Structure-Odor Activity Studies on Monoterpenoid Mercaptans Synthesized by Changing the Structural Motifs of the Key Food Odorant 1-p-Menthene-8-thiol

Changes in microbial diversity and volatile metabolites during the fermentation of Bulang pickled tea

The present study aimed to determine microbial community, short-chain fatty acids (SCFAs), and volatilome of Bulang pickled tea during fermentation. Sequencing of 16S rRNA and ITS revealed that Bualng pickled tea was dominated by Lactobacillus plantarum, unclassified Enterobacteriaceae, unclassified Debaryomyces, Candida metapsilosis, Cladosporium sphaerospermum, and unclassified Aspergillus. The overall contents of SCFAs increased, with acetic acid showing the highest content. A total of 398 differential volatile metabolites were detected using differential metabolomics analysis. Out of these different volatile compounds, ten key volatile compounds including (Z)-4-heptenal, 1-(2-thienyl)-ethanone, 5-methyl-(E)-2-hepten-4-one, 2-ethoxy-3-methylpyrazine, p-cresol, 2-methoxy-phenol, ethy-4-methylvalerate, 3-ethyl-phenol, p-menthene-8-thiol, and 2-s-butyl-3-methoxypyrazinewere were screened based on odor activity value (OAV). The Spearman correlation analysis showed a high correlation of SCFAs and volatile compounds with microorganisms, especially L. plantarum and C. sphaerospermum. This study provided a theoretical basis for elucidating the flavor quality formation mechanism of Bulang pickled tea.

Determining Ultra-Low Organic Molecular Odor Thresholds in Air Helps Identify the Most Potent Fungal Aroma Compound

The determination of odor threshold values can be performed in various matrices, including air, and serves as a parameter to compare the potencies of odorous compounds. Typically, the odor thresholds in air are determined by gas chromatography-olfactory (GC-O) and referenced to an internal standard, most often (E)-dec-2-enal. Herein, a direct gas chromatography-flame ionization detector-olfactory analysis method for the determination of odor thresholds in air is reported. As model substrates for this novel approach, naturally occurring substances (R)-1-p-menthene-8-thiol as well as (3S,3aS,6R,7aS)-3,6-dimethyl-3a,4,5,6,7,7a-hexahydro-3H-1-benzofuran-2-one were used. The latter compound was synthesized from (-)-isopulegol and exhibited an extremely low odor recognition threshold of 1.9 × 10-6 ng L-1 air, the lowest value reported for a fungal aroma compound thus far.

Monoterpene Thiols: Synthesis and Modifications for Obtaining Biologically Active Substances

Monoterpene thiols are one of the classes of natural flavors that impart the smell of citrus fruits, grape must and wine, black currants, and guava and are used as flavoring agents in the food and perfume industries. Synthetic monoterpene thiols have found an application in asymmetric synthesis as chiral auxiliaries, derivatizing agents, and ligands for metal complex catalysis and organocatalysts. Since monoterpenes and monoterpenoids are a renewable source, there are emerging trends to use monoterpene thiols as monomers for producing new types of green polymers. Monoterpene thioderivatives are also known to possess antioxidant, anticoagulant, antifungal, and antibacterial activity. The current review covers methods for the synthesis of acyclic, mono-, and bicyclic monoterpene thiols, as well as some investigations related to their usage for the preparation of the compounds with antimicrobial properties.

Impact of crossflow microfiltration on aroma and sensory profiles of a potential functional citrus-based food

BACKGROUND

Citrus juices can be cold-concentrated by crossflow microfiltration (CMF) in order to obtain functional foods enriched in carotenoids, flavonoids and pectins. The work aimed to characterize the organoleptic quality of this type of micronutrient-dense foods through their aroma profile and sensory analysis. Two citrus concentrates with and without a diafiltration step were compared.

RESULTS

Both citrus products were very different, linked to aroma compound, sugar and organic acid contents. Due to its sugar/acidity balance and its better aromatic profile responsible for the citrus-floral flavour, the concentrate without diafiltration was preferred by the sensory panel. Thanks to a simple transfer model, we showed that retention of volatiles clearly varied from one aroma compound to another. The terpene hydrocarbons were the most retained by the membrane during CMF, probably because they were strongly associated with insoluble solids by adsorption.

CONCLUSION

Even though the process modified their organoleptic profiles, both citrus-based products were well rated and can be consumed directly as pleasant functional drinks. © 2022 Society of Chemical Industry.

Odor Characteristics of Novel Non-Canonical Terpenes

Several non-canonical, methylated terpenes have been described as products of genetically modified Escherichia coli recently, and the aroma properties of 28 odor-active methylated derivatives of prenol, isoprenol, bornane, camphene, carene, citronellol, fenchol, geraniol, limonene, linalool, terpineol, and farnesol were characterized for the first time in the current study. Twelve methylated monoterpenes exhibited a particularly intense and pleasant odor and were therefore chosen for the determination of their respective odor thresholds (OTs) in comparison to their non-methylated equivalents. In addition to the determination of OTs based on the literature value for the internal standard, (2E)-decenal, the threshold values of the compounds with individually determined OTs of the participants were calculated. This enabled a more precise identification of the OTs. Among the non-canonical terpenes, the lowest OTs in the air were found for 2-methyllinalool (flowery, 1.8 ng L⁻¹), 2-methyl-α-fenchol (moldy, 3.6 ng L⁻¹), 2-methylgeraniol (flowery, 5.4 ng L⁻¹), 2-methylcitronellol (citrus-like, 7.2 ng L⁻¹), and 4-methylgeraniol (citrus-like, 16 ng L⁻¹). The derivatives of geraniol, linalool, and citronellol showed very pleasant odor impressions, which could make them interesting for use as flavoring agents in the flavor and fragrance industry.

Occurrence of Marine Ingredients in Fragrance: Update on the State of Knowledge

The fragrance field of perfumes has attracted considerable scientific, industrial, cultural, and civilizational interest. The marine odor is characterized by the specific smell of sea breeze, seashore, algae, and oyster, among others. Marine odor is a more recent fragrance and is considered as one of the green and modern fragrances. The smells reproducing the marine environment are described due to their content of Calone 1951 (7-methyl-2H-1,5-benzodioxepin-3(4H)-one), which is a synthetic compound. In addition to the synthetic group of benzodioxepanes, such as Calone 51 and its derivatives, three other groups of chemical compounds seem to represent the marine smell. The first group includes the polyunsaturated cyclic ((+)-Dictyopterene A) and acyclic (giffordene) hydrocarbons, acting as pheromones. The second group corresponds to polyunsaturated aldehydes, such as the (Z,Z)-3,6-nonadienal, (E,Z)-2,6-nonadienal, which are most likely derived from the degradation of polyunsaturated fatty acids. The third group is represented by small molecules such as sulfur compounds and halogenated phenols which are regarded as the main flavor compounds of many types of seafood. This review exposes, most notably, the knowledge state on the occurrence of marine ingredients in fragrance. We also provide a detailed discussion on several aspects of essential oils, which are the most natural ingredients from various marine sources used in fragrance and cosmetics, including synthetic and natural marine ingredients.

Hydroxylamine-Derived Reagent as a Dual Oxidant and Amino Group Donor for the Iron-Catalyzed Preparation of Unprotected Sulfinamides from Thiols

An iron catalyzed reaction for the selective transformation of thiols (-SH) to sulfinamides (-SONH2 ) by a direct transfer of -O and free -NH2 groups has been developed. The reaction operates under mild conditions using a bench stable hydroxylamine derived reagent, exhibits broad functional group tolerance, is scalable and proceeds without the use of any precious metal catalyst or additional oxidant. This novel, practical reaction leads to the formation of two distinct new bonds (S=O and S-N) in a single step to chemoselectively form valuable, unprotected sulfinamide products. Preliminary mechanistic studies implicate the role of the alcoholic solvent as an oxygen atom donor.

Basis set dependence of S[double bond, length as m-dash]O stretching frequencies and its consequences for IR and VCD spectra predictions

Benchmarking functionals and basis sets for the computational prediction of molecular properties is usually done on very small model systems. Larger organic molecules containing heavier second row atoms are not the typical model structures. We herein present the first survey of basis sets and functionals for the prediction of the IR and VCD spectra of chiral tosylates and sulfinates as we noted drastic deviations between computed harmonic frequencies obtained at B3LYP/6-311++G(2d,p) level of theory and those observed in experimental solution phase IR and VCD spectra. We show that the harmonic frequencies of the asymmetric and symmetric S[double bond, length as m-dash]O stretching modes of tosylates are predicted at significantly too low vibrational frequencies if the employed basis set does not provide higher order polarization functions. The results of our benchmarks show that at least the 6-311G(3df,2dp) basis (or equivalent Dunning and Ahlrichs variants) should be used.

Chirality: An Important Phenomenon Regarding Biosynthesis, Perception, and Authenticity of Flavor Compounds

Chirality plays an important role in flavor research. This will be outlined using selected examples from the key areas analysis, authenticity assessment, biogenesis, and odor perception. Developments of analytical techniques, in particular the use of chiral stationary phases in capillary gas chromatography, enabled the determination of naturally occurring configurations of chiral volatiles at trace levels. Thus, knowledge of pathways and enzymes involved in the biogenesis of chiral substances was acquired, and enantioselective analysis has become a prominent tool in the authenticity assessment of flavorings. Increasing information is also available on structure-odor relationships of chiral flavor compounds and the influence of their configurations on odor thresholds and odor properties. A substantial extension of these data sets and a further understanding of the role of chirality in the perception of aroma compounds is expected from studies on the enantioselectivity of odorant receptor systems. Developments in these areas will be put into a historical perspective, recent progress will be emphasized, and data gaps will be described.

Copper-mediated thiol potentiation and mutagenesis-guided modeling suggest a highly conserved copper-binding motif in human OR2M3

Sulfur-containing compounds within a physiological relevant, natural odor space, such as the key food odorants, typically constitute the group of volatiles with the lowest odor thresholds. The observation that certain metals, such as copper, potentiate the smell of sulfur-containing, metal-coordinating odorants led to the hypothesis that their cognate receptors are metalloproteins. However, experimental evidence is sparse-so far, only one human odorant receptor, OR2T11, and a few mouse receptors, have been reported to be activated by sulfur-containing odorants in a copper-dependent way, while the activation of other receptors by sulfur-containing odorants did not depend on the presence of metals. Here we identified an evolutionary conserved putative copper interaction motif CC/CSSH, comprising two copper-binding sites in TMH5 and TMH6, together with the binding pocket for 3-mercapto-2-methylpentan-1-ol in the narrowly tuned human receptor OR2M3. To characterize the copper-binding motif, we combined homology modeling, docking studies, site-directed mutagenesis, and functional expression of recombinant ORs in a cell-based, real-time luminescence assay. Ligand activation of OR2M3 was potentiated in the presence of copper. This effect of copper was mimicked by ionic and colloidal silver. In two broadly tuned receptors, OR1A1 and OR2W1, which did not reveal a putative copper interaction motif, activation by their most potent, sulfur-containing key food odorants did not depend on the presence of copper. Our results suggest a highly conserved putative copper-binding motif to be necessary for a copper-modulated and thiol-specific function of members from three subfamilies of family 2 ORs.

Partial and Total Solvent-Free Limonene’s Hydrogenation: Metals, Supports, Pressure, and Water Effects

Bio-based solvents menthene and menthane were obtained through limonene’s partial and total hydrogenation under various catalytic conditions. Heterogeneous catalysts based on different active metals and supports (carbon, alumina, and silica) were systematically tested for solvent-free total and partial hydrogenation of limonene under high and low hydrogen pressure. Influences of these catalysts on the formation of menthene, menthane, and cymene, a dehydrogenated product, were determined. The impact of water addition on the conversion and selectivity of the catalysts was also investigated. Amongst all tested catalysts, Rh/Alumina which was never tested for total and partial hydrogenation of limonene was the most effective as 1-menthene was quantitatively produced at low pressure (0.275 MPa) while menthane was mostly obtained at a higher pressure (2.75 MPa). Water addition on Rh/Alumina favoured menthene production even at high pressure. To propose menthane, menthene, and menthane/menthene mixture as an alternative to fossil-based solvents such as n-hexane for the extraction of natural products, β-carotene, vanillin, and rosmarinic acid solubilizations have been investigated. If a modeling approach using COSMO-RS software predicted a comparable solubilization of these 3 compounds for the 3 solvents, experimental assays revealed that menthene solubilizes β-carotene, vanillin, and rosmarinic acid three to five times better than n-hexane.

Nickel-Catalyzed Inter- and Intramolecular Aryl Thioether Metathesis by Reversible Arylation

A nickel‐catalyzed aryl thioether metathesis has been developed to access high‐value thioethers. 1,2‐Bis(dicyclohexylphosphino)ethane (dcype) is essential to promote this highly functional‐group‐tolerant reaction. Furthermore, synthetically challenging macrocycles could be obtained in good yield in an unusual example of ring‐closing metathesis that does not involve alkene bonds. In‐depth organometallic studies support a reversible Ni⁰/Ni^II pathway to product formation. Overall, this work not only provides a more sustainable alternative to previous catalytic systems based on Pd, but also presents new applications and mechanistic information that are highly relevant to the further development and application of unusual single‐bond metathesis reactions.

Analysis of Potent Odour-Active Volatile Thiols in Foods and Beverages with a Focus on Wine

Certain volatile thiols are some of the most potent odour-active molecules that are found in nature. Thiols play significant roles in the aroma qualities of a range of foods and beverages, including wine, with extremely low odour detection thresholds (nanogram per litre range). A fundamental understanding of their formation, fate, and impact essentially depends on the development of suitable analytical methods. The analysis of volatile thiols in foods and beverages is a challenging task when considering (1) the complexity of food and beverage matrices and (2) that thiols are highly reactive, low molecular-weight volatiles that are generally present at trace to ultra-trace concentrations. For the past three decades, the analytical evaluation of volatile thiols has been intensively performed in various foods and beverages, and many novel techniques related to derivatisation, isolation, separation, and detection have been developed, particularly by wine researchers. This review aims to provide an up-to-date overview of the major analytical methodologies that are proposed for potent volatile thiol analysis in wine, foods, and other beverages. The analytical challenges for thiol analysis in foods and beverages are outlined, and the main analytical methods and recent advances in methodology are summarised and evaluated for their strengths and limitations. The key analytical aspects reviewed include derivatisation and sample preparation techniques, chromatographic separation, mass spectrometric detection, matrix effects, and quantitative analysis. In addition, future perspectives on volatile thiol research are also suggested.

Screening for Novel Mercaptans in 26 Fruits and 20 Wines Using a Thiol-Selective Isolation Procedure in Combination with Three Detection Methods

A selection of 46 fruits and wines were screened for mercaptans using a thiol-selective isolation procedure. The identification was based on three methods: gas chromatography-olfactometry (GC-O), gas chromatography-sulfur chemiluminescence detection (GC-SCD), and two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOF/MS). A database constructed from analytical and sensory data for more than 300 sulfur compounds was used to confirm the identifications. Overall, 11 mercaptans were characterized in the fruits or wines for the first time. In addition, 1- p-menthene-9-thiol identified in kumquat is the first report in any food thus far. Thus, besides 1- p-menthene-8-thiol, p-menthane-8-thiole-3-one, and 3-mercapto-3,7-dimethyl-6-octenyl acetate, a fourth naturally occurring mercapto terpenoid occurs in fruits.

Structure/Odor Activity Studies on Aromatic Mercaptans and Their Cyclohexane Analogues Synthesized by Changing the Structural Motifs of Naturally Occurring Phenyl Alkanethiols

Following a structure/odor activity approach as previously published, the present study was focused on three aromatic thiols also identified as food odorants, namely 1-phenylethane-1-thiol, phenyl methanethiol, and 2-phenylethanethiol. Their structures were systematically modified to receive 16 new sulfur-containing benzene derivatives. A determination of odor thresholds indicated that none of its homologues elicited a lower odor threshold than 1-phenylethane-1-thiol, and an enantiospecific synthesis, elucidated that its ( S)-enantiomer turned out to be the compound with by far the lowest odor threshold of 0.00025 ng/L in air. Within the homologous series of the ω-phenylalkane-1-thiols as well as the 1-phenylalkane-1-thiols the threshold values increased constantly with an elongation of the side chain. Among the respective cyclohexane derivatives, the tendencies with respect to thresholds and odor properties were comparable. The odor thresholds and odor qualities of the aromatic thiols were quite similar to those of their heterocyclic analogues considered in a previous publication. In addition, spectroscopic data for 28 new sulfur-containing compounds were generated, which might be helpful in the identification of such sulfur containing odorants occurring in trace levels in foods.

Molecular Basis of Mammalian Odor Discrimination: A Status Report

Humans have 396 unique, intact olfactory receptors (ORs), G-protein coupled receptors (GPCRs) containing receptor-specific binding sites; other mammals have more. Activation of these transmembrane proteins by an odorant initiates a signaling cascade, evoking an action potential leading to perception of a smell. Because the number of distinguishable odorants vastly exceeds the number of ORs, research has focused on mechanisms of recognition and signaling processes for classes of odorants. In this review, selected recent examples will be presented of "deorphaned" mammalian receptors, where the OR ligands (odorants) as well as key aspects of receptor-odorant interactions were identified using odorant-mediated receptor activation data together with site-directed mutagenesis and molecular modeling. Based on cumulative evidence from OR deorphaning and olfactory receptor neuron activation studies, a receptor-ligand docking model rather than an alternative bond vibration model is suggested to best explain the molecular basis of the exquisitely sensitive odor discrimination in mammals.

Structure-Odor Correlations in Homologous Series of Mercapto Furans and Mercapto Thiophenes Synthesized by Changing the Structural Motifs of the Key Coffee Odorant Furan-2-ylmethanethiol

Furan-2-ylmethanethiol (2-furfurylthiol; 2-FFT, 1) is long-known as a key odorant in roast and ground coffee and was also previously identified in a wide range of thermally treated foods such as meat, bread, and roasted sesame seeds. Its unique coffee-like odor quality elicited at very low concentrations, and the fact that only a very few compounds showing a similar structure have previously been described in foods make 1 a suitable candidate for structure-odor activity studies. To gain insight into the structural features needed to evoke a coffee-like odor at low concentrations, 46 heterocyclic mercaptans and thio ethers were synthesized, 32 of them for the first time, and their odor qualities and odor thresholds were determined. A movement of the mercapto group to the 3-position kept the coffee-like aroma but led to an increase in odor threshold. A separation of the thiol group from the furan ring by an elongation of the carbon side chain caused a loss of the coffee-like odor and also led to an increase in odor thresholds, especially for ω-(furan-2-yl)alkane-1-thiols with six or seven carbon atoms in the side chain. A displacement of the furan ring by a thiophene ring had no significant influence on the odor properties of most of the compounds studied, but the newly synthesized longer-chain 1-(furan-2-yl)- and 1-(thiophene-2-yl)alkane-1-thiols elicited interesting passion fruit-like scents. In total, only 4 out of the 46 compounds also showed a coffee-like odor quality like 1, but none showed a lower odor threshold. Besides the odor attributes, also retention indices, mass spectra, and NMR data of the synthesized compounds were elaborated, which are helpful in possible future identification of these compounds in trace levels in foods or other materials.

Structure-Odor Correlations in Homologous Series of Mercaptoalkanols

To study the influence of molecular structure on the sensory properties of mercaptoalkanols, homologous series of 1-mercaptoalkan-3-ols, 3-mercaptoalkan-1-ols, 2-mercaptoalkan-1-ols, 4-mercaptoalkan-2-ols, 3-mercapto-3-methylalkan-1-ols, 1-mercapto-2-methylalkan-3-ols, 3-mercapto-2-methylalkan-1-ols, and 2-ethyl-3-mercaptoalkan-1-ols were synthesized. Odor thresholds in air and odor qualities were determined, and the results obtained were correlated to the chemical structures. Sensory properties were strongly influenced by steric effects: All homologous series revealed a minimum in odor thresholds between five and seven carbon atoms, and increasing the length of the carbon chain led to an exponential increase in odor thresholds. The olfactory power of the thiols was considerably improved by methyl or ethyl substitution in the α-position to the thiol group as well as by an additional methyl or ethyl group at the mercapto-containing carbon atom. By using comparative molecular similarity indices analysis, a 3D-quantitative structure-activity relationship model could be created, which was able to predict the odor thresholds of mercaptoalkanols in good agreement with the experimental results. Retention indices, NMR data, and mass spectra for 49 mercaptoalkanols, most of them synthetically prepared for the first time, are supplied.

The role of metals in mammalian olfaction of low molecular weight organosulfur compounds

Covering: up to the end of 2017While suggestions concerning the possible role of metals in olfaction and taste date back 50 years, only recently has it been possible to confirm these proposals with experiments involving individual olfactory receptors (ORs). A detailed discussion of recent experimental results demonstrating the key role of metals in enhancing the response of human and other vertebrate ORs to specific odorants is presented against the backdrop of our knowledge of how the sense of smell functions both at the molecular and whole animal levels. This review emphasizes the role of metals in the detection of low molecular weight thiols, sulfides, and other organosulfur compounds, including those found in strong-smelling animal excretions and plant volatiles, and those used in gas odorization. Alternative theories of olfaction are described, with evidence favoring the modified "shape" theory. The use of quantum mechanical/molecular modeling (QM/MM), site-directed mutagenesis and saturation-transfer-difference (STD) NMR is discussed, providing support for biological studies of mouse and human receptors, MOR244-3 and OR OR2T11, respectively. Copper is bound at the active site of MOR244-3 by cysteine and histidine, while cysteine, histidine and methionine are involved with OR2T11. The binding pockets of these two receptors are found in different locations in the three-dimensional seven transmembrane models. Another recently deorphaned human olfactory receptor, OR2M3, highly selective for a thiol from onions, and a broadly-tuned thiol receptor, OR1A1, are also discussed. Other topics covered include the effects of nanoparticles and heavy metal toxicants on vertebrate and fish ORs, intranasal zinc products and the loss of smell (anosmia).

Piperitone Profiling in Fine Red Bordeaux Wines: Geographical Influences in the Bordeaux Region and Enantiomeric Distribution

Piperitone was recently identified in red Bordeaux wines, and this study was designed to further explore its contribution to wine aroma. Firstly, a geographical origin effect was detected within the Bordeaux region (left versus right banks of the Gironde estuary), with significantly higher mint aroma intensities and piperitone levels in wines from the left bank (Medoc appellations). Statistical analysis of chemical and sensory data highlighted a correlation with the proportion of Cabernet Sauvignon in the wine blends. Accordingly, it was suggested that these sensory and chemical differences may be of varietal origin. Secondly, the contribution of piperitone to minty aromas in the aging bouquet was confirmed by combining both sensory and chemical data from a wide range of red Bordeaux wines. Subsequent investigation of the enantiomeric distribution of piperitone found a significantly higher proportion of the (+)-(6S) enantiomeric form in wines displaying a strong aging bouquet.

Smelling Sulfur: Copper and Silver Regulate the Response of Human Odorant Receptor OR2T11 to Low-Molecular-Weight Thiols

Mammalian survival depends on ultrasensitive olfactory detection of volatile sulfur compounds, since these compounds can signal the presence of rancid food, O₂ depleted atmospheres, and predators (through carnivore excretions). Skunks exploit this sensitivity with their noxious spray. In commerce, natural and liquefied gases are odorized with t-BuSH and EtSH, respectively, as warnings. The 100-million-fold difference in olfactory perception between structurally similar EtSH and EtOH has long puzzled those studying olfaction. Mammals detect thiols and other odorants using odorant receptors (ORs), members of the family of seven transmembrane G-protein-coupled receptors (GPCRs). Understanding the regulator cofactors and response of ORs is particularly challenging due to the lack of X-ray structural models. Here, we combine computational modeling and site-directed mutagenesis with saturation transfer difference (STD) NMR spectroscopy and measurements of the receptor response profiles. We find that human thiol receptor OR2T11 responds specifically to gas odorants t-BuSH and EtSH requiring ionic copper for its robust activation and that this role of copper is mimicked by ionic and nanoparticulate silver. While copper is both an essential nutrient for life and, in excess, a hallmark of various pathologies and neurodegenerative diseases, its involvement in human olfaction has not been previously demonstrated. When screened against a series of alcohols, thiols, sulfides, and metal-coordinating ligands, OR2T11 responds with enhancement by copper to the mouse semiochemical CH₃SCH₂SH and derivatives, to four-membered cyclic sulfide thietane and to one- to four-carbon straight- and branched-chain and five-carbon branched-chain thiols but not to longer chain thiols, suggesting compact receptor dimensions. Alcohols are unreactive.