Configurations and Sensory Properties of the Stereoisomers of 2,6-Dimethyl-4-propyl-1,3-oxathiane and 2,4-Dimethyl-6-propyl-1,3-oxathiane.

The heterocyclic compounds 2,6-dimethyl-4-propyl-1,3-oxathiane 1 and 2,4-dimethyl-6-propyl-1,3-oxathiane 2 were obtained by condensing 4-mercapto-2-heptanol and 2-mercapto-4-heptanol, respectively, with acetaldehyde. For both, separation of the eight stereoisomers was achieved via capillary gas chromatography using heptakis(diethyl-tert-butyldimethylsilyl)-ß-cyclodextrin as the chiral stationary phase. Their configurations were assigned by combinations of enzyme-catalyzed kinetic resolutions, HPLC separations, and assessments of NMR data. The odor thresholds and odor qualities of the stereoisomers were determined by capillary gas chromatography/olfactometry. The odor thresholds of the stereoisomers of 2 were generally higher than those of 1. For both oxathianes, the stereoisomers in which all substituents are in equatorial positions showed the highest odor thresholds. Most of the stereoisomers of 1 exhibited pleasant flowery, fruity, or sweet nuances; the stereoisomers of 2 were mainly characterized by descriptors, such as broth, mushroom, or pungent. The data demonstrate the impact of the positions of substituents and their spatial orientations on the sensory properties of 1,3-oxathianes.

Odorant metabolism of the olfactory cleft mucus in idiopathic olfactory impairment patients and healthy volunteers.

BACKGROUND: It remains unclear whether the metabolic activity of nasal mucus in the olfactory and respiratory areas is different. Moreover, age- and olfaction-related changes may affect metabolism. METHODS: Hexanal, octanal, and 2-methylbutanal were selected for in vitro metabolism analysis and compared between the olfactory cleft and respiratory mucus of participants < 50-year-old with normal olfaction using gas chromatography mass spectrometry. The metabolic activity of hexanal in the olfactory cleft mucus was further compared between three groups, (1) normal olfaction, age < 50 years old, (2) normal olfaction, age ≥50 years old, and (3) idiopathic olfactory impairment. To characterize the enzyme(s) responsible for aldehyde reduction, we also tested if epalr22897estat and 3,5-dichlorosalicylic acid, types of reductase inhibitors, affect metabolism. RESULTS: Conversion of aldehydes to their corresponding alcohols was observed in the olfactory cleft and respiratory mucus. The metabolic production of hexanol, octanol, and 2-methybutanol was significantly higher in the olfactory cleft mucus than in the respiratory mucus (p < 0.01). The metabolic conversion of hexanal to hexanol in the mucus of the idiopathic olfactory impairment group was significantly lower than that in the age-matched normal olfaction group. Excluding the nicotinamide adenine dinucleotide phosphate (NADPH) regenerating system from the reaction mixture inhibited metabolism. The addition of either epalr22897estat or 3,5-dichlorosalicylic acid did not inhibit this metabolic conversion. CONCLUSIONS: The enzymatic metabolism of odorants in the olfactory cleft mucus is markedly higher than in the respiratory mucus and decreases in patients with idiopathic olfactory impairment.

Analytical and Sensory Characterization of the Stereoisomers of 3-Mercaptocycloalkanones and 3-Mercaptocycloalkanols.

3-Mercaptocycloalkanones and 3-mercaptocycloalkanols (chain lengths C5-C7) were obtained by addition of thioacetic acid to the respective 2-cycloalken-1-ones and subsequent enzyme-mediated hydrolysis and reduction with LiAlH4, respectively. The stereoisomers were separated via capillary gas chromatography using chiral stationary phases. Their configurations were determined based on 1H NMR data and enzyme-catalyzed kinetic resolutions. Odor thresholds and odor qualities were assessed by capillary gas chromatography/olfactometry. Compared to the analogous acyclic 4-mercapto-2-alkanones and 4-mercapto-2-alkanols, the cyclic polyfunctional thiols lacked fruity, tropical notes; the perceived odor properties ranged from cooked, roasted vegetables and meat types to onion-related notes. The odor thresholds of the enantiomers of the 3-mercaptocycloalkanones were mainly impacted by their ring size rather than their configuration. For the 3-mercaptocycloalkanols, the (S)-configuration at the stereogenic center bearing the thiol group and the relative configuration of the second asymmetric center with the hydroxyl group were of importance for low odor thresholds.

