Enantioselective synthesis of 6-methyloctanal and 8-methyldecanal, the characteristic aroma components in yuzu Citrus junos, and the analysis of their enantiomeric compositions in yuzu essential oil.

6-Methyloctanal and 8-methyldecanal are the characteristic aroma components of yuzu Citrus junos. However, their absolute configurations and enantiomeric compositions in yuzu essential oil have not been analyzed. A concise enantioselective synthesis of both aldehydes was successfully carried out to determine their absolute configurations and enantiomeric compositions. Both aldehydes in yuzu essential oil were found to be (S)-form with high enantiomeric excess.

Concise enantioselective synthesis of wine lactone via intramolecular Diels-Alder reaction.

An enantioselective synthesis of (3S,3aS,7aR)-wine lactone, a major aroma component of white wine and citrus juices, was achieved starting from (S)-2-methyl-3-butenoic acid. An intramolecular Diels-Alder reaction was employed as a key step.

Identification of Odor-Active Compounds Released from a Damaged Plant of the Asian Skunk Cabbage Symplocarpus renifolius.

Symplocarpus renifolius (Asian skunk cabbage) is a perennial herb of the Araceae family. As its common name implies, this plant produces a strong garliclike irritant odor with a rotten note when the plant parts are crushed. To elucidate the odor characteristics, the volatile compounds released from crushed plant parts (rhizome, petioles, and leaf blades) of S. renifolius were identified by a dynamic headspace method. Fifteen sulfur compounds were identified as odor-active compounds by gas chromatography-mass spectrometry-olfactometry (GC-MS-O). The sulfur compounds may be responsible for the strong odor emitted by crushed skunk cabbage. Many of the compounds lack a carbon-carbon bond, and all of the carbon atoms are connected to sulfur. This is regarded as the characteristic structure of the sulfur compounds released from the damaged plant parts of the skunk cabbage. Nine of the sulfur compounds were detected in all three of the plant parts analyzed in this study: hydrogen sulfide, methanethiol, 1-hexanethiol, methyl dithioformate, 2,4-dithiapentane, dimethyl trisulfide, methional, 2,3,5-trithiahexane, and tris(methylthio)methane. Methyl dithioformate and methylthiomethyl dithioformate were identified for the first time as natural products.

Identification of an olfactory signal molecule that activates the central regulator of reproduction in goats.

Pheromone signals regulate conspecific behavior and physiology [1]. Releaser pheromones induce specific behavior by exerting acute effects on the neuronal response, whereas primer pheromones induce physiological changes with long-lasting effects by changing the neuroendocrine status of the recipients. In mammals, although several types of releaser pheromones have been identified [2-5], the identities of primer pheromones, as well as their mechanisms of action, remain largely unknown [6]. In sheep and goats, the seasonally anestrous endocrine state of females is changed to the estrous state upon exposure to male scents [7, 8]. This so-called "male effect" is one of the most conspicuous primer pheromone effects in mammals [9, 10]. In this study, we have identified an olfactory signal molecule that activates the central regulator of reproduction, the gonadotropin-releasing hormone (GnRH) pulse generator, in goats. Using gas chromatography-mass spectrometry to analyze male goat headspace volatiles, we identified several ethyl-branched aldehydes and ketones. We electrophysiologically demonstrated that one of these compounds, 4-ethyloctanal, activates the GnRH pulse generator in female goats. This is the first report of an olfactory molecule that has been shown to activate the central reproductive axis, and this discovery will provide a new direction for primer pheromone research.

Factors influencing the formation of medicinal off-flavor from ascorbic acid and α,ß-unsaturated aldehydes.

In a previous study, the formation and formation pathway of 6-propylbenzofuran-7-ol as an off-flavor compound formed from ascorbic acid and (E)-hex-2-enal in a test apple beverage were investigated. In the present study, elucidating the pH, temperature, and (E)-hex-2-enal and ascorbic acid concentrations that lead to the generation of 6-propylbenzofuran-7-ol in various model solutions was performed. The quantities of 6-propylbenzofuran-7-ol generated in model solutions increased when the initial concentrations of (E)-hex-2-enal and ascorbic acid increased, and linear correlations between the quantities and concentrations were observed. The quantity of 6-propylbenzofuran-7-ol steadily rose in step with pH from pH 2.24, peaked at pH 3.73, and steadily decreased in step with pH to pH 4.24. The quantity of 6-propylbenzofuran-7-ol increased sharply as the storage temperature rose. Meanwhile, no 6-propylbenzofuran-7-ol was produced when short-duration heating processes such as heat sterilization were applied. These results will allow to us prevent the occurrence of off-flavor by regulating the initial pH, storage temperature, and (E)-hex-2-enal and ascorbic acid concentrations. Reactions with ascorbic acid and other α,ß-unsaturated aldehydes were also investigated, and it was confirmed that corresponding 6-alkylbenzofuran-7-ols were formed.

Elucidating the formation pathway of the off-flavor compound 6-propylbenzofuran-7-ol.

