Abstract 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.

Colloidal copper dispersions are prepared by reducing copper(II) ions in water with sodium tetrahydroborate or hydrazine in the presence of various protective polymers. The polymers are poly(N-vinyl-2-pyrrolidone), poly(vinyl alcohol), poly(methyl vinyl ether), poly(potassium vinyl sulfate), dextrin, amylopectin, methylamylopectin, methylcellulose, ethylcellulose, and (2-hydroxyethyl)cellulose. The dispersions are black, reddish dark brown, or reddish brown homogeneous solutions, and are stable under nitrogen at room temperature for more than three months. Electron diffraction experiments indicate that copper atoms in the colloidal particles are arranged in an ordered way, which is almost identical with that in a crystal of bulk copper metal. At the charged molar ratio 40 of the monomeric residue of the protective polymer to copper(II) ion, the average diameters of the copper particles, prepared by use of sodium tetrahydroborate, range from 50 to 150 Å, depending on the polymer used. With the use of poly(N-vinyl-2-pyrrolidone) as protective polymer, the size of the copper particles monotonously increases with increase in the degree of polymerization of the polymer and also with decrease in the amount of the polymer.

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.

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