-19 (Figure 3), were individually collected into ice-
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cooled glass flasks. The corresponding fractions obtained from 15 HPLC runs were
151
combined, and after the solvent was removed in vacuum and freeze dried twice,
152
were directly used for taste dilution analysis (TDA).
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Isolation of Taste Compounds 1, 2, and 4-13 from Fractions II-8 to II-19.
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An aliquot (1.0 g) of fraction II isolated from orange molasses was dissolved in a
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mixture (50/50; v/v; 10 mL) of methanol and aqueous formic acid (0.1 % in water; pH
156
2.5), membrane filtered, and aliquots (300 µL) were separated by HPLC on a 250 x
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10 mm i.d., 5 µm, Microsorb–MV RP-18 column (Varian, Germany). Monitoring the
158
effluent at 272 nm, chromatography was performed with a mixture (20/80; v/v) of
159
acetonitrile and aqueous formic acid (0.1% in water), increasing the acetonitrile
160
content to 30% within 15 min, then to 40% within 40 min, and, finally, to 100% within
161
additional 15 min, thereafter elution with acetonitrile for 5 min at a flow rate of
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3.5 mL/min. A total of 12 peaks evaluated with high TD-factors were collected and,
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after separating the solvent in vacuum, the residues were freeze-dried twice to give
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the taste compounds limonin (1), nomilin (2), 5,6,7,3´,4´-pentamethoxyflavone
165
(sinensetin, 4), 5,6,7,4´-tetramethoxyflavone (scutellarein tetramethylether, 5),
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5,6,7,8,4´-pentamethoxyflavone (tangeretin, 6), 5,7,8,3´,4´-pentamethoxyflavone (7),
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3,5,7,8,3´,4´-hexamethoxyflavone
168
5,6,7,8,3´,4´-hexamethoxyflavone
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3,5,6,7,8,3´,4´-heptamethoxyflavone
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pentamethoxyflavone (13) in a purity of >99% (HPLC/DAD). LC-MS/MS and 1D/2D-
171
NMR spectroscopic data of 1, 2, and 4-13, respectively, were identical to those
172
reported earlier29-33 and are given as supplementary information.
(8),
3,5,6,7,3´,4´-hexamethoxyflavone
(9),
5,7,8,4´-tetramethoxyflavone
(11),
(10), (12),
and
4´-hydroxy-5,6,7,8,3´-
173
Isolation of Taste Compounds 14 and 15 in Fractions II-3 and II-4. An
174
aliquot (1.2 g) of fraction II isolated from molasses was taken up in a mixture (40/60,
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v/v; 15 mL) of methanol and aqueous formic acid (0.1% in water) and was applied
176
onto the top of a water-cooled 300 x 35 mm i.d. glass column, which was filled with a
177
slurry of RP-18 material (25-40 µm) conditioned with methanol/0.1% aqueous formic
178
acid (40/60, v/v). Chromatography was performed with a flow rate of 2 mL/min using
179
mixtures of methanol and 0.1% aqueous formic acid to give fractions II-A (40/60, v/v;
180
100 mL), II-B (50/50, v/v; 100 mL), II-C (60/40, v/v; 100 mL), II-D (70/30, v/v;
181
100 mL), II-E (80/20, v/v; 100 mL), followed by methanol (100 mL) and acetonitrile
182
(100 mL) to afford fractions II-F and II-G, respectively. After separating the individual
183
fractions from solvent in vacuum, the residues were analysed by analytical RP-
184
HPLC. Fractions II-A to II-D, containing the taste compounds located in HPLC/TDA
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fractions II-3 and II-4, were combined, taken up in water (20 mL) and separated by
186
gel adsorption chromatography (GAC) on a 380 x 40 mm i.d. column (Amersham
187
Pharmacia Biotech, Sweden) filled with a slurry of Sephadex LH-20 (Amersham
188
Pharmacia Biotech) in 0.1% aqueous formic acid. Chromatography was performed at
189
a flow rate of 2 mL/min starting with aqueous formic acid (0.1% in water, 250 mL),
190
followed by mixtures of methanol and 0.1% aqueous formic acid: 20/80 (v/v, 250 mL),
191
40/60 (v/v, 250 mL), 60/40 (v/v, 250 mL), and 80/20 (v/v, 250 mL). Monitoring the
192
effluent at 272 nm, a total of 12 GAC fractions were collected (30 min each),
193
separated from solvent in vacuum, and freeze dried twice. GAC fractions no. 3 and 4,
194
containing the taste compounds located in HPLC/TDA fractions II-3 and II-4, were
195
dissolved in a mixture (50/50; v/v) of methanol and aqueous formic acid (0.1% in
196
water), membrane filtered, and aliquots (250 µL) were separated by means of semi-
197
preparative HPLC on a 250 x 10 mm i.d., 5 µm, RP-18 column (Varian, Germany).
198
Monitoring the effluent at 272 nm, chromatography was performed at a flow rate of
199
3.5 mL/min with a mixture (10/90, v/v) of acetonitrile and aqueous formic acid (0.1%
200
in water), increasing the acetonitrile content to 20% within 10 min, holding for 20 min,
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then increasing to 30% within 20 min, and, finally, to 100% within 2 min. The two
202
main peaks were collected, the solvents separated in vacuum, and the remaining
203
residue
204
pentamethoxyflavone-3-O-ß-D-glucopyranoside
205
hexamethoxyflavone-3-O-ß-D-glucopyranoside
206
(HPLC/ELSD, HPLC/MS, 1H NMR). The spectroscopic data of 15 were well in line
207
with those reported earlier34 and are given as supplementary information.
208
5,6,7,3´,4´-Pentamethoxyflavone-3-O-β-D-glucopyranoside,
of
each
fraction
was
freeze-dried
twice
to
(14) (15)
in
give
and a
5,6,7,3´,4´5,6,7,8,3´,4´-
purity
14,
of
>
99%
Figure
2:
209
LC/MS (ESI+): m/z 551 ([M+H]+), 573 ([M+Na]+), 389 ([M-Hexose+H]+); 1H NMR (400
210
MHz, DMSO-d6, COSY): δ [ppm] 3.11 [m, 2H, H-C(3´´), H-C(5´´)], 3.24 [m, 2H, H-
211
C(2´´), H-C(4´´)], 3.38 [m, 1H, H-C(6´´)], 3.57 [d, 1H, J=10.0 Hz, H-C(6´´)], 3.77 [s,
212
3H, H-C(7a)], 3.83 [s, 3H, H-C(5a)], 3.84 [s, 3H, H-C(3´a)], 3.85 [s, 3H, H-C(4´a)],
213
3.94 [s, 3H, H-C(6a)], 5.51 [d, 1H, J=7.4 Hz, H-C(1´´)], 7.11 [d, 1H, J=8.5, H-C(5´)],
214
7.19 [s, 1H, H-C(8)], 7.66 [dd, 1H, J=2.0, 8.5 Hz, H-C(6´)], 7.94 [d, 1H, J=2.0 Hz, H-
215
C(2´)];
216
57.2 [C(6a)], 61.1 [C(6´´)], 61.4 [C(7a)], 62.4 [C(5a)], 70.2-78.2 [C(3´´), C(5´´)], 74.7
217
[C(2´´)], 77.3 [C(4´´)], 97.8 [C(8)], 101.2 [C(1´´)], 111.7 [C(5´´)], 112.6 [C(10)], 113.2
218
[C(2´)], 122.2 [C(6´)], 137.1 [C(3)], 140.1 [C(6)], 149.0 [C(3´)], 150.0 [C(4´)], 153.2
219
[C(5)], 157.9 [C(7)], 172.4 [C(4)].
