Red wine was stored in different oak barrels or in stainless steel, and samples were taken for two years to determine 79 aroma compounds. Aging in oak affects 41 compounds. The type of wood affects 11 compounds. At least seven different processes seem to take place concurrