Compositional changes of the floral scent volatile emissions from Asian skunk cabbage (Symplocarpus renifolius, Araceae) over flowering sex phases.

INTRODUCTION: Flowering of the Asian skunk cabbage (Symplocarpus renifolius, Araceae) shows a sequential expression of female, bisexual and male sex phases. The protogynous thermogenic inflorescence has unpleasant odours, but the contributing chemical composition is poorly understood. OBJECTIVE: To determine the volatile composition of odour emissions from each S. renifolius flowering phase. METHODOLOGY: The dynamic headspace method was used to collect floral volatiles from six intact S. renifolius inflorescences in their natural habitat. Collected volatiles from the three flowering phases were analysed using gas chromatography-mass spectrometry/olfactometry (GC-MS/O). RESULTS: Female-phase inflorescences were characterised by an earthy-rotten-minty odour, while male-phase inflorescences typically exhibited a rotten-oily odour. Approximately 160 compounds were detected in volatiles from the three phases. Common to all phases were 3-methylbutyl 3-methylbutanoate, 1,8-cineole, dimethyl disulphide and sabinene, together accounting for 52 to 54% of total volatiles. GC-MS/O revealed that at least 28 volatiles including eight S-containing compounds contributed to the unpleasant odour of S. renifolius. Among them, dimethyl disulphide (onion-like), methional (potato-like), and the tentatively identified methyl dithioformate (garlic-like) were intense odour-active compounds in each floral phase. Additionally, 2-isopropyl-3-methoxypyrazine (IPMP) was a major contributor to the earthy odour that was characteristic of the female phase. CONCLUSIONS: No marked changes were observed in floral volatile compositions over the three flowering phases of S. renifolius. Instead, flower phase-dependent proportional changes of minor components (e.g. IPMP and 2,3-butanedione) altered the odour characteristics between the female and male phases.

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.

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.