13
C NMR (100 MHz, DMSO-d6, HMQC, HMBC): δ [ppm] 55.9 [C(3´a), C(4´a)],
220
Identification of the Carbohydrate Moiety in Glycosides. An aliquot (200-
221
500 µg) of each purified glycoside was taken up in water (500 µL), hydrochloric acid
222
(4.0 mol/L; 500 µL) was added, and the mixture was heated at 110° C. After 1 h, the
223
solution was diluted with water (500 mL) and analysed for monosaccharides by
224
means of high-performance ion chromatography (HPIC) as reported earlier.35
225
Isolation of Limonoides (1, 2, 22) and Their Glucosides (3, 16-21) from
226
Orange Seeds. Orange seeds (100 g) were washed with water (2 x 150 mL) and
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acetone (2 x 150 mL), frozen in liquid nitrogen, ground in a laboratory mill, and then
228
refluxed with methanol (2 x 300 mL) for 4 h at 60° C. The organic layer was filtered,
229
the solvent was removed in vacuum to obtain a crude isolate, which was taken up in
230
water (10 mL) and, then, applied onto the top of a water-cooled 300 x 35 mm i.d.
231
glass column filled with an aqueous slurry of Amberlite XAD-2 (BDH Chemicals Ltd,
232
Poole, England). Chromatography was performed with a flow rate of 5 mL/min using
233
the following sequence of solvents: water (200 mL; XAD fraction I), methanol/water
234
mixtures 20/80 (v/v, 200 mL; XAD fraction II), 40/60 (v/v, 200 mL; XAD fraction III),
235
60/40 (v/v, 200 mL, fraction IV, 80/20 (v/v, 200 mL, XAD fraction V), followed by
236
methanol (200 mL; XAD fraction VI). The individual XAD fractions I-VI were
237
separated from solvent in vacuum, followed by freeze-drying. XAD fractions I-IV and
238
V/VI, respectively, were dissolved in a 20/80 and 90/10 (v/v) mixture of methanol and
239
aqueous formic acid (0.1% in water), and, after membrane filtration, aliquots (250 µL)
240
were fractionated by means of semi-preparative HPLC on a 250 x 10 mm i.d., 5 µm,
241
Microsorb – MV RP-18 column (Varian, Germany). Monitoring the effluent by means
242
of UV detection (220 nm) and evaporative light scattering detection (ELSD), the XAD
243
fractions I-IV were separated at a flow rate of 3.5 mL/min with a gradient system
244
starting with a mixture (10/90, v/v) of acetonitrile/0.1% aqueous formic acid,
245
increasing the acetonitrile content to 20% within 15 min, holding for 10 min,
246
increasing to 30% within 15 min and, then, to 100% within another 5 min, thereafter
247
keeping the mobile phase constant for 5 min. A total of seven peaks were collected
248
into ice-cooled flasks. To gently remove formic acid, the collected fractions were put
249
on top of Strata C 18-E cartridges (10 g, 60 mL, 55 µm, Giga Tubes, Phenomenex,
250
Germany) pre-conditioned with water. After flushing the cartridges with water (150
251
mL), the cartridges were dried by a stream of nitrogen and, then, the target
252
compounds were eluted with methanol. The solvent was separated under vacuum,
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the residue of each fraction was taken up with water (5 mL) and freeze-dried twice to
254
afford limonin-17-O-β-D-glucopyranoside (3, Figure 1), deacetylnomilinic acid 17-O-
255
β-D-glucopyranoside (16), epiisoobacunoic acid 17-O-β-D-glucopyranoside (17),
256
methyldeacetylnomilinate
257
glucopyranoside
258
opbacunone 17-O-β-D-glucopyranoside (21) as white amorphous powders in a purity
259
of >99 % (HPLC/ELSD, HPLC/MS, 1H NMR).
(19),
17-O-β-D-glucopyranoside nomilinic
acid
(18),
nomilin
17-O-β-D-glucopyranoside
17-O-β-D(20),
and
260
XAD fractions V and IV were combined and separated using the same
261
chromatographic system, but another gradient system operated with a flow rate of
262
3.5 mL/min: starting with a mixture (30/70, v/v) of acetonitrile/0.1% aqueous formic
263
acid, the acetonitrile content was increased to 40% within 15 min, then to 80% within
264
10 min, and, finally, to 100% within additional 10 min. The effluent of three peaks
265
were collected and separated from solvent in vacuum to give limonin (1), nomilin (2),
266
and deacetylnomilin (22) in a purity of >99% (HPLC/ELSD, HPLC/MS, 1H NMR).
267
Sensory Analyses. General Conditions, Panel Training. Twelve subjects (six
268
women and six men, age 25-35 years), who have given the informed consent to
269
participate the sensory tests of the present investigation and have no history of
270
known taste disorders, participated for at least two years in weekly training sessions
271
to recognize the taste of aqueous solutions of reference compounds using nose-clips
272
as reported earlier.27,28,36-39 Prior to sensory analysis, the fractions or isolated
273
compounds were confirmed to be essentially free of solvents and buffer compounds
274
used as reported earlier.27,28 In order to minimize the uptake of any compound to the
275
best of our knowledge, all the sensory analyses were performed by using the sip-
276
and-spit method, that means the test materials were not swallowed but expectorated.
277
Taste Profile Analysis. Whilst wearing nose-clips, aliquots (5 mL) of orange
278
juice and, after 1+1 dilution with water, orange molasses were presented to the
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sensory panelists who were asked to briefly swirl around the sample in the mouth for
280
5 s and, then, to expectorate. Using this sip-and-spit method, the panelists were
281
asked to score the taste qualities astringent, bitter, sour, and sweet on a scale from 0
282
(not detectable) to 5.0 (strong taste impression) (Table 1).
283
Taste Dilution Analysis (TDA). HPLC fractions were dissolved in bottled water
284
(8 mL; pH 4.5) in their “natural” concentration ratio, that means in the ratios as they
285
were obtained from orange juice, and, then, stepwise diluted 1+1 with bottled water
286
(pH 4.5). The serial dilutions of each fraction were presented in order of ascending
287
concentrations to the trained sensory panellists who were asked to evaluate the taste
288
impressions and to determine the dilution at which a taste difference between the
289
diluted extract and the blank (control) could just be detected by means of a half-
290
tongue-test. These so-called taste dilution (TD)-factors40 evaluated by four different
291
assessors in three different sessions did not differ by more than plus/minus one
292
dilution step and were averaged (Table 2).
293
Recognition Threshold Concentrations. Bitter recognition thresholds were
294
determined using a whole mouth sip-and-spit approach based on an ascending
295
three-alternative forced-choice procedure with bottled water (pH 4.5) as the solvent
296
as reported earlier.27,37,39 Threshold concentrations of astringent compounds were
297
determined in bottled water (pH 4.5) by means of the so-called half-tongue test using
298
ascending 1+1 dilutions of the samples following the procedure reported earlier.36
299
The threshold value of the sensory group was approximated for each compound by
300
averaging the threshold values of the individuals (geometric mean) in three
301
independent sessions (Table 3). Values between individuals and separate sessions
302
did not differ more than plus or minus one dilution step; that is, a threshold value of
303
24 µmol/L for 4 represents a range of 12-48 µmol/L.
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304
Taste Re-Engineering Experiments. To prepare basic taste recombinants
305
(expt. 1), hesperidin, carbohydrates, and organic acids summarized in groups III-V
306
(Table 4) were dissolved in their “natural” concentrations in bottled water and the pH-
307
value of this solution was adjusted to 3.5 by the addition of trace amounts of an
308
aqueous formic acid solution (1.0 mol/L). Towards stepwise preparation of the total
309
taste recombinant, the basic taste recombinant solution was spiked with the limonoid
310
glucosides 3, 16, 17, and 19-21 (expt. 2), with limonoid glucosides 3, 16, 17, and 19-
311
21 and limonoid aglycons 1 and 2 (expt. 3), with limonoid glucosides 13, 16, 17, and
312
19-21 and polymethoxylated flavones 4-10, 12, 14, and 15 (expt. 4), and with
313
limonoid aglycons 1 and 2, limonoid glucosides 3, 16, 17,and 19-21, as well as the
314
polymethoxylated flavones 4-10, 12, 14, and 15 (expt. 5). Each taste recombinant
315
was sensorialy evaluated and compared to authentic orange juice by means of taste
316
profile analysis.
