The Tannins of Oak Heartwood: Structure, Properties, and Their Influence on Wine Flavor

Abstract

Barrels used for the aging of wines and spirits are constructed from oak heartwood. Heartwood tannins (soluble polyphenols) are the most abundant of the constituents of oak wood that are potentially extracted into the wine during aging. The structure and properties of these compounds are reviewed, as are methods of analysis. Studies of the natural variation in the concentration of heartwood tannins are summarized and show that levels are highly variable within and between individual trees and especially between wood provenance and species. Despite the abundance of heartwood tannins and their solubility in the wines, the concentration in oak-aged wines is generally lower then expected and their influence on wine flavor remains uncertain. Different studies and hypotheses concerning the influence of oak tannins during wine aging are reviewed and conclusions drawn on our current understanding of their role.