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Proanthocyanidins from Quercus petraea Bark.

E Pallenbach1, E Scholz, M König

  • 1Institut für Pharmazeutische Biologie, Schänzlestraße 1, D(W)-7800 Freiburg, Federal Republic of Germany.

Planta Medica
|June 1, 1993
PubMed
Summary

Oak bark tannins, primarily oligomeric proanthocyanidins, were analyzed. Despite low abundance, these compounds significantly contribute to the astringency of Quercus petraea bark extracts.

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Area of Science:

  • Phytochemistry
  • Natural Products Chemistry
  • Plant Biochemistry

Background:

  • Oak bark (Quercus petraea) is a rich source of tannins, known for their diverse biological activities and sensory properties.
  • Understanding the specific composition and contribution of different tannin types is crucial for various applications, including food, beverage, and pharmaceutical industries.
  • Oligomeric proanthocyanidins are a significant class of tannins with complex structures and potent astringency.

Purpose of the Study:

  • To isolate and characterize flavanols and oligomeric proanthocyanidins from Quercus petraea bark.
  • To determine the structural features, including degree of polymerization and monomeric composition, of these compounds.
  • To quantify the contribution of oligomeric proanthocyanidins to the overall astringency of oak bark tannins.

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Main Methods:

  • Isolation of flavanols and oligomeric proanthocyanidins from oak bark.
  • Characterization using acid hydrolysis, gel permeation chromatography, haemanalysis, and thiolytic degradation.
  • Assessment of astringency contribution from different tannin fractions.

Main Results:

  • Oligomeric proanthocyanidins from Quercus petraea bark exhibited an average polymerization degree of 6.1, with a procyanidin to prodelphinidin ratio of 6:4.
  • The predominant monomeric units were (+)-catechin, (-)-epicatechin, and (+)-gallocatechin, with minor amounts of galloylated derivatives.
  • Although comprising only 23% of water-soluble tannins, oligomeric proanthocyanidins accounted for 55% of the total astringency.

Conclusions:

  • Quercus petraea bark contains both hydrolyzable and condensed tannins, with oligomeric proanthocyanidins being key contributors to astringency.
  • The structural characteristics of these oligomeric proanthocyanidins, particularly their degree of polymerization and monomeric composition, influence their sensory impact.
  • This study highlights the significant role of specific tannin subclasses in defining the organoleptic properties of oak bark extracts.