Abstract
Grape tannin content and composition studies have been limited to a few varieties. To determine whether grape tannin composition was generic or variety specific, we examined skin tannins in 36 cultivars. Tannins were analyzed by phloroglucinolysis and gel permeation chromatography (GPC). Concentration ranged from 1.3 (Malbec) to 15.6 mg/g fresh weight of skin (Red Emperor). Catechin terminal subunits ranged from 45.6% (Lagrein) to 78.5% (Black Rose), epicatechin from 19.0% (Black Rose) to 49.7% (Lagrein) and epicatechin-gallate from 1.0% (Muscat Hamburg) to 8.4% (Barbera). Epigallocatechin was not observed as a terminal subunit. Catechin extension subunits ranged from 1.4% (Pinot Gris) to 6.2% (Beauty Seedless), epicatechin from 36.3% (Cienna) to 69.1% (Cardinal), epicatechin-gallate from 2.3% (Pinot Gris) to 11.5% (Beauty Seedless) and epigallocatechin from 22.6% (Cardinal) to 53.7% (Cienna). Average polymer length by phloroglucinolysis ranged from 6.6 (Malbec) to 46.1 subunits (Lagrein). In the GPC fraction containing the largest polymers, lengths ranged from 38.1 (Barbera) to 112.4 subunits (Flame Seedless). Polymer lengths were in many cases greater than previous reports. While there was inter-specific variation in tannin content and composition, terminal units were generally catechin or epicatechin and extension subunits epicatechin or epigallocatechin. A weak relationship was observed between total tannin and average polymer length (R2=0.46). Tannin content and polymer length was often higher in table grapes than winegrapes. Understanding tannin diversity between grape varieties underpins planting, harvesting and blending decisions to produce wines that meet consumer expectations.
Keywords: Condensed tannin, Vitis vinifera, gel permeation chromatography, HPLC, phloroglucinolysis, polymer length distribution.
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