Metabolism of Odorant Molecules in Human Nasal/Oral Cavity Affects the Odorant Perception.

In this study, we examined the mode of metabolism of food odorant molecules in the human nasal/oral cavity in vitro and in vivo. We selected 4 odorants, 2-furfurylthiol (2-FT), hexanal, benzyl acetate, and methyl raspberry ketone, which are potentially important for designing food flavors. In vitro metabolic assays of odorants with saliva/nasal mucus analyzed by gas chromatography mass spectrometry revealed that human saliva and nasal mucus exhibit the following 3 enzymatic activities: (i) methylation of 2-FT into furfuryl methylsulfide (FMS); (ii) reduction of hexanal into hexanol; and (iii) hydrolysis of benzyl acetate into benzyl alcohol. However, (iv) demethylation of methyl raspberry ketone was not observed. Real-time in vivo analysis using proton transfer reaction-mass spectrometry demonstrated that the application of 2-FT and hexanal through 3 different pathways via the nostril or through the mouth generated the metabolites FMS and hexanol within a few seconds. The concentration of FMS and hexanol in the exhaled air was above the perception threshold. A cross-adaptation study based on the activation pattern of human odorant receptors suggested that this metabolism affects odor perception. These results suggest that some odorants in food are metabolized in the human nasal mucus/saliva, and the resulting metabolites are perceived as part of the odor quality of the substrates. Our results help improve the understanding of the mechanism of food odor perception and may enable improved design and development of foods in relation to odor.

Structural analysis and taste evaluation of γ-glutamyl peptides comprising sulfur-containing amino acids.

The structures, flavor-modifying effects, and CaSR activities of γ-glutamyl peptides comprising sulfur-containing amino acids were investigated. The chemical structures, including the linkage mode of the N-terminal glutamic acid, of γ-L-glutamyl-S-(2-propenyl)-L-cysteine (γ-L-glutamyl-S-allyl-L-cysteine) and its sulfoxide isolated from garlic were established by comparing their NMR spectra with those of authentic peptides prepared using chemical methods. Mass spectrometric analysis also enabled determination of the linkage modes in the glutamyl dipeptides by their characteristic fragmentation. In sensory evaluation, these peptides exhibited flavor-modifying effects (continuity) in umami solutions less pronounced but similar to that of glutathione. Furthermore, the peptides exhibited intrinsic flavor due to the sulfur-containing structure, which may be partially responsible for their flavor-modifying effects. In CaSR assays, γ-L-glutamyl-S-methyl-L-cysteinylglycine was most active, which indicates that the presence of a medium-sized aliphatic substituent at the second amino acid residue in γ-glutamyl peptides enhances CaSR activity.

Umami compounds enhance the intensity of retronasal sensation of aromas from model chicken soups.

We examined the influence of taste compounds on retronasal aroma sensation using a model chicken soup. The aroma intensity of a reconstituted flavour solution from which glutamic acid (Glu), inosine 5'-monophosphate (IMP), or phosphate was omitted was significantly lower (p<0.05) than that of the model soup. The aroma intensity of 0.4% NaCl solution containing the aroma chicken model (ACM) with added Glu and IMP was significantly higher (p<0.05) than that of 0.4% NaCl solution containing only ACM. The quantitative analyses showed that adding monosodium glutamate (MSG) to aqueous aroma solution containing only ACM enhanced the intensity of retronasal aroma sensation by 2.5-folds with increasing MSG concentration from 0% to 0.3%. Sensation intensity using an umami solution with added MSG and IMP was significantly higher than that with only MSG when the MSG concentration was 0.05%, 0.075%, or 0.1%. However, it plateaued when MSG concentration was beyond 0.3%.

Characterization of the key aroma compounds in pork soup stock by using an aroma extract dilution analysis.

The aroma extract dilution analysis of an extract prepared from pork stock and subsequent experiments led to the identification of 15 aroma-active compounds in the flavor dilution factor range of 64-2048. Omission experiments to select the most aroma-active compounds from the 15 odor compounds suggested acetol, octanoic acid, δ-decalactone, and decanoic acid as the main active compounds contributing to the aroma of pork stock. Aroma recombination, addition, and omission experiments of these four aroma compounds in taste-reconstituted pork stock showed that each compound had an individual aroma profile. A comparison of the overall aroma between this recombined mixture and pork stock showed strong similarity, suggesting that the key aroma compounds had been successfully identified.

Characterization of the key aroma compounds in beef extract using aroma extract dilution analysis.

Aroma extract dilution analysis (AEDA) of an ether extract prepared from beef extract (BE) and subsequent identification experiments led to the determination of seven aroma-active compounds in the flavor dilution (FD) factor range of 32-128. Omission experiments to select the most aroma-active compounds from the seven aroma compounds suggested that 2,3,5-trimethyl pyrazine, 1-octen-3-ol, 3-methylbutanoic acid, and 4-hydroxy-2,5-dimethyl-3(2H)-furanone were the main active compounds contributing to the aroma of BE. Aroma recombination, addition, and omission experiments of the four aroma compounds in taste-reconstituted BE showed that each compound had an individual aroma profile. A comparison of the overall aroma between this recombination mixture and BE showed a high similarity, suggesting that the key aroma compounds had been identified successfully.

Action of transglucosidase from Aspergillus niger on maltoheptaose and [U-(13)C]maltose.

Oligosaccharides synthesized from a mixture of maltoheptaose and [U-(13)C]maltose with transglucosidase [EC 2.4.1.24] from Aspergillus niger were investigated. When the reaction mixture was incubated at 15 degrees C for 1h, several types of oligosaccharides with DP (degree of polymerization) 2 to DP8 containing alpha-D-Glcp-(1-->6)-maltoheptaose were detected by liquid chromatography-mass spectrometry (LC-MS) and methylation analysis. Most of these compounds consisted of alpha-(1-->4) linkages in the main chain and alpha-(1-->6) linkages at the non-reducing ends. However, when the reaction mixture was incubated for 96h, most of these products were converted into oligosaccharides with DP2 to DP5 consisting of only alpha-(1-->6) linkages. These results suggested that A. niger transglucosidase rapidly transferred glucosyl residues to maltooligosaccharides, and gradually hydrolyzed both alpha-(1-->4) linkages and alpha-(1-->6) linkages at the non-reducing end, and transformed these into smaller molecules of mainly alpha-(1-->6) linkages.

Stereochemical course of the generation of 3-mercaptohexanal and 3-mercaptohexanol by beta-lyase-catalyzed cleavage of cysteine conjugates.

The product resulting from the reaction between E-2-hexenal and l-cysteine was shown to be a diastereoisomeric mixture of 2-(2-S-l-cysteinylpentyl)-1,3-thiazolidine-4-carboxylic acid 1. Treatment of the conjugate with two sources of cysteine-S-conjugate beta-lyase (tryptophanase from E. coli and a crude enzyme extract prepared from Eubacterium limosum) resulted in the formation of 3-mercaptohexanal. The reaction proceeded with a slight preference for the (S)-configured product, however, with low conversion rate. The role of 3-S-l-cysteinylhexanal 2 as substrate for beta-lyases was demonstrated by in situ generation of 2 from 3-S-(N-acetyl-l-cysteinyl)hexanal using acylase. Opposite enantioselectivity was observed for the liberation of 3-mercaptohexanol from 3-S-l-cysteinylhexanol 5 by the enzyme preparations from Eubacterium limosum and tryptophanase. Various yeasts produced 3-mercaptohexanol starting from 1 as well as from 5. The reactions proceeded without preferential formation of one of the enantiomers.

Stereoselectivity of the generation of 3-mercaptohexanal and 3-mercaptohexanol by lipase-catalyzed hydrolysis of 3-acetylthioesters.

The enantioselectivity of the generation of 3-mercaptohexanal and 3-mercaptohexanol, two potent sulfur-containing aroma compounds, by lipase-catalyzed hydrolysis of the corresponding 3-acetylthioesters was investigated. The stereochemical course of the kinetic resolutions was followed by capillary gas chromatography using modified cyclodextrins as chiral stationary phases. The enzyme preparations tested varied significantly in terms of activity and enantioselectivity (E). The highest E value (E = 36) was observed for the hydrolysis of 3-acetylthiohexanal catalyzed by lipase B from Candida antarctica resulting in (S)-configured thiol products. Immobilization of the enzyme (E = 85) and the use of tert-butyl alcohol as cosolvent (E = 49) improved the enantioselectivity. Modification of the acyl moiety of the substrate (3-benzoylthiohexanal) had no significant impact. The sulfur-containing compounds investigated possess attractive odor properties, and only one of the enantiomers exhibits the pleasant citrus type note.