Flavor Effects and Adsorptive Properties of Purified Fractions of Grape-Seed Phenols

¹ Department of Viticulture and Enology, University of California, Davis.

Grape seed phenolics were extracted and separated into three distinct groups of compounds. The separated fractions were evaluated in turn for their sensory effects in water and in wine with respect to their thresholds for astringency, bitterness, and effect on taste interaction with acidity. If was found that the three fractions have considerably different flavor effects, both qualitatively and quantitatively. The catechin fraction was bitter but not astringent at levels likely to occur in wine. The leucoanthocyanin fraction and the condensed tannin fraction were both bitter and astringent. All three affected the judges' reaction to acidity by moderating the apparent sour taste when added at levels near threshold. All three fractions appear capable of important effects on these quality factors at levels which may occur in white wines and certainly do occur in rosé and red wines. The condensed tannin fraction contributed the most intense flavor effects on a weight basis, and the evident flavor effects of the phenolic substances in wine were found to be 10 to 100 times as intense as previously noted. The levels present in a red wine are about 10 to 100 times the level barely detectable in a white wine.

A comparison was made of the adsorptive properties of casein, gelatin, isinglass, nylon, and insoluble polyvinylpyrrolidone with respect to these phenolic fractions. Large differences were found in relative affinity for the different phenolic fractions. The PVP had a greater capacity in phenol per unit weight than did nylon, and the proteins were greater yet. The synthetic resins had a greater affinity for the smaller molecules than for the condensed tannins, and the proteins were the reverse, under the conditions tested. The resins had considerable affinity for all the fractions in column tests, but, as fining agents the proteins showed no affinity for the catechin fraction under our conditions.

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