317
Dose/Response Recordings. Serial 1:1 dilutions of (i) limonoid aglycons (1, 2),
318
(ii) glucoside 3, (iii) polymethoxylated flavones (4-8, 10, 11, 12, and 15), (iv) mixtures
319
of limonin (1) and polymethoxylated flavones (4-7, 9, 10, 12, and 15; each in its
320
“natural” concentration ratios as given in Table 4; 0.25, 0.50, and 0.75 mg/100 g),
321
and (v) mixtures of nomilin (2) and polymethoxylated flavones (4-7, 9, 10, 12, and 15;
322
in their concentration ratio as present in orange juice; Table 4; 0.50 mg/100 g) were
323
used to perform human dose-response recordings following the procedure reported
324
earlier.38,39 Using a half-tongue tasting procedure, the solutions of the individual
325
compounds were applied in binary combinations to one side of the tongue and
326
panelists were asked to determine which side showed the stronger sensation. In
327
order to assess reliable taste intensities of compounds, the taste intensity of the
328
individual compounds were compared on a five-point scale (0.25 scale units) to
329
reference solutions containing caffeine in concentrations of 0.75 (0), 1.5 (0.25), 3.0
13 Environment ACS Paragon Plus
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330
(0.5), 6.0 (1.0), 12 (2.25), 24 (3.0), 48 (3.5), and 96 mmol/L (4.5) with pre-defined
331
bitter intensities given in parenthesis. After the taste intensity of each compound at its
332
maximum solubility had been rated, the taste intensities of the other dilutions were
333
determined by using the half-tongue tasting method so that one dilution of an
334
individual compound was rated against the intensity of another dilution of the same
335
compound, and by cross-checking the taste intensity between the different
336
compounds at the same dilution. The dose/response functions of three individual
337
sessions were averaged. Human response functions with dose-over-threshold (DoT)-
338
factors on the x-axis and taste intensities on the y-axis were recorded for each
339
individual subject in triplicates.
340
Bitterness Enhancement of Limonin Solutions by Polymethoxylated Flavones.
341
Solutions of limonin (16 µmol/L) spiked with individual flavones (4-8, 10, 11, 12, and
342
15; 0.5 mg/100 g) were presented to the sensory panelists who were asked to rate
343
the bitter taste intensity against a reference solution containing limonin (16.0 µmol/L).
344
The sensory data were determined as the mean of triplicates.
345
Sensory Interactions between Sugars, Limonoids, and Polymethoxylated
346
Flavones. A matrix A of 81 test samples was prepared with varying levels (6.1, 6.6,
347
7.1, 7.6, 8.1, 8.6, 9.1, 9.6, 10.1 g/100 g) of a glucose/fructose/sucrose mixture (molar
348
ratio 1.02:1.23:1.18 as found for OJ1, Table 4) and varying concentrations (0.25,
349
0.55, 0.65, 0.75, 0.85, 1.25, 1.65, 2.05, 2.45, and 2.85 mg/100 g) of
350
polymethoxylated flavones (4-10, 12, 14, and 15, in “natural” concentration ratios as
351
given for OJ1 in Table 4) while the levels of a limonin/nomilin mixture (molar ratio
352
1:2; 0.6 mg/100g), citric acid (640 mg/g), and malic acid (170 mg/100 g) were kept
353
constant. In addition, two additional sets of 56 test samples (matrices B and C) were
354
prepared with varying levels of the glucose/fructose/sucrose mixture detailed above
355
and varying concentrations (0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 mg/100 g) of a
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limonin/nomilin mixture (molar ratio 1:2), while the concentration of citric acid (640
357
mg/g) and malic acid (170 mg/100 g) was kept constant and that of flavones adjusted
358
to 1.1 (matrix B) or 0.6 mg/100 g (matrix C). Randomly selected samples out of the
359
matrices A and B were presented to the trained sensory panelists who were asked to
360
judge the bitter taste intensity of each test solution on a scale from 0 (no taste
361
observed) to 10 (extremely intense bitterness) by comparing it with an ascending
362
dilution series of limonin as the reference. To avoid fatigue of the panelists, a
363
maximum of eight solutions per day were presented to the sensory panel. The data
364
were calculated as the means of duplicates, intensity values between trained
365
individuals and separate sessions did not differ more than plus or minus 0.4 units.
366
Quantitative Analysis. Polymethoxylated Flavones. For quantitative analysis
367
of compounds 4-13, an aliquot of orange juice (15 g) was spiked with an internal
368
standard solution (1 mL) of 4′-methoxyflavone (16 mg) in methanol/water (80/20, v/v;
369
100 mL), diluted with water to about 25 mL, and equilibrated for 30 min upon
370
ultrasonification. After centrifugation (15 min, 1006 × g), an aliquot (10 mL) of the
371
supernatant was applied on top of a water-conditioned Strata C 18-E, 55 µm, RP-18-
372
cartridge (10 g, 60 mL, Giga Tubes, Phenomenex), followed by elution with water (30
373
mL). The target analytes were then eluted with methanol (40 mL), the solvent was
374
separated in vacuum, the residue was taken up in water/methanol (80/20, v/v; 1.0
375
mL), and an aliquot (20-30 µL) was analyzed by means of HPLC-DAD on a 125 x
376
4.6 mm i.d., 5 µm, Microsorb – MV 100-5C 18 column (Varian, Germany). Operating
377
with a flow rate of 1.0 mL/min and monitoring the effluent at 256 nm, chromatography
378
was performed by means of a solvent gradient using 0.03 % aqueous formic acid
379
(solvent A) and acetonitrile (solvent B): 0 min (A/B, 80/20, v/v) - 15 min (A/B, 70/30,
380
v/v) - 52 min (A/B, 62/38, v/v) - 60 min (A/B, 40/60, v/v) - 65 min (A/B, 40/60, v/v) - 67
381
min (A/B, 0/100, v/v). Quantitation was performed by means of HPLC-DAD with
15 Environment ACS Paragon Plus
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382
external 5-point calibration using standard solutions of polymethoxylated flavones in
383
concentrations between 0.1 and 30.0 mg/L.
384
For the quantitative analysis of the flavone glucosides 14 and 15, orange juice
385
(25 g) was made up to 50 mL with methanol, ultrasonificated for 20 min, centrifuged
386
(15 min, 1006 × g), membrane filtered, and, then, an aliquot (5 µL) of the supernatant
387
was analyzed by means of HPLC-MS/MS in the ESI+ mode on a 150 x 2 mm, 4 µm,
388
Synergi Fusion column (Phenomenex) equipped with a 4 x 2 mm pre-column of the
389
same type operated at a flow rate of 250 µL/min. Using aqueous formic acid (0.1 %)
390
as solvent A and acetonitrile as solvent B, chromatography was done as follows: 0
391
min (A/B, 60/40, v/v) – 10 min (A/B, 40/60, v/v) – 12 min (A/B, 40/60, v/v) – 15 min
392
(A/B, 0/100, v/v). Compounds 14 (m/z 551.3→389.2; +90/+30/+15) and 15 (m/z
393
581.3→419.2; +90/+30/+15) were analyzed in the positive electrospray ionization
394
mode (ESI+) using the mass transitions and declustering potential (DP, in V), collision
395
energy (CE, in V), and cell exit potential (CXP, in V) given in parentheses. Analysis
396
was performed with external 5-point-standard calibration with standard solutions of
397
14 and 15 in concentrations between 0.70 and 14.7 mg/L.
