Kinetics of phenolic extraction during aging of model wine solution and white wine in oak barrels

The objective of the present work was to study the mechanism and kinetics o f phenolic extraction from wood to wine during aging in barrels and to deve lop a mathematical model capable of representing this migration. French oak barrels of 20-L capacity were filled with a model wine solution and the ex traction of phenolics was measured as a function of time and location withi n the barrel. Strong concentration gradients were observed during the first few days, indicating fast dissolution of phenolics from the wood surface a nd relatively slow diffusion through the unstirred liquid. From the second week onwards, the gradients were found to decrease rapidly, indicating that the main resistance to phenolic transfer was on the wood side. Examination of the extraction data indicated that mass transfer rate was controlled by the rate of liquid penetration into the wood rather than by diffusional tr ansport of phenolics. A model, based on penetration-rate controlled release of wood phenolics, adequately predicted the shape of phenolic extraction c urves as well as the actual phenolic extraction from new and once-used barr els into the model wine solution. In the case of white wine aged in new bar rels, the model satisfactorily predicts the shape of the extraction curve, but overestimates the actual quantity of phenolics that were extracted afte r several weeks of contact. This may be due to differences in liquid penetr ation and phenolics solubilization rate between the model solution and whit e wine.