The compounds responsible for the bitter off-taste of processed onion ( Allium cepa) were studied. Using a series of sensory-guided HPLC fractionations, the existence of nine groups of hitherto unknown sulfur compounds has been revealed. On the basis of spectroscopic data (MS, NMR, and IR), it was found that these compounds, trivially named allithiolanes A-I, are members of a large family of structurally closely related derivatives of 3,4-dimethylthiolane S-oxide, with the general formulas of C xH yO2S4, C xH yO3S5, and C xH yO4S6 ( x = 10-18, y = 18-30). The presence of multiple stereoisomers was observed for each group of allithiolanes. Allithiolanes possess an unpleasantly bitter taste with detection thresholds in the range of 15-30 ppm. Formation pathways of these newly discovered sulfur compounds were proposed.
Keywords: Allium cepa; allithiolane; bitter; cepaene; cepathiolane; garlicnin; isoalliin; off-taste; onion; onionin; taste-dilution analysis; thiolane S-oxide.