398
Limonoid Aglycones 1 and 2. Orange juice (20 g) was made up to 50 mL with
399
methanol, ultrasonificated for 20 min, centrifuged (10 min, 1006 × g), membrane
400
filtered, and, then, an aliquot (5 µL) was analyzed by means of HPLC-MS/MS in the
401
ESI+-mode on a 150 x 2 mm, 4 µm, Synergi Fusion column (Phenomenex) equipped
402
with a 4x2 mm pre-column of the same type operated at a flow rate of 250 µL/min.
403
Using aqueous formic acid (0.1 %) as solvent A and acetonitrile as solvent B,
404
chromatography was done as follows: 0 min (A/B, 85/15, v/v) – 5 min (A/B, 55/45,
405
v/v) – 15 min (A/B, 30/70, v/v) – 18 min (A/B, 0/100, v/v). Compounds 1 (m/z
406
471.0→425.0; +40/+30/+15) and 2 (m/z 515.0→469.0; +70/+30/+15) were analyzed
407
in the ESI+ mode using the mass transitions and declustering potential (DP, in V),
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408
collision energy (CE, in V), and cell exit potential (CXP, in V) given in parentheses.
409
Analysis was performed with external 6-point-standard calibration with standard
410
solutions of 1 and 2 in concentrations between 0.3 and 12.0 mg/L.
411
Limonoid Glucosides 3, 16, 17, 19-21. Orange juice (20 g) was made up to 100
412
mL with methanol, ultrasonificated for 20 min, 1:20 diluted with water, centrifuged (10
413
min, 3000 rpm), membrane filtered, and, then, an aliquot (5 µL) of the supernatant
414
was analyzed by means of HPLC-MS/MS in the ESI--mode on a 150x2 mm, 4 µm,
415
Synergi Fusion column (Phenomenex) equipped with a 4x2 mm pre-column of the
416
same type operated at a flow rate of 250 µL/min. Using aqueous formic acid (0.1 %)
417
as solvent A and acetonitrile as solvent B, chromatography was done as follows: 0
418
min (A/B, 60/40, v/v) – 10 min (A/B, 45/55, v/v) – 13 min (A/B, 0/100, v/v) – 18 min
419
(A/B, 0/100, v/v). The target compounds were analyzed in the negative electrospray
420
ionization mode (ESI-) using the mass transitions and declustering potential (DP, in
421
V), collision energy (CE, in V), and cell exit potential (CXP, in V) given in
422
parentheses: 3 (m/z 649.0→605.0; -70/-30/-15), 16 (m/z 669.0→609.0; -50/-40/-15),
423
17 (m/z 651.0→591.0; -60/-40/-15), 19 (m/z 693.0→59.0; -80/-102/-15), 20 (m/z
424
711.0→59.0; -65/-100/-15), 21 (m/z 633.0→59.0; -80/-86/-15). Analysis was
425
performed with external 6-point-standard calibration with standard solutions of each
426
limonoid glucoside in concentrations between 0.06 and 4.2 mg/L.
427
Hesperidin (23). Orange juice (10 g) was centrifuged (15 min, 1006 × g), the
428
residue was suspended in an aqueous ammonium oxalate solution (0.025 mol/L; 10
429
mL), followed by centrifugation, and was then extracted with dimethyl formamide (10
430
mL), followed by centrifugation. The combined supernatants were made up to 50 mL
431
with aqueous formic acid (0.01 mol/L in water) and an aliquot (20 µL) was analyzed
432
by means of HPLC-DAD on a 125 x 4.6 mm i.d., 5 µm, Microsorb – MV 100-5C 18
433
column (Varian, Germany). Operating with a flow rate of 1.0 mL/min and monitoring
17 Environment ACS Paragon Plus
Journal of Agricultural and Food Chemistry
434
the effluent at 280 nm, chromatography was performed isocratically with a mixture
435
(79/21, v/v) of 0.1 % aqueous formic acid and acetonitrile. Quantitation was
436
performed by means of HPLC-DAD with external 5-point-calibration using hesperidin
437
standard solutions in concentrations between 20.0 and 140.0 mg/L.
438
Organic Acids and Carbohydrates. Organic acids and carbohydrates were
439
quantitated by means of high-performance ion chromatography (HPIC) on a ICS-
440
2500 ion chromatography system (Dionex, Idstein, Germany) equipped with a 250 x
441
2 mm IonPac AS11-HC column (Dionex) and a 250 x 2 mm CarboPac PA-10 column
442
(Dionex), respectively, following the protocols reported earlier.35
443
High Performance Liquid Chromatography (HPLC). For isolation of
444
polymethoxylated flavones, the HPLC apparatus (Jasco, Groß-Umstadt, Germany)
445
consisted of two pumps (PU 2086/2087), a gradient mixer (1000 µL), a Rheodyne
446
injector (250 or 300 µL loop), and a multiwavelength detector (MD 2010plus, Jasco,
447
Germany) monitoring the effluent in a wavelength range between 220 and 500 nm.
448
For isolation of limonoides and their glucosides, the HPLC apparatus consisted of
449
two pumps (Sykam, S 1122), a gradient mixer (Sunchrom, dynamic/statistic gradient
450
mixer), a Rheodyne injector (250 µL loop), an autosampler (Spark, Midas 380), a
451
diode array detector (Sunchrom, SpectraFlow 600 DAD), monitoring the effluent in a
452
wavelength range between 200 and 600 nm, a splitting module (Upchurch, P-470
453
graduated microsplitter), and a type Sedex 85 LT-ELSD evaporative light scattering
454
detector (S.E.D.E.R.E.) equipped with a nebulizer (p = 2,6 bar, T = 40 °C, 2.5-
455
200 µL/min). For quantitative analysis of polymethoxylated flavones and hesperidine,
456
an analytical HPLC (Merck Hitachi, Eching) was equipped with two type L-7100a
457
pumps, a type L-7612a degasser, a type L-7200 autosampler, and a type L-7450
458
diode array detector (DAD).
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459
Liquid Chromatography/Mass Spectrometry (LC/MS). Electrospray ion-
460
ization (ESI) spectra of target compounds were acquired on an API 4000 Q-Trap
461
LC/MS/MS system (AB Sciex Instruments, Darmstadt, Germany) with direct loop
462
injection of the sample (2-20 µL). For HPLC-MS/MS analyses the API 4000 Q-Trap
463
LC/MS/MS system was coupled to an Agilent 1100 HPLC system equipped with a
464
150 x 2 mm, 4 µm, Synergi Fusion column (Phenomenex) and a 4 x 2 mm pre-
465
column of the same type. Ionization was performed by means of electrospray
466
ionization (ESI). The spray voltage was set at –4500 V in the ESI--mode and at 5500
467
V in the ESI+-mode. Nitrogen served as curtain gas (20 psi), the declustering
468
potential was set at –10 to –30 V in the ESI- mode and 30-40 V in the ESI+-mode.
469
The mass spectrometer was operated in the full scan mode monitoring positive or
470
negative ions. Fragmentation of [M-H]- and [M+H]+ molecular ions into specific
471
product ions was induced by collision with nitrogen (4 x 10-5 torr) and a collision
472
energy of -20 to -70 V or 20 to 60 V.
473
Nuclear Magnetic Resonance Spectroscopy (NMR). Samples were dissolved
474
in DMSO-d6 or MeOD-d4 with tetramethylsilane as internal standard, placed into 178
475
x 5 mm i.d. NMR tubes (Schott Professional), and were then analyzed by means on a
476
DPX 400 NMR spectrometer (Bruker, Rheinstetten, Germany). Data processing was
477
performed by using the NMR Software Mestre-C.
478 479 480
RESULTS AND DISCUSSION
481 482
To identify the compounds responsible for the typical taste of orange juice,
483
orange juice OJ1, prepared from Valencia late oranges harvested in November 2003,
19 Environment ACS Paragon Plus
Journal of Agricultural and Food Chemistry
484
was presented to trained sensory panelists who were asked to rate the intensities of
485
the taste descriptors sweet, sour, bitter, and astringent on a linear scale from 0 (not
486
detectable) to 5 (strongly detectable). High scores were found for the intensities of
487
sourness (2.3) and sweetness (2.0), followed by bitterness and astringency judged
488
with somewhat lower intensities of 1.8 and 1.1, respectively (Table 1). To investigate
489
the hydrophobicity of the key taste molecules, juice OJ1 was separated by means of
490
sequential solvent fractionation.
