Sensory-guided decomposition of red currant juice (Ribes rubrum) and structure determination of key astringent compounds

Sequential application of solvent extraction, gel permeation chromatography, and RP-HPLC in combination with taste dilution analyses, followed by LC-MS and 1D/2D NMR experiments, led to the discovery and structure determination of 25 key astringent compounds of red currant juice. Besides several flavonol glycosides, in particular, 3-carboxymethyl-indole-1-N-beta-D-glucopyranoside, 3-methylcarboxymethyl-indole-1-N-beta-D-glucopyranoside, and a family of previously not identified compounds, namely, 2-(4-hydroxybenzoyloxymethyl)-4-beta-D-glucopyranosyloxy-2(E)-butenenitrile, 2-(4-hydroxy-3-methoxybenzoyloxymethyl)-4-beta-D-glucopyranosyloxy-2(E)-butenenitrile, (E)-6-3-hydroxy-4-(O-beta-D-glucopyranosyl)phenyl]-5-hexen-2-one named dehydrorubrumin, and (3E,5E)-6-3-hydroxy-4-(O-beta-D-glucopyranosyl)phenyl]-3,5-hexadien-2-one named rubrumin, have been identified. Determination of the oral astringency thresholds by means of the half-tongue test revealed that the lowest thresholds of 0.3 and 1.0 nmol/L were found for the nitrogen-containing 3-carboxymethyl-indole-1-N-beta-D-glucopyranoside and 3-methylcarboxymethyl-indole-1-N-beta-D-glucopyranoside, which do not belong to the group of plant polyphenols.