Sensory-guided decomposition of red currant juice (Ribes rubrum) and structure determination of key astringent compounds

Sequential application of solvent extraction, gel permeation chromatography, and RP-HPLC in combination with taste dilution analyses, followed by LC-MS and 1D/2D NMR experiments, led to the discovery and structure determination of 25 key astringent compounds of red currant juice. Besides several flavonol glycosides, in particular, 3-carboxymethyl-indole-1-N-beta-D-glucopyranoside, 3-methylcarboxymethyl-indole-1-N-beta-D-glucopyranoside, and a family of previously not identified compounds, namely, 2-(4-hydroxybenzoyloxymethyl)-4-beta-D-glucopyranosyloxy-2(E)-butenenitrile, 2-(4-hydroxy-3-methoxybenzoyloxymethyl)-4-beta-D-glucopyranosyloxy-2(E)-butenenitrile, (E)-6-[3-hydroxy-4-(O-beta-D-glucopyranosyl)phenyl]-5-hexen-2-one named dehydrorubrumin, and (3E,5E)-6-[3-hydroxy-4-(O-beta-D-glucopyranosyl)phenyl]-3,5-hexadien-2-one named rubrumin, have been identified. Determination of the oral astringency thresholds by means of the half-tongue test revealed that the lowest thresholds of 0.3 and 1.0 nmol/L were found for the nitrogen-containing 3-carboxymethyl-indole-1-N-beta-D-glucopyranoside and 3-methylcarboxymethyl-indole-1-N-beta-D-glucopyranoside, which do not belong to the group of plant polyphenols.     

Isolation, structure determination, synthesis, and sensory activity of N-phenylpropenoyl-L-amino acids from cocoa (Theobroma cacao)

Application of chromatographic separation and taste dilution analyses recently revealed besides procyanidins a series of N-phenylpropenoyl amino acids as the key contributors to the astringent taste of nonfermented cocoa beans as well as roasted cocoa nibs. Because these amides have as yet not been reported as key taste compounds, this paper presents the isolation, structure determination, and sensory activity of these amino acid amides. Besides the previously reported (-)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-3-hydroxy-L-tyrosine (clovamide), (-)-N-[4'-hydroxy-(E)-cinnamoyl]-L-tyrosine (deoxyclovamide), and (-)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-L-tyrosine, seven additional amides, namely, (+)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-L-aspartic acid, (+)-N-[4'-hydroxy-(E)-cinnamoyl]-L-aspartic acid, (-)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-L-glutamic acid, (-)-N-[4'-hydroxy-(E)-cinnamoyl]-L-glutamic acid, (-)-N-[4'-hydroxy-(E)-cinnamoyl]-3-hydroxy-L-tyrosine, (+)-N-[4'-hydroxy-3'-methoxy-(E)-cinnamoyl]-L-aspartic acid, and (+)-N-[(E)-cinnamoyl]-L-aspartic acid, were identified for the first time in cocoa products by means of LC-MS/MS, 1D/2D-NMR, UV-vis, CD spectroscopy, and polarimetry, as well as independent enantiopure synthesis. Using the recently developed half-tongue test, human recognition thresholds for the astringent and mouth-drying oral sensation were determined to be between 26 and 220 micromol/L (water) depending on the amino acid moiety. In addition, exposure to light rapidly converted these [E]-configured N-phenylpropenoyl amino acids into the corresponding [Z]-isomers, thus indicating that analysis of these compounds in food and plant materials needs to be performed very carefully in the absence of light to prevent artifact formation.     

Application of a molecular sensory science approach to alkalized cocoa (Theobroma cacao): structure determination and sensory activity of nonenzymatically C-glycosylated flavan-3-ols

Application of comparative taste dilution analyses on nonalkalized and alkalized cocoa powder revealed the detection of a velvety, smoothly astringent tasting fraction, which was predominantly present in the alkalized sample. LC-MS/MS analysis, 1D- and 2D-NMR, and CD spectroscopy as well as model alkalization reactions led to the unequivocal identification of the velvety, smoothly astringent molecules as a series of catechin- and epicatechin-C-glycopyranosides. Besides the previously reported (-)-epicatechin-8-C-beta-D-galactopyranoside, additional flavan-3-ol-C-glycosides, namely, (-)-epicatechin-8-C-beta-D-glucopyranoside, (-)-catechin-8-C-beta-D-glucopyranoside, (-)-catechin-6-C-beta-D-glucopyranoside, (-)-epicatechin-6-C-beta-D-glucopyranoside, (-)-catechin-8-C-beta-D-galactopyranoside, (-)-catechin-6-C-beta-D-galactopyranoside, (-)-catechin-6-C,8-C-beta-D-diglucopyranoside, (-)-epicatechin-6-C,8-C-beta-D-digalactopyranoside, (-)-catechin-6-C,8-C-beta-D-digalactopyranoside, and epicatechin-6-C,8-C-beta-D-diglucopyranoside, were identified for the first time in cocoa. Most surprisingly, these phenol glycoconjugates were demonstrated by model experiments to be formed via a novel nonenzymatic C-glycosylation of flavan-3-ols. Using the recently developed half-tongue test, human recognition thresholds for the astringent and mouth-drying oral sensation were determined to be between 1.1 and 99.5 micro mol/L (water) depending on the sugar and the intramolecular binding position as well as the aglycone.     

Spectroscopic identification and antioxidant activity of glucosylated carotenoid metabolites from Cydonia vulgaris fruits

Carotenoid metabolites are common plant constituents with significant importance for the flavor and aroma of fruits. Three new carotenoid derivatives, (2E,4E)-8-hydroxy-2,7-dimethyl-2,4-decadiene-1,10-dioic acid 1-O-beta-D-glucopyranosyl ester (1), (2Z,4E)-8-beta-D-glucopyranosyloxy-2,7-dimethyl-2,4-decadiene-1,10-dioic acid (3), and 3,9-dihydroxymegastigmast-5-ene-3-O-[beta-D-glucopyranosyl-(1-->6)]-beta-D-glucopyranoside (5), as well as three known compounds, have been isolated from the ethanolic extract of peels of Cydonia vulgaris, the fruit of a shrub belonging to the same family as the apple. All the compounds were identified by spectroscopic techniques, especially 1D and 2D NMR. Antioxidant activities of all the isolated metabolites were assessed by measuring their ability to scavenge DPPH radical and superoxide radical (O2*-) and to induce the reduction of Mo(VI).     

Sensory-guided decomposition of roasted cocoa nibs (Theobroma cacao) and structure determination of taste-active polyphenols

Sequential application of solvent extraction, gel permeation chromatography, and RP-HPLC in combination with taste dilution analyses, followed by LC-MS and 1D/2D-NMR experiments and thiolytic degradation, revealed that, besides theobromine and caffeine, the flavan-3-ols epicatechin, catechin, procyanidin B-2, procyanidin B-5, procyanidin C-1, [epicatechin-(4beta-->8)](3)-epicatechin, and [epicatechin-(4beta-->8)](4)-epicatechin were among the key compounds contributing to the bitter taste as well as the astringent mouthfeel imparted upon consumption of roasted cocoa. In addition, a series of quercetin, naringenin, luteolin, and apigenin glycopyranosides as well as a family of not previously identified amino acid amides, namely, (+)-N-[4'-hydroxy-(E)-cinnamoyl]-L-aspartic acid, (+)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-L-aspartic acid, (-)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-L-glutamic acid, (-)-N-[4'-hydroxy-(E)-cinnamoyl]-L-glutamic acid, (-)-N-[4'-hydroxy-(E)-cinnamoyl]-3-hydroxy-L-tyrosine, (+)-N-[4'-hydroxy-3'-methoxy-(E)-cinnamoyl]-L-aspartic acid, and (+)-N-(E)-cinnamoyl-L-aspartic acid, have been identified as key astringent compounds of roasted cocoa. Furthermore, (-)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-3-hydroxy-l-tyrosine (clovamide), (-)-N-[4'-hydroxy-(E)-cinnamoyl]-L-tyrosine (deoxyclovamide), and (-)-N-[3',4'-dihydroxy-(E)-cinnamoyl]-L-tyrosine, reported previously as antioxidants, have been found as contributors of cocoa's astringent taste. By means of the half-tongue test, the taste thresholds of flavan-3-ols and glycosides have been determined.     

Activity-guided identification of (S)-malic acid 1-O-D-glucopyranoside (morelid) and gamma-aminobutyric acid as contributors to umami taste and mouth-drying oral sensation of morel mushrooms (Morchella deliciosa Fr.)

Although morel mushrooms are widely used as tasty ingredients in savory dishes, knowledge of the key compounds evoking their attractive taste is still very fragmentary. In the present study, taste activity-guided fractionation of an aqueous morel extract by means of the recently developed taste dilution analysis (TDA) enabled the localization of several umami-like-tasting fractions as well as a fraction imparting an intense mouth-drying sensation to the oral cavity. Hydrophilic interaction liquid chromatography (HILIC), LC-MS, and amino acid analysis led to the successful identification of gamma-aminobutyric acid as the chemical inducer of the mouth-drying and mouth-coating oral sensations imparted by the morel extract. Besides the well-known umami-like taste contributors L-glutamic acid, L-aspartic acid, and succinic acid, an additional HILIC fraction was isolated and evaluated as tasting umami-like. LC-MS and NMR studies revealed that this fraction consisted of a mixture of (S)-malic acid 1-O-alpha-D-glucopyranoside and (S)-malic acid 1-O-beta-D-glucopyranoside, the structure of which could be successfully confirmed by independent synthesis. To the best of our knowledge, this morel-derived glycoside, which we named (S)-morelid, has previously not been reported in any food products. Sensory analysis of aqueous solutions of the compounds identified revealed threshold concentrations of 0.02 mmol/L for the mouth-drying effect of gamma-aminobutyric acid and 6.0 mmol/L for the umami-like, slightly sour taste of (S)-morelid.     

Structure determination of 3-O-caffeoyl-epi-gamma-quinide, an orphan bitter lactone in roasted coffee

Recent investigations on the bitterness of coffee as well as 5- O-caffeoyl quinic acid roasting mixtures indicated the existence of another, yet unknown, bitter lactone besides the previously identified bitter compounds 5- O-caffeoyl- muco-gamma-quinide, 3- O-caffeoyl-gamma-quinide, 4- O-caffeoyl- muco-gamma-quinide, 5- O-caffeoyl- epi-delta-quinide, and 4- O-caffeoyl-gamma-quinide. In the present study, this orphan bitter lactone was isolated from the reaction products generated by dry heating of 5- O-caffeoylquinic acid model, and its structure was determined as the previously unreported 3- O-caffeoyl- epi-gamma-quinide by means of liquid chromatography-mass spectrometry (LC-MS) and one-/two-dimensional NMR experiments. The occurrence of this bitter lactone, exhibiting a low bitter recognition threshold of 58 micromol/L, in coffee beverages could be confirmed by LC-MS/MS (negative electrospray ionization) operating in the multiple reaction monitoring mode.     

Structure determination and sensory analysis of bitter-tasting 4-vinylcatechol oligomers and their identification in roasted coffee by means of LC-MS/MS

Aimed at elucidating intense bitter-tasting molecules in coffee, various bean ingredients were thermally treated in model experiments and evaluated for their potential to produce bitter compounds. As caffeic acid was found to generate intense bitterness reminiscent of the bitter taste of a strongly roasted espresso-type coffee, the reaction products formed were screened for bitter compounds by means of taste dilution analysis, and the most bitter tastants were isolated and purified. LC-MS/MS as well as 1-D/2-D NMR experiments enabled the identification of 10 bitter compounds with rather low recognition threshold concentrations ranging between 23 and 178 micromol/L. These bitter compounds are the previously unreported 1,3-bis(3',4'-dihydroxyphenyl) butane, trans-1,3-bis(3',4'-dihydroxyphenyl)-1-butene, and eight multiply hydroxylated phenylindanes, among which five derivatives are reported for the first time. In addition, the occurrence of each of these bitter compounds in a coffee brew was verified by means of LC-MS/MS (ESI-) operating in the multiple reaction monitoring (MRM) mode. The structures of these bitter compounds show strong evidence that they are generated by oligomerization of 4-vinylcatechol released from caffeic acid moieties upon roasting.     

Systematic studies on structure and physiological activity of cyclic alpha-keto enamines, a novel class of "cooling" compounds.

3-Methyl- and 5-methyl-2-(1-pyrrolidinyl)-2-cyclopenten-1-one were recently identified as intense cooling compounds in roasted dark malt. To gain more insights into the molecular requirements of these compounds for imparting a cooling sensation, 26 cyclic alpha-keto enamine derivatives were synthesized, and their physiological cooling activities were evaluated. Any modification of the amino moiety, the carbocyclic ring size, or incorporation of additional methyl groups led to a significant increase of the cooling threshold. Insertion of an oxygen atom into the 2-cyclopenten-1-one ring, however, increased the cooling activity, e.g., the cooling threshold of the 5-methyl-4-(1-pyrrolidinyl)-3(2H)-furanone was found to be 16-fold below the threshold concentration determined for the 3-methyl-2-(1-pyrrolidinyl)-2-cyclopenten-1-one. Shifting the oxygen atom from the 4- into the 5-position of the cyclopentenone ring resulted in a even more drastic increase in cooling activity, e.g., the 4-methyl-3-(1-pyrrolidinyl)-2(5H)-furanone exhibited the strongest cooling effect at the low oral threshold concentration of 0.02-0.06 mmol/L, which is 35-fold below the value determined for (-)-menthol. In contrast to the minty smelling (-)-menthol, most of the alpha-keto enamines were found to be virtually odorless but impart a sensation of "cooling" to the oral cavity as well as to the skin, thus illustrating that there is no physiological link between cooling activity and mint-like odors.     

Structural and sensory characterization of key pungent and tingling compounds from black pepper (Piper nigrum L.)

To gain a more comprehensive knowledge on whether, besides the well-known piperine, other compounds are responsible for the pungent and tingling oral impression imparted by black pepper, an ethanol extract prepared from black pepper (Piper nigrum L.) was screened for its key sensory-active nonvolatiles by application of taste dilution analysis (TDA). Purification of the compounds perceived with the highest sensory impact, followed by LC-MS and 1D/2D NMR experiments as well as synthesis, led to the structure determination of 25 key pungent and tingling phytochemicals, among which the eight amides 1-(octadeca-2E,4E,13Z-trienyl)piperidine, 1-(octadeca-2E,4E,13Z-trienyl)pyrrolidine, (2E,4E,13Z)-N-isobutyl-octadeca-2,4,13-trienamide, 1-(octadeca-2E,4E,12Z-trienoyl)-pyrrolidine, 1-(eicosa-2E,4E,15Z-trienyl)piperidine, 1-(eicosa-2E,4E,15Z-trienyl)pyrrolidine, (2E,4E,15Z)-N-isobutyl-eicosa-2,4,15-trienamide, and 1-(eicosa-2E,4E,14Z-trienoyl)-pyrrolidine were not yet reported in literature. Sensory studies by means of a modified half-tongue test revealed recognition thresholds ranging from 3.0 to 1150.2 nmol/cm2 for pungency and from 520.6 to 2162.1 nmol/cm2 for the tingling orosensation depending on their chemical structure.     

Enzymatic O-methylation of flavanols changes lag time, propagation rate, and total oxidation during in vitro model triacylglycerol-rich lipoprotein oxidation

3'-O-Methyl derivatives of flavan-3-ols, (+)-catechin (C), (-)-epicatechin (EC), and (-)-catechin gallate (CG) were prepared enzymatically. Hexanal (EC and CG family, 5 mmol/L) and conjugated diene (C and EC family, 0.25-10 mmol/L) formation following CuSO4-mediated triacylglycerol-rich lipoprotein oxidation was measured. All EC and CG compounds significantly reduced hexanal formation (p < 0.02). O-Methylation improved the ability of CG (more polar) while reducing the ability of EC (less polar) to limit hexanal formation. 3'-O-methyl EC was 18% (p < 0.001) and 4'-O-methyl 65% (p < 0.001) less able than EC to suppress hexanal formation. At >1 micromol/L all EC and C compounds significantly increased lag time. Parent compounds were more effective (> 4-fold increase) than metabolites (1.5-fold increase). Parent compounds did not influence propagation rate (DeltaOD/min). At >1 mmol/L O-methylated EC and C reduced propagation by 20-40% (p < 0.01). Notably, at 0.25 mmol/L O-methylated EC and C increased propagation rates 22% (p < 0.01) despite prolonging lag time.     