491
Solvent Fractionation of Orange Juice OJ1. Orange juice was extracted
492
with solvents of increasing polarity to obtain the pentane soluble fraction I, the
493
dichloromethane soluble fraction II, the ethyl acetate soluble fraction III and the
494
remaining aqueous fraction IV, which were freed from solvent in high-vacuum, taken
495
up in water in their “natural” concentration and, after adjustment to pH 4.5, were
496
presented to the trained sensory panel for taste profile analysis (Table 1). Fraction IV
497
was judged with the highest scores for sweetness (2.5) and sourness (1.5), whereas
498
bitterness (0.5) and astringency (0.4) were only marginally detected. In comparison,
499
the taste profile of fraction II was dominated by an intense bitter taste (3.3)
500
accompanied by velvety-like astringent mouthfeel (1.1). Also fraction III exhibited
501
some pronounced bitterness (1.1) and astringency (1.0) with only low ratings for
502
sourness (0.9) and sweetness (0.1). In comparison, the most hydrophobic fraction I
503
showed only faint taste activity.
504
Although fraction II showed by far the highest impact for bitterness and
505
astringency, the yield of this fraction isolated from orange juice were too low to allow
506
straightforward structure elucidation of the key taste molecules by means of NMR
507
spectroscopy. As preliminary taste profile analyses revealed orange molasses, a
508
main by-product of the citrus industry, as a rich source for bitter and astringent
509
compounds judged with intensities of 4.1 and 3.1 (Table 1), orange molasses was
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Journal of Agricultural and Food Chemistry
510
separated by means of sequential solvent extraction in order to obtain the candidate
511
bitter and astringent compounds in higher amounts. Matching well with the sensory
512
analysis of the fractions isolated from orange juice, fraction II isolated from orange
513
molasses showed high scores for bitterness (4.5) and astringency (2.4).
514
Comparison of the RP-HPLC chromatogram of fraction II isolated from orange
515
juice with that obtained from orange molasses demonstrated that the presence of the
516
same compounds in both bitter fractions, just differing in the quantity of the individual
517
molecules (data not shown). Due to the higher yield of fraction II from orange
518
molasses, the isolation and structure determination of bitter and astringent
519
compounds was performed using fraction II from orange molasses.
520
Identification of Sensometabolites in Fraction II. To sort out the strongly
521
taste-active compounds from the bulk of less taste-active or tasteless substances,
522
fraction II was separated by RP-HPLC to give 19 fractions (II/1 - II/19; Figure 3),
523
which were collected individually, freeze-dried, taken up in water in their “natural”
524
concentrations and, then, used for taste dilution analysis (Table 2). Although each
525
individual fraction exhibited some bitterness and astringency, fractions II/3, II/8-11,
526
II/14, and II/16 were judged with the highest TD-factors ranging from 128 to 512 for
527
velvety-like astringency. In addition, fractions II/8-12 and II/16 were evaluated as the
528
most intense bitter fractions showing bitter taste even in a dilution of 1:32 to 1:256,
529
respectively. Due to their taste impact, the following identification experiments were
530
targeted towards the key taste compounds in fractions II/3, II/4, and II/8-19.
531
HPLC-DAD, HPLC-ELSD, LC-MS, and NMR analysis, followed by co-
532
chromatography with reference materials led to the identification of the triterpene
533
lactones limonin (1) and nomilin (2) as key bitter compounds in HPLC fractions II/12
534
and II/17 and the polymethoxylated flavones 5,6,7,3′,4′-pentamethoxyflavone (4),
535
5,6,7,4′-tetramethoxyflavone
(5),
and
5,6,7,8,4′-pentamethoxyflavone
21 Environment ACS Paragon Plus
(6)
as
Journal of Agricultural and Food Chemistry
536
astringent and bitter tastants in fractions II/10, II/15, and II/18, respectively (Table 2).
537
Moreover, LC-MS/MS,
538
correlation experiments (HMQC, HMBC) revealed the unequivocal identification of
539
5,7,8,3′,4′-pentamethoxyflavone (7, fraction II/8), 3,5,7,8,3′,4′-hexamethoxyflavone
540
(8, fraction II/9), 3,5,6,7,3′,4′-hexamethoxyflavone (9, fraction II/1), 5,6,7,8,3′,4′-
541
hexamethoxyflavone (10, fraction II/14), 5,7,8,4′-tetramethoxyflavone (11, fraction
542
II/13), 3,5,6,7,8,3′,4′-heptamethoxyflavone (12, fraction II/16), and 4′-hydroxy-
543
5,6,7,8,3′-pentamethoxyflavone (13, fraction II/19), respectively. Although 4-13 were
544
identified earlier in literature,29-32 their bitter taste has not yet been reported.
1
H/13C NMR, homonuclear (COSY) and heteronuclear
545
Due to their low yield and impurity, tastants 14 and 15 isolated from fractions
546
II/3 and II/4 were enriched by means of flash-chromatography on RP-18 and gel
547
adsorption chromatography on Sephadex LH-20, and then purified by re-
548
chromatography (RP-HPLC). LC/MS analysis of the compound 14 revealed the
549
pseudomolecular ions m/z 551 ([M+H]+), 573 [M+Na]+, and 589 [M+K]+, thus
550
indicating a molecular weight of 550 Da. MS/MS fragmentation experiments with the
551
[M+H]+ ion demonstrated the cleavage of a hexose moiety by the loss of 162 amu.
552
The 1H NMR as well as the COSY spectrum indicated a total of 25 protons, four of
553
which resonated between 7.11 and 7.94 ppm as expected for aromatic protons and
554
five signals, each integrating for three protons, showed chemical shifts between 3.77
555
and 3.94 ppm as expected for methoxy groups. The remaining protons observed
556
between 3.11 and 3.57 ppm as well as at 5.51 ppm indicated the presence of a
557
hexose moiety linked at position 3 of the methoxyflavone skeleton. For final
558
identification of the hexose moiety, sugar analysis was performed in an acidic
559
hydrolysate of 14 by means of high-performance ion chromatography (HPIC).
560
Comparison of retention time and co-chromatography with a glucose reference
561
revealed glucose as the sugar moiety and led to the identification of 5,6,7,3′,4′-
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562
pentamethoxyflavone-3-O-β-D-glucopyranoside (14, Figure 2), which to the best of
563
our knowledge has not been previously reported in literature. Rather similar NMR
564
data were obtained from compound 15, just lacking one aromatic proton and
565
exhibiting an additional methoxy group at position 8. Acidic hydrolysis, followed by
566
HPIC analysis, as well as comparison of spectroscopic data with those reported in
567
literature34
568
hexamethoxyflavone-3-O-β-D-glucopyranoside (15).
revealed the astringent and bitter tastant 15 as
5,6,7,8,3′,4′-
569
Isolation of Limonoids and their Glucosides. Compared to fraction II, the
570
bitter and astringent tasting compounds in orange juice fraction III were expected to
571
be more hydrophilic, but their low amounts did not allow a straightforward structure
572
elucidation. As preliminary LC-MS screening experiments located limonin-17-O-ß-D-
573
glucopyranoside in fraction III, the following experiments were targeted towards the
574
isolation of a series of limonoid glycosides from orange seeds as these are known as
575
a rich source of limonoid glucosides.41,42 Once isolated, these glucosides should be
576
used as reference materials for sensory analysis and as external standards for their
577
accurate quantitative analysis in orange juice.
578
A methanol extract prepared from freshly ground orange seeds was separated
579
on an XAD resin, followed by RP-HPLC purification to afford a total of three limonoid
580
aglycones and seven glucosides (Figure 4). By means of HPLC-MS/MS and 1D/2D-
581
NMR experiments and comparison with literature data, these bitter and astringent
582
compounds were identified as limonin 17-ß-D-glucopyranoside, 3,17 deacetylnomilinic
583
acid
584
glucopyranoside, 17,43 methyldeacetylnomilinic acid 17-O-ß-D-glucopyranoside, 18,21
585
nomilin-17-O-ß-D-glucopyranoside, 19,20 nomilinic acid 17-O-ß-D-glucopyranoside,
586
20,43 obacunon 17-O-ß-D-glucopyranoside, 21,20 deacetylnomilin, 22,21 limonin (1),
587
and nomilin (2), respectively (Figure 1).
17-O-ß-D-glucopyranoside,
16,43
epiisoobacunoic
23 Environment ACS Paragon Plus
acid
17-O-ß-D-
Journal of Agricultural and Food Chemistry
588
Identification of Sensometabolites in Orange Juice Fractions III and IV.
589
RP-HPLC/DAD and RP-HPLC-MS/MS analysis, followed by co-chromatography with
590
the reference compounds isolated from orange seeds and molasses, respectively,
591
led to the identification of the limonoid aglycones 1 and 2, the limonoid glucosides 3,
592
16, 17, and 19 - 21, and low amounts of the polymethoxylated flavones 4-10, 12, 14,
593
and 15 in the astringent and bitter tasting orange juice fraction III. In addition,
594
degustation experiments revealed an intense astringent orosensation imparted by the
595
major peak detected at 280 nm. By comparison of chromatographic and LC-MS/MS
596
data with those reported in the literature,44 followed by co-chromatography with the
597
reference compound, this astringent compound was identified as hesperidin (23).
598
Sensory analysis revealed a recognition threshold of 11 µmol/L for the astringent
599
sensation induced by this flavanone glycoside.
600
In order to analyze the major compounds imparting the sour and sweet taste
601
impression induced by juice fraction IV, organic acids and carbohydrates were
602
analyzed by means of HPIC. Citric acid, malic acid, and isocitric acid were identified
603
as major organic acids and glucose, fructose, and sucrose as predominating
604
carbohydrates.
605
Sensory Activity of Astringent and Bitter Sensometabolites. After
606
checking the purity of all compounds (HPLC-MS, 1H NMR), recognition threshold
607
concentrations for bitter taste and the astringent oral sensation were determined in
608
aqueous solutions by means of a three-alternative forced choice test and a half-
609
tongue test, respectively (Table 3). The polymethoxylated flavones 4-15 imparted a
610
velvety-like astringency to the oral cavity at very low threshold concentrations ranging
611
from 4 to 51 µmol/L. With the exception of 3,5,7,8,3′,4′-hexamethoxyflavone (8) and
612
5,7,8,4′-tetramethoxyflavone (11), all the other flavones exhibited also a long-lasting
613
bitterness with recognition thresholds between 19 and 103 µmol/L depending on the
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Journal of Agricultural and Food Chemistry
614
chemical structure. Structure/activity considerations indicated an influence of the
615
methoxylation pattern on the bitterness of the flavones. Tetramethoxylation of the B-
616
ring was found to slightly increase thresholds when compared to structures
617
decorated with methoxy groups only at positions 5, 6, and 7. For example,
618
compounds 4 and 5 exhibiting three methoxygroups at the B ring imparted somewhat
619
lower bitter thresholds of 56 and 44 µmol/L, respectively, when compared to 10 (103
620
µmol/L) and 6 (93 µmol/L) just differing by one additional methoxy group at position 8
621
(Figure 2). Whereas the methoxylation pattern of the C-ring did not seem to
622
influence the bitter threshold, the substitution of one methoxy group by a hydroxyl
623
function induced a decrease of the bitter threshold concentration, e.g. the bitter
624
threshold
625
hexamethoxyflavone 10. Also position 3 was found to play an important role in the
626
perceived bitterness of these flavones. The threshold of 19 and 24 µmol/L found for 9
627
and 12 showed that methoxylation at position 3 led to lower thresholds compared to
628
glucosylation as found in 14 and 15 exhibiting thresholds of 61 and 78 µmol/L,
629
respectively.
of
13
was
found
to
be
4
times
lower
when
compared
to
630
Among the limonoid aglycones (Table 3), limonin (1) and nomilin (2) were
631
found to induce bitterness at rather low recognition thresholds of 4 and 13 µmol/L,
632
thus confirming literature data.9,10 In comparison, deacetylnomilin (22) did not show
633
any bitter taste up to the maximum test concentration of 106 µmol/L, but imparted an
634
intense astringent and mouth-drying orosensation above 13 µmol/L. The limonoid
635
glucosides showed a silky astringent orosensation with low threshold concentrations
636
between 5 and 27 µmol/L for compounds 3, 16, 17, and 21, and somewhat higher
637
astringency thresholds of 205 and 404 µmol/L for compounds 20 and 19, respectively
638
(Table 3). In addition, the sensory panel reported compounds 3, 16, 17, and 21 to
639
induce a distinct bitter taste perceived at the posterior tongue.
25 Environment ACS Paragon Plus
Journal of Agricultural and Food Chemistry
640
Quantitation of Sensometabolites in Orange Juices. After identification of
641
the non-volatile sensometabolites and determination of their taste threshold
642
concentrations, polymethoxylated flavones were analyzed by means of HPLC/DAD,
643
limonoid aglycones, limonoid glucosides, as well as the polymethoxylated flavone
644
glucosides by means of HPLC-MS/MS, and carbohydrates and organic acids by
645
means of HPIC in the orange juices OJ1 and OJ2 prepared from Valencia oranges
646
harvested in November 2003 and March 2006, respectively.
647
The limonoid aglycones 1 and 2, summarized in group I in Table 4, were
648
found in concentrations of 13.1 and 3.8 µmol/L in OJ1, whereas OJ2 contained
649
comparatively low amounts of 2.1 and 0.1 µmol/L, respectively. To evaluate the taste
650
impact of these sensometabolites, the dose-over-threshold (DoT)-factors were
651
calculated as the ratio of the concentration to the taste threshold of a compound.
652
Only limonin (1) exceeded its threshold concentration in OJ1 and was evaluated with
653
a DoT-factor of 3.3, followed by nomilin (2) with a DoT-factor of 0.3 only (Table 4). In
654
comparison, none of these limonoids exceeded their threshold in OJ2.
655
The astringent and bitter tasting limonoid glucosides 3, 16, 17, and 19-21 as
656
well as the polymethoxylated flavones 4-10, 12, 14, and 15 were summarized in
657
tastant group II (Table 4). The limonoid glucosides were found in both juices in high
658
concentrations, e.g. 139.2 or 113.9 µmol/L of 3 and 93.7 or 58.5 µmol/L of 20 was
659
found in OJ1 or OJ2, respectively. A comparatively low concentration of 4.8 µmol/L
660
was determined for glucoside 21. Calculation of DoT-factors for the astringent
661
orosensation revealed a value of 8.9 and 7.2 for 3 in OJ1 and OJ2, respectively
662
(Table 4). Interestingly, glucoside 3 even exceeded its threshold concentration for
663
bitterness by a factor of 1.3 and 1.1, respectively, thus given a first evidence for its
664
contribution to the bitter taste of orange juices. The concentration of flavones 4-10,
665
12, 14, and 15 ranged between 0.02 and 5.8 µmol/L for OJ1 and between 0.03 and
26 Environment ACS Paragon Plus
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Journal of Agricultural and Food Chemistry
666
10.8 µmol/L for OJ2, respectively. Compound 10 was identified as the major flavone
667
in both juices, followed by compound 12 with 4.6 and 6.6 µmol/L and by compound 4
668
with 3.0 and 10.3 µmol/L in OJ1 and OJ2, respectively. Calculation of DoT-factors
669
revealed that exclusively 3,5,6,7,8,3′,4′-heptamethoxyflavone (12) reached its
670
astringency threshold in both orange juices. As none of the polymethoxylated
671
flavones were found in supra-threshold concentrations for bitter taste, the individual
672
derivatives cannot be considered as primary taste contributors to bitter taste of
673
orange juice. However, recent cell-based experiments demonstrated that structurally
674
related bitter compounds do additively co-activate bitter receptors.45 Therefore, the
675
structural similarity of the group of polymethoxylated flavones might enable an
676
additive taste contribution of 4-10, 12, 14, and 15, respectively.
