The effect of successive uses of oak barrels on the extraction of oak-related volatile compounds from wine

Changes in the concentration of furfuryl compounds, guaiacol and 4-methylguaiacol, cis- and trans-oak lactone, and vanillin were studied during the maturation of wines in oak barrels of different origins (French oak and American oak). The rate at which these volatile compounds accumulated in the wines followed a curve that fits to mathematical models. The changes in concentration were usually consistent with diffusion kinetics. When the barrels are new, the concentration gradient between the wood surface and the wine is high, so diffusion occurs quickly, especially in the case of lactones, the extraction rate of which could be fitted to a power curve. After some time of maturation, the concentration gradient is not so large and furthermore, the wood surface may be exhausted of some compounds so the rate of extraction decreases. During the second fill, the rate of accumulation of these compounds in the wine was almost linear, especially during the first 6 months of storage, and the quantities measured in wine were much lower. The concentration of some compounds, increased during the final 3 months (between days 180 and 270), exhibiting a quadratic curve when plotted. Furfuryl aldehydes behaved differently as they are extracted very rapidly from the first millimetres of wood surface and transformed into furfuryl alcohol, a compound with no organoleptic significance.