Why do oak barrels change the taste of wine?
A barrel is a slow wine-flavouring machine: it lets in a whisper of air, lets out some water and alcohol, and adds vanilla from the wood.
▶ Start the storyA barrel changes wine in three ways at once: it lets in a little air, it lets some water and alcohol evaporate, and it adds flavours from the wood itself. Oak is used in winemaking to vary the colour, flavour, tannin profile and texture of wine.
The air part is the surprising one. The wood of an oak barrel is porous, so evaporation and oxygenation occur, but typically not at levels that would cause oxidation or spoilage. Evaporation lets the wine concentrate its flavour and aroma compounds, and small amounts of oxygen soften its tannins.
The wood part is the familiar one. The in the oak interact to produce vanilla-type flavours, and can give an impression of tea notes or sweetness. The degree of on the barrel's inside also changes the tannin levels and how strong the wood flavours are.
Winemakers did not work all this out from chemistry. They noticed. Oak has been prevalent in winemaking for at least two millennia, spreading in the Roman Empire, because winemakers found that wine kept in oak barrels became softer and, in some cases, better-tasting. Before oak, Herodotus noted, the Mesopotamians had used palm-wood barrels to carry wine along the Euphrates.
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One more thing to know, because it trips up many people: butter flavours do not come from oak. They come from , which is produced during , the step in which the wine's tart malic acid is converted to softer lactic acid.
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Recap
A barrel breathes, shrinks the wine, and flavours it with phenols from the wood.
💡 A trick to remember it · Barrel = breathe, shrink, flavour: it lets air in a drop at a time, lets water and alcohol out, and lends vanilla.
Surprising fact · Butter flavours come from diacetyl made during malolactic fermentation, not from the oak.
Sources (2)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.