677
High concentrations of 650.8 and 752.5 µmol/L were found for the velvety
678
astringent hesperidin (23) in OJ1 and OJ2, respectively (Table 4). Calculation of the
679
DoT-factors for astringency revealed a value of 59.2 and 68.4, respectively, thus
680
implying 23 as a key contributor to the astringency of orange juices.
681
Among the carbohydrates (group IV, Table 4), sucrose, fructose, and glucose
682
were found almost in equimolar comcentrations. Calculation of DoT-factors indicated
683
sucrose (4.6 and 3.6) and fructose (2.2 and 1.6) as the key sweet contributors in OJ1
684
and OJ2, respectively, whereas glucose just reached its threshold concentrations.
685
In tastant group V, comprising the organic acids, high concentrations of
686
33.9/31.8 and 11.2/17.2 mmol/L were found for citric acid and malic acid in OJ1/2,
687
respectively (Table 4). Both acids exceeded their individual taste threshold in both
688
orange juices, whereas the minor isocitric acid was present in subthreshold
689
concentrations.
690
Re-Engineering the Sensometabolome of Orange Juices OJ1 and OJ2. In
691
order to functionally validate the analytical data obtained, taste re-engineering
27 Environment ACS Paragon Plus
Journal of Agricultural and Food Chemistry
692
experiments were performed. First, a cocktail of hesperidin, carbohydrates, and
693
organic acids, each in its “natural” concentration (Table 4), was dissolved in bottled
694
water and the pH was adjusted to 3.5. Aliquots of this basic taste recombinant
695
solution were then spiked with the limonoid glucosides 3, 16, 17, and 19-21 (expt. 2),
696
with limonoid glucosides 3, 16, 17,and 19-21 and limonoid aglycons 1 and 2 (expt. 3),
697
with limonoid glucosides 3, 16, 17,and 19-21 and polymethoxylated flavones 4-10,
698
12, 14, and 15 (expt. 4), and with limonoid aglycons 1, 2, limonoid glucosides 3, 16,
699
17, 19-21, and polymethoxylated flavones 4-10, 12, 14, 15 (expt. 5.). The
700
recombinant solutions prepared in expts. 1-5 were then sensorially evaluated and
701
compared to authentic orange juices OJ1 and OJ2, respectively, by means of taste
702
profile analysis (Figure 5).
703
Although a triangle test showed a significantly different overall taste profile of
704
OJ1 and OJ2 (p = 0.01), both authentic juices were judged with rather similar
705
intensities for astringency and bitterness with scores of about 1.1 (OJ1) and 1.8
706
(OJ2), respectively (Figure 5, A). It is however interesting to notice, that the
707
bitterness of OJ1 was perceived on the entire tongue, whereas the bitter taste of OJ2
708
was located mainly at the posterior part of the tongue and was perceived as more
709
long-lasting. OJ1 was rated with higher scores for sourness (2.9) than OJ1 (2.3),
710
which was perceived as more sweet (2.0) when compared to OJ2 (1.7).
711
Sensory analysis of the basic taste recombinants containing organic acids,
712
sugars, and hesperidin demonstrated a good match with the authentic juices OJ1
713
and OJ2 with regard to sour taste, whereas sweetness was evaluated with slightly
714
higher scores in the recombinants (expt. 1; Figure 5, B). Surprisingly, astringency
715
was judged with rather low intensities of 0.3 and 0.4, respectively, although the
716
astringent hesperidin was present in both tastant cocktails.
717
Addition of limonoid glucosides 3, 16, 17, and 19-21 (expt. 2) to the basic
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Journal of Agricultural and Food Chemistry
718
recombinant solutions did not show any significant influence on sweetness and
719
sourness. However, astringency and bitterness intensities increased to 0.8/0.6 and
720
0.7/0.4 for OJ1/2, although the taste intensities of the authentic juices were not yet
721
reached (Figure 5, C). Interestingly, all members of the sensory panel reported that
722
the recombinant containing the limonoid glucosides were perceived as “more rich”,
723
“more complex”, and “more full-bodied”. These sensory data nicely confirm the
724
proposed sensory contribution of these glucosides and, in particular, of compound 3
725
on the basis of calculated DoT-factors (Table 4).
726
In expt. 3, the basic recombinant was spiked with limonoid glucosides and
727
limonoid aglycons. Sensory analysis revealed a significant increase of bitterness
728
from 0.7 (Figure 5, B) to 1.5 (Figure 5, D) for OJ1 whereas the bitter taste intensity
729
of the recombinant of OJ2 was only marginally increased from 0.4 to 0.7. This is well
730
in line with the high DoT-factor calculated for limonin (1) in OJ1 when compared to
731
OJ2. Compared to expt. 2 (Figure 5, C), the bitterness perceived for the partial OJ1
732
recombinant in the presence of the limonoid aglycones 1 and 2 was described as
733
unpleasant and biting bitter and, in addition, the intensity of the sweetness was
734
slightly suppressed (Figure 5, D). This confirms the previous finding that
735
concentrations of 1 above 6 ppm will reduce the acceptability of orange juices.46
736
Neither astringency, nor sourness was affected significantly by the presence of these
737
triterpenoids.
738
In expt. 4, the taste recombinant containing the limonoid glucosides were
739
spiked with the polymethoxylated flavones instead of limonoid aglycons (expt. 3).
740
When compared to expt. 2 (Figure 5, C), the astringency of the recombinants
741
increased to 1.0 and 1.1, respectively, thus matching the astringency scores of the
742
authentic juices OJ1 and OJ2 (Figure 5, E). In addition, a significant increase of
743
bitterness (0.4→1.4) was observed for the partial recombinant of OJ2, whereas the
29 Environment ACS Paragon Plus
Journal of Agricultural and Food Chemistry
744
influence of bitter taste for the OJ1 sample was only marginal. This is well in line with
745
the higher content of 4-10, 12, 14, and 15 in OJ2 (Table 4) and, for the first time,
746
unequivocally demonstrates the contribution of these polymethoxylated flavones to
747
the astringency as well as the bitter taste of orange juice. Sensory analysis of total
748
recombinants, including all taste compounds summarized in Table 4 (expt. 5),
749
revealed an excellent match between the authentic juices OJ1 and OJ2 and their
750
corresponding taste recombinants (Figure 5, F), thus confirming that the key
751
sensometabolites of orange juice were successfully identified and quantified.
752
It is interesting to notice that, although the taste profiles of OJ1 and OJ2 are
753
almost identical (Figure 5, A), the quantitative composition of the sensometabolome
754
in both juices is very different (Table 4). This clearly indicates that not a single
755
compound such as, e.g. limonin (1), can be considered a suitable marker compound
756
to analytically measure bitterness. More important, the taste percept of orange juice
757
seems to be created by a rather complex interplay of limonin, limonoid glucosides,
758
polymethoxylated flavones, organic acids, and sugars and, therefore, the prediction
759
of the taste profile requires the measurement of quantitative alterations of the orange
760
sensometabolome as well as a new quality of knowledge on the interactions between
761
the tastant groups involved.
762
Dose-Response Functions of Orange Sensometabolites. In order to
763
address the non-linear relationship between concentration and bitterness intensity,
764
dose-response recordings were performed.38,39 Limonoid aglycones 1 and 2 as well
765
as the glucoside 3 followed rather different dose/response functions. Compound 1
766
reached a taste intensity of 2.8 at its maximum solubility exceeding the threshold
767
concentration by 16 times (Figure 6, A). Although nomilin (2) was water-soluble up to
768
a 32-fold threshold concentration, its maximum sensory response was evaluated with
769
an intensity of 2.2 only. Compared to 1 and 2, the taste intensity of the glucoside 3
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770
Journal of Agricultural and Food Chemistry
increased only slightly with increasing DoT-factors.
771
Although polymethoxylated flavones were recently shown by cell-based
772
assays to activate hTAS2R14 as the primary bitter taste receptor with 5,6,7,8,3´,4´-
773
hexamethoxyflavone (10) showing a low EC50 value of 14 µmol/L,47 the
774
polymethoxylated flavones 4-7, 9, 10, 12, and 15 showed variations in their dose-
775
response functions strongly depending on their chemical structure (Figure 6, B). In
776
particular, the perception of 15 is reflected in a rather high slope with increased taste
777
intensities at higher concentration, e.g. 15 showed the highest taste intensity of 3.25
778
at a concentration exceeding the threshold concentration by a factor of 32. Also
779
compounds 9, 10, and 5 reached a high maximum sensory response of 3.0, 3.0, and
780
3.1, respectively, whereas compounds 4 and 6 showed comparatively flat dose-
781
response functions even at concentration exceeding the threshold by a factor of 64.
782
To investigate the bitterness enhancement of limonoids 1 and 2 by
783
polymethoxylated flavones as found by the taste re-engineering experiments (Figure
784
5), dose-response functions were recorded with solutions of 1 or 2 in the absence
785
and presence of a mixture of flavones at levels of 0.25, 0.50, and 0.75 mg/100 g,
786
respectively (Figure 6, B). Confirming the taste re-engineering data (Figure 5), the
787
bitter taste intensity of 1 was increased by the addition of polymethoxylated flavones,
788
e.g. the presence of these flavones in sub-threshold concentrations (0.75 mg/100 g)
789
increased the perceived bitterness of a solution of 1, which exceeded its bitter
790
threshold by a factor of two (DoT = 2), from 0.4 to 0.8 (Figure 6, C). Even more
791
pronounced, the bitter taste of a solution of 2 (DoT = 2) was significantly enhanced
792
from 1.0 to 1.5 in the presence of the flavones (0.5 mg/100 mL).
793
In order to investigate the influence of the flavone structure on their bitter taste
794
enhancing activity, aqueous solutions of 1 (16 µmol/L) spiked with individual
795
polymethoxylated flavones were sensorially evaluated and compared to a solution of
31 Environment ACS Paragon Plus
Journal of Agricultural and Food Chemistry
Page 32 of 58
796
1 in the absence of any flavone (control). While the control solution of 1 was judged
797
with an intensity of 0.9, addition of the individual flavones at a level of 0.5 mg/100 g
798
increased the perceived bitterness depending on the chemical structure (Figure 7).
799
Compounds 4, 7, and 8 were identified to affect the bitterness of 1 most intensively,
800
e.g. the presence of 7 enhanced the bitter intensity of limonin from 0.9 to 1.7. In
801
comparison, 5 did not show any significant effect in bitterness enhancement.
802
Hierarchical
Cluster
Analysis
and
Sensomics
Heatmapping
of
In order to gain a first insight into the
803
Sensometabolites in Orange Juices.
804
naturally occurring quantitative variation of the sensometabolome in orange juices,
805
limonoid aglycons (1, 2), polymethoxylated flavones (4-10, 12), limonoid glucosides
806
(3, 16, 17, 19-21), and the sugars sucrose, glucose, and fructose were quantitatively
807
determined in a total of 15 orange juices, namely OJ1-OJ15. To examine the
808
multivariate distances between the individual sensometabolites in the orange juice
809
samples, a hierarchical cluster analysis was performed on the basis of the scaled
810
DoT-factors determined for each compound. The results were visualized in a
811
sensomics heatmap that was combined with hierarchical agglomerative clustering of
812
the sensometabolites 1-10, 12, 16, 17, and 19-21. The cluster analysis quantifies the
813
degree of similarity between the sensometabolites by calculating the distance
814
between all possible pairs of molecules. The two most similar sensometabolites were
815
then grouped together and the distance measure recalculated. This iterative process
816
was continued until all sensometabolites were members of a single cluster. This
817
resulting hierarchical clustering is visually displayed as a dendrogram (Figure 8). The
818
closer the sensometabolites are to each other in the dendrogram, the smaller the
819
differences in their DoT-factor pattern throughout the set of orange juices. The
820
hierarchical analysis arranged the sensometabolites into two large clusters. Cluster 1
821
consisted of the group of polymethoxylated flavones (4-10, 12) which showed high
32 Environment ACS Paragon Plus
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Journal of Agricultural and Food Chemistry
822
DoT-factors in OJ5, OJ7, and OJ12 and rather low values in OJ1, OJ3, and OJ4
823
when compared to the other juice samples. The limonoid aglycons were grouped in
824
subcluster 2a with DoT-factors ranging from the lowest values of 0.5 (1) and 249
9 4 6 13 21 24 21 20 19 51 44 13
4 13 >106
n.d. n.d. 13
limonoid aglycones limonin, 1 nomilin, 2 deacetylnomilin, 22 limonoid glucosides epiisoobacunoic acid 17-O-β-D-glcp, 17 obacunon-17-O-β-D-glcp, 21 deacetylnomilinic acid 17-O-β-D-glcp, 16 limonin-17-O-β-D-glcp, 3 methyldeacetylnomilinic acid 17-O-β-D-glcp, 18 nomilin-17-O-β-D-glcp, 19 nomilinic acid 17-O-β-D-glcp, 20 1129 1130 1131 1132 1133
a
42 49 68 106 >182 >793 >702
16 15 27 17 5 404 205
Compound numbering refers to structures given in Figures 1 and 2. b Taste threshold concentrations were determined by means of a three alternative forced choice test in bottled water. c The recognition threshold concentrations were determined by means of a half-tongue test in bottled water. glcp: glucopyranoside, n.d.: not detectable.
1134 1135 1136 1137
47 Environment ACS Paragon Plus
Journal of Agricultural and Food Chemistry
1138 1139
Table
4.
Concentrations and Dose-over-Threshold-(DoT)-factors Compounds in Orange juices OJ1 and OJ2
taste compound (no.)a
limonin (1) nomilin (2)
13.1 (3.3) 3.8 (0.3)
group II (astringent and bitter) limonin 17-β-D-glcp (3) nomilinic acid 17-β-D-glcp (20) nomilin 17-β-D-glcp (19) deacetylnomilinic acid 17-β-D-glcp (16) epiisoobacunoic acid 17-β-D-glcp (17) obacunon 17-β-D-glcp (21) 5,6,7,8,3´,4´-hexamethoxyflavone (10) 3,5,6,7,8,3´,4´-heptamethoxyflavone (12) 5,6,7,3´,4´-pentamethoxyflavone (4) 5,6,7,8,4´-pentamethoxyflavone (6) 5,6,7,4´-tetramethoxyflavone (5) 3,5,6,7,3´,4´-hexamethoxyflavone (9) 5,7,8,3´,4´-pentamethoxyflavone (7) 3,5,7,8,3´,4´-hexamethoxyflavone (8) 5,6,7,8,3´,4´-hexamethoxyflavone-3-O-βD-glcp (15) 5,6,7,3´,4´-pentamethoxyflavone-3-O-β-Dglcp (14)
hesperidin (23) group IV (sweet) sucrose fructose glucose group IV (sour) citric acid malic acid isocitric acid
Taste
OJ2
conc. [µmol/L] (DoTbitter)
group I (bitter)
group III (velvety astringent)
of
conc. [µmol/L] (DoT)b in OJ1
1140 1141 1142 1143 1144 1145
Page 48 of 58
2.1 (0.5) 0.1 (