Understanding the Effect of Barrel-to-Barrel Variation on Color and Phenolic Composition of a Red Wine
Aging in oak barrels is a traditional and widespread practice in winemaking worldwide. Alternative containers, such as stainless steel tanks, concrete vessels, or polyethylene tanks, surpass barrels in some respects, like price, hygiene and material homogeneity. Nevertheless, barrels are still firmly established in quality wine production due to their positive influence on the organoleptic quality and complexity of wine.
Various phenomena related to physical and chemical characteristics of the oak are directly responsible for these effects. First, there is water and ethanol non-negligible evaporation due to the porosity of the wood,3 as well as some wine absorption by the wood (especially in new barrels).
Second, there is the transfer of extractable compounds, such as ellagitannins and volatile substances, like guaiacol, eugenol, ethyl- and vinyl-phenols, as well as oak lactones (ß-methyl-y-octalactone) and furfural (-derivates). The total amount, though, is limited and quickly reduced by the extraction process into wine. The extracted substances influence sensations, such as astringency and mouthfeel, and increase aroma intensity and complexity.
Third, moderate oxygen permeation and diffusion, through the wood, promote different reactions of oxidation, polymerization, co-pigmentation and condensation, involving anthocyanins and proanthocyanins, which stabilize the color and reduce astringency. Storage in barrels accelerates the natural sedimentation of unstable colloidal matter, thus contributing to wine stability and limpidity.