In a previous work, we identified 6-propylbenzofuran-7-ol as an off-flavor compound formed from ascorbic acid and (E)-hex-2-enal in a test apple beverage. In this study, we elucidate the pathway by which 6-propylbenzofuran-7-ol formed. Isotope labeling studies revealed that the propyl group of 6-propylbenzofuran-7-ol derives from (E)-hex-2-enal and that 6-propylbenzofuran-7-ol contains carbons 2-6 of ascorbic acid. Two compounds, namely, 2,3-dihydro-6-propylbenzofuran-3,7-diol and 3-(2-furoyl)hexanal, were identified as byproducts of a model reaction of ascorbic acid and (E)-hex-2-enal. Each of these compounds was dissolved in an aqueous solution of citric acid and stored at 60 °C for 1 week. After storage, 6-propylbenzofuran-7-ol was detected from a solution of 2,3-dihydro-6-propylbenzofuran-3,7-diol, but not from 3-(2-furoyl)hexanal. 6-Propylbenzofuran-7-ol was formed by isolating tricyclic hemiacetal lactone derived from the Michael addition of ascorbic acid to (E)-hex-2-enal, mixing the tricyclic hemiacetal lactone with the aqueous solution of citric acid, and applying heat. This confirmed that 6-propylbenzofuran-7-ol was formed via the Michael adduct.

Identification of the medicinal off-flavor compound formed from ascorbic acid and (E)-hex-2-enal.

A test apple beverage made up of apple juice (20%), high-fructose corn syrup (11.5%), citric acid (0.43%), trisodium citrate (0.02%), apple-odor flavor (0.1%), and ascorbic acid (0.02%) was stored at 40 °C and then analyzed for the change of odor in the beverage. Although no thermoacidophilic bacteria (TAB) were detected, a medicinal off-flavor was perceived after the 8 weeks of storage. Model experiments on the ingredients of the test apple beverage revealed that the off-flavor compound had been formed by ascorbic acid and (E)-hex-2-enal. Synthesis and NMR (¹H, ¹³C, HMQC, and HMBC) analyses identified the compound as 6-propylbenzofuran-7-ol. The odor quality, retention index (RI), and mass spectrum of synthetic 6-propylbenzofuran-7-ol were identical with those of the medicinal odor compound from the test apple beverage. Sensory evaluation revealed the recognition thresholds for medicinal odor were 31.4 ppb in water and 24.0 ppb in apple beverage, and the detection thresholds were 19.6 ppb in water and 8.6 ppb in apple beverage, respectively. The quantified concentration of 6-propylbenzofuran-7-ol formed in test apple beverage was 90 ppb, approximately. This concentration was well above the odor threshold, so it was concluded that the compound was the source of the medicinal off-flavor.

Novel key aroma components of galbanum oil.

Galbanum oil is composed of monoterpenes in large amounts and pyrazines in small amounts. Although the monoterpenes are the main components of galbanum oil, they hardly contribute to the distinct galbanum aroma. The scanty amounts of pyrazines, in contrast, contribute significantly to the aroma. Considering the complexity and potency of the odor, the essential oil was assumed to contain so far not identified compounds with high odor contribution. By the gas chromatography-mass spectrometry-olfactometry (GC-MS-O) analysis of galbanum oil, fruity-green-balsamic notes were detected at two different retention times. The mass spectra (MS) of the newly discovered notes were elucidated by conducting multidimensional (MD) GC-MS-O. By analyzing the MS data, six chemical structures were proposed: (6E/Z,8E)-undeca-6,8,10-trien-2-one, (6E/Z,8E)-undeca-6,8,10-trien-3-one, and (6E/Z,8E)-undeca-6,8,10-trien-4-one. The compounds were then synthesized in an attempt to match the MS, retention indices (RI), and odor qualities. The MD-GC-MS-O analyses of the candidate compounds led to the identification of the novel key aroma compounds (6Z,8E)-undeca-6,8,10-trien-3-one and (6Z,8E)-undeca-6,8,10-trien-4-one in galbanum oil.

Novel character impact compounds in Yuzu (Citrus junos Sieb. ex Tanaka) peel oil.

Yuzu ( Citrus junos Sieb. ex Tanaka), a tree-grown fruit similar to a kind of sour orange, is widely used in Japanese food/cooking for its pleasant flavor. To clarify the odor-active volatiles that differentiate yuzu from other citrus fruits, sensory evaluations were conducted on yuzu peel oil. The results revealed that the polar part of yuzu peel oil was the source of the characteristic aroma of fresh yuzu fruit. By aroma extract dilution analysis (AEDA) of the polar volatile part of yuzu peel oil, seven odorants were newly identified as odor-active volatiles in yuzu peel oil in the highest flavor dilution (FD) factors of 128 and 32: oct-1-en-3-one, (E)-non-4-enal, (E)-dec-4-enal, 4-methyl-4-mercaptopentan-2-one, (E)-non-6-enal, (6Z,8E)-undeca-6,8,10-trien-3-one (Yuzunone), and (6Z,8E)-undeca-6,8,10-trien-4-ol (Yuzuol). Among the most odor-active volatiles in yuzu, (E)-non-6-enal and Yuzunone were identified for the first time solely in yuzu peel oil and not in the peel of other citrus species, and Yuzuol was identified for the first time in nature. Sensory evaluation of yuzu aroma reconstitutions revealed that the newly identified compound, Yuzunone, contributes greatly to the distinct yuzu aroma.