Orally administered delphinidin 3-rutinoside and cyanidin 3-rutinoside are directly absorbed in rats and humans and appear in the blood as the intact forms

Four components of black currant anthocyanins (BCA), delphinidin 3-O-beta-rutinoside (D3R), cyanidin 3-O-beta-rutinoside (C3R), delphinidin 3-O-beta-glucoside (D3G), and cyanidin 3-O-beta-glucoside (C3G), were found to be directly absorbed and distributed to the blood and excreted into urine as the glycosylated forms. In a rat study, following oral administration of purified D3R, C3R, and C3G (800 micromol/kg of body weight), the anthocyanins were detected in the plasma and the C(max) values were 580 +/- 410, 850 +/- 120, and 840 +/- 190 nmol/L, respectively, 0.5-2.0 h after administration. In a human study, when a mixture of BCA [6.24 micromol (3.58 mg) consisting of 2.75 micromol (1.68 mg) of D3R, 2.08 micromol (1.24 mg) of C3R, 1.04 micromol (0.488 mg) of D3G, and 0.37 micromol (0.165 mg) of C3G/kg of body weight)] was orally ingested by eight volunteers, D3R, C3R, D3G, and C3G were detected in the plasma and urine. The plasma C(max) values were 73.4 +/- 35.0, 46.3 +/- 22.5, 22.7 +/- 12.4, and 5.0 +/- 3.7 nmol/L, respectively, 1.25-1.75 h after intake, and the cumulative excretion of the four compounds in urine in the period 0-8 h after intake was 0.11 +/- 0.05% of the dose ingested. These results indicate that 3-O-beta-rutinosyl anthocyanins were directly absorbed and distributed to the blood.     

Aroma properties of a homologous series of 2,3-epoxyalkanals and trans-4,5-epoxyalk-2-enals

A few odor-active epoxyaldehydes, formed during lipid peroxidation, have recently been reported as intense aroma compounds in foods. However, very little is known about their flavor properties in general. Syntheses of homologous trans-2,3-epoxyalkanals (C(6)-C(12)) and trans-4,5-epoxy-(E)-2-alkenals (C(7)-C(12)) followed by structural characterization using mass spectrometry (MS/EI; MS/CI) and (1)H NMR measurements were performed. An evaluation of their odor qualities and odor thresholds by gas chromatography-olfactometry revealed the following: within the trans-2,3-epoxyalkanals, the odor quality changed from grassy for the compounds with six and seven carbon atoms to citrus-like or soapy for aldehydes with eight and more carbon atoms. The odor thresholds lay in the range of 3-15 ng/L (in air) and were nearly identical within the series; however, a slight minimum was measured for trans-2,3-epoxyoctanal to trans-2,3-epoxydecanal. In the series of the trans-4,5-epoxyalk-(E)-2-enals the C(10) compound was characterized by the lowest odor threshold of 0.6-2.5 pg/L of air. However, all trans-4,5-epoxy-alk-(E)-2-enals smelled intensely metallic.     

Determination of benzoxazinone derivatives in plants by combining pressurized liquid extraction-solid-phase extraction followed by liquid chromatography-electrospray mass spectrometry

A new analytical method based on the use of pressurized liquid extraction (PLE) followed by solid-phase extraction with LiChrolut RP C18 cartridges was evaluated for the sample preparation, extraction, and cleanup of eight naturally occurring benzoxazinone derivatives, 2-beta-D-glucopyranosyloxy-4-hydroxy-1,4-benzoxazin-3-one, 2-beta-D-glucopyranosyloxy-4-hydroxy-7-methoxy-1,4-benzoxazin-3-one, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one, 2-hydroxy-1,4-benzoxazin-3-one, 2-hydroxy-7-methoxy-1,4-benzoxazin-3-one, benzoxazolin-2-one, and 6-methoxybenzoxazolin-2-one in plant samples. Afterward, liquid chromatography-electrospray mass spectrometry, using the selected ion monitoring mode and internal standard (2-MeO-DIBOA, indoxyl-beta-D-glucoside, and quercetin-3-O-rutinoside) quantification method was performed. This paper demonstrates the effectiveness of the PLE method, in conjunction with sensitive and specific mass spectrometric detection, for the quantitative recovery of compounds of the benzoxazinone class from plants. The recoveries of the analytes ranged from 66 to 110% with coefficients of variation ranging from 1 to 14%. This method gave detection limits between 1 and 27 microg/g. The method was applied to foliage and roots of three different wheat cultivars, and the analytes were detected in the range of 11-3261 microg/g of dry weight.     

