Isolation, Structure Determination, and Sensory Activity of Mouth-Drying and Astringent Nitrogen-Containing Phytochemicals Isolated from Red Currants (Ribes rubrum)

Application of chromatographic separation and taste dilution analyses recently revealed, besides a series of flavon-3-ol glycosides and ( E )/( Z )-aconitic acid, four nitrogen-containing phytochemicals as the key astringent and mouth-drying compounds in red currants ( Ribes rubrum ). The isolation and structure determination of the astringent indoles 3-carboxymethyl-indole-1- N -β- d -glucopyranoside ( 1 ) and 3-methylcarboxymethyl-indole-1- N -β- d -glucopyranoside ( 2 ), as well as the astringent, noncyanogenic nitriles 2-(4-hydroxybenzoyloxymethyl)-4-β- d -glucopyranosyloxy-2( E )-butenenitrile ( 3 ) and 2-(4-hydroxy-3-methoxybenzoyloxymethyl)-4-β- d -glucopyranosyloxy-2( E )-butenenitrile ( 4 ) by means of 1D/2D NMR, LC-MS/MS, and UV−vis spectroscopy are reported. The structures of compounds 1 and 2 were confirmed by synthesis. Using the recently developed half-tongue test, human recognition thresholds for the astringent and mouth-drying nitrogen compounds were determined to be between 0.0003 and 5.9 μmol/L (water). In particular, the extraordinarily low threshold of 0.0003 μmol/L evaluated for the indole 1 represents the lowest recognition threshold of any astringent phytochemical reported to date. Keywords: Red currants; astringency; taste dilution analysis; half-tongue test; 3-carboxymethyl-indole-1- N -β- d -glucopyranoside; 3-methylcarboxymethyl-indole-1- N -β- d -glucopyranoside; 2-(4-hydroxybenzoyloxymethyl)-4-β- d -glucopyranosyloxy-2( E )-butenenitrile; 2-(4-hydroxy-3-methoxybenzoyloxymethyl)-4-β- d -glucopyranosyloxy-2( E )-butenenitrile