Oak Flavour: Tannins and other phenolics

tannins and other phenolics act as a reservoir to balance the oxidative/reductive reactions of the wine, protecting it from oxidation and lessening the chance of unpleasant reductive aromas. Hydrolysable tannins derived from oak lignin are known as ellagitannins. Their concentration decreases with heavy toasting. It is worth noting that wine in the barrel is biologically active. The yeasts that effect the fermentation of sugars to alcohol also transform some of these directly extracted oak compounds into other compounds with flavours different from the original. The furfurals, for instance, are derived from hemicellulose, and are transformed by the yeasts into compounds which have a range of flavours from smoky to coffee. For example, during barrel fermentation of white wines, furfurylrthiol (FFT) is formed by yeast from the furfural released by toasted staves. Furfural levels increase with the duration of the toasting.