Reinvestigation of the bitter compounds in carrots (Daucus carota L.) by using a molecular sensory science approach

In order to reinvestigate the key molecules inducing bitter off-taste of carrots ( Daucus carota L.), a sensory-guided fractionation approach was applied to bitter carrot extracts. Besides the previously reported bitter compounds, 6-methoxymellein (1), falcarindiol (2), falcarinol (3), and falcarindiol-3-acetate (4), the following compounds were identified for the first time as bitter compounds in carrots with low bitter recognition thresholds between 8 and 47 micromol/L: vaginatin (5), isovaginatin (6), 2-epilaserine oxide (7), laserine oxide (8), laserine (14), 2-epilaserine (15), 6,8-O-ditigloyl- (9), 6-O-angeloyl-, 8-O-tigloyl- (10), 6-O-tigloyl-, 8-O-angeloyl- (11), and 6-, 8-O-diangeloyl-6 ss,8alpha,11-trihydroxygermacra-1(10) E,4 E-diene (12), as well as 8-O-angeloyl-tovarol (13) and alpha-angeloyloxy-latifolone (16). Among these bitter molecules, compounds 9, 10, 13, and 16 were not previously identified in carrots and compounds 6, 11, and 12 were yet not reported in the literature.     

Antimutagenic activity of flavonoids from Pogostemon cablin

A methanol extract from Pogostemon cablin showed a suppressive effect on umu gene expression of SOS response in Salmonella typhimurium TA1535/pSK1002 against the mutagen 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (furylfuramide). The methanol extract was re-extracted with hexane, dichloromethane, butanol, and water. A dichloromethane fraction showed a suppressive effect. Suppressive compounds against furylfuramide in the dichloromethane fraction were isolated by SiO(2) column chromatography and identified as 7,4'-di-O-methyleriodictyol (1), 7, 3',4'-tri-O-methyleriodictyol (2), and 3,7,4'-tri-O-methylkaempferol (3). In addition, three flavonoids, ombuine (4), pachypodol (5), and kumatakenin (6), were isolated and identified from the dichrolomethane fraction. Compounds 1 and 3 suppressed >50% of the SOS-inducing activity at <0.6 micromol/mL, and the ID(50) values of both compounds were 0.25 micromol/mL. Compound 2 showed a weakly suppressive effect (17%) at a concentration of 0.6 micromol/mL, and compounds 4-6 did not. These compounds were also assayed with 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1), which requires liver metabolizing enzymes. Compounds 3-6 suppressed >80% of the SOS-inducing activity of Trp-P-1 at <0.06 micromol/mL, and compounds 1 and 2 suppressed 87 and 63% at a concentration of 0.3 micromol/mL. In addition, these compounds were assayed with activated Trp-P-1, and the suppressed effects of these compounds were further decreased when compared to Trp-P-1. The antimutagenic activities of these compounds against furylfuramide, Trp-P-1, and activated Trp-P-1 were assayed by the Ames test using S. typhimurium TA100.     

Comparison of odor-active compounds from six distinctly different rice flavor types

Using a dynamic headspace system with Tenax trap, GC-MS, GC-olfactometry (GC-O), and multivariate analysis, the aroma chemistry of six distinctly different rice flavor types (basmati, jasmine, two Korean japonica cultivars, black rice, and a nonaromatic rice) was analyzed. A total of 36 odorants from cooked samples were characterized by trained assessors. Twenty-five odorants had an intermediate or greater intensity (odor intensity >or= 3) and were considered to be major odor-active compounds. Their odor thresholds in air were determined using GC-O. 2-Acetyl-1-pyrroline (2-AP) had the lowest odor threshold (0.02 ng/L) followed by 11 aldehydes (ranging from 0.09 to 3.1 ng/L), guaiacol (1.5 ng/L), and 1-octen-3-ol (2.7 ng/L). On the basis of odor thresholds and odor activity values (OAVs), the importance of each major odor-active compound was assessed. OAVs for 2-AP, hexanal, ( E)-2-nonenal, octanal, heptanal, and nonanal comprised >97% of the relative proportion of OAVs from each rice flavor type, even though the relative proportion varied among samples. Thirteen odor-active compounds [2-AP, hexanal, ( E)-2-nonenal, octanal, heptanal, nonanal, 1-octen-3-ol, ( E)-2-octenal, ( E, E)-2,4-nonadienal, 2-heptanone, ( E, E)-2,4-decadienal, decanal, and guaiacol] among the six flavor types were the primary compounds explaining the differences in aroma. Multivariate analysis demonstrated that the individual rice flavor types could be separated and characterized using these compounds, which may be of potential use in rice-breeding programs focusing on flavor.     

Identification of the astringent taste compounds in black tea infusions by combining instrumental analysis and human bioresponse

Application of taste dilution analyses on freshly prepared black tea infusions revealed neither the high molecular weight thearubigen-like polyphenols nor the catechins and theaflavins, but a series of 14 flavon-3-ol glycosides as the main contributors to the astringent taste perceived upon black tea consumption. Among these glycosides, the apigenin-8-C-[alpha-l-rhamnopyranosyl-(1-->2)-O-beta-d-glucopyranoside] was identified for the first time in tea infusions. Depending on the structure, the flavon-3-ol glycosides were found to induce a velvety and mouth-coating sensation at very low threshold concentrations, which were far below those of catechins or theaflavins; for example, the threshold of 0.001 micromol/L found for quercetin-3-O-[alpha-l-rhamnopyranosyl-(1-->6)-O-beta-d-glucopyranoside] is 190000, or 16000 times below the threshold determined for epigallocatechin gallate or theaflavin, respectively. Moreover, structure/activity considerations revealed that, besides the type of flavon-3-ol aglycon, the type and the sequence of the individual monosaccharides in the glycosidic chain are key drivers for astringency perception of flavon-3-ol glycosides.     

Development of a class-selective enzyme-linked immunosorbent assay for mercapturic acids in human urine

Epidemiological and toxicological studies often require the analysis of large numbers of samples for biological markers of exposure. The goal of this work was to develop a class-selective ELISA to detect groups of structurally closely related mercapturic acids with small nonpolar S-substituents. An assay was developed with strong recognition for mercapturates including S-benzylmercapturic acid (IC50 = 0.018 micromol/L), S-n-hexylmercapturic acid (IC50 = 0.021 micromol/L), S-phenylmercapturic acid (IC50 = 0.024 micromol/L), and S-cyclohexylmethylmercapturic acid (IC50 = 0.042 micromol/L). The same assay also showed weaker recognition for S-(1-hydroxynaphthal-2-yl)mercapturic acid and S-allylmercapturic acid (IC50 = 1.1 and 1.7 micromol/L, respectively). Subtle modifications to the hapten linker structure of the coating antigen proved to have a strong impact on the selectivity and the specificity of the assay. A slightly modified assay showed high recognition for S-benzylmercapturic acid (IC50 = 0.018 micromol/L) and weaker recognition for seven other mercapturic acids (IC50 = 0.021-10 micromol/L). Strong positive assay responses were detected in 12 urine samples obtained from persons with no known occupational exposure to exogenous electrophilic xenobiotics. Solid phase extraction and cross-reactivity indicated that the presumptive immunoreactive materials were similar in size and polarity to S-benzylmercapturic acid. The assay was more selective to mercapturic acids than the spectrophotometric thioether assay.     

Assessment of the aroma impact of major odor-active thiols in pan-roasted white sesame seeds by calculation of odor activity values

Eleven odor-active thiols, namely, 2-methyl-1-propene-1-thiol, (Z)-3-methyl-1-butene-1-thiol, (E)-3-methyl-1-butene-1-thiol, (Z)-2-methyl-1-butene-1-thiol, (E)-2-methyl-1-butene-1-thiol, 2-methyl-3-furanthiol, 3-mercapto-2-pentanone, 2-mercapto-3-pentanone, 4-mercapto-3-hexanone, 3-mercapto-3-methylbutyl formate, and 2-methyl-3-thiophenethiol, recently identified in an extract prepared from white sesame seeds, were quantitated in sesame using stable isotope dilution analyses. For that purpose, the following deuterium-labeled compounds were synthesized and used as internal standards in the quantitation assays: [2H6]-2-methyl-1-propene-1-thiol, [2H3]-(E)- and [2H3]-(Z)-2-methyl-1-butene-1-thiol, [2H3]-2-methyl-3-furanthiol, [2H2]-3-mercapto-2-pentanone, [2H3]-4-mercapto-3-hexanone, [2H6]-3-mercapto-3-methylbutyl formate, and [2H3]-2-methyl-3-thiophenethiol. On the basis of the results obtained, odor activity values (OAVs) were calculated as ratio of the concentration and odor threshold of the individual compounds in cooking oil. According to their high OAVs, particularly the 3-methyl-1-butene-1-thiols (OAV: 2400) and the 2-methyl-1-butene-1-thiols (OAV: 960) were identified as the most odor-active compounds in pan-roasted white sesame seeds. These compounds were therefore suggested to be mainly responsible for the characteristic but rather unstable sulfury aroma of freshly pan-roasted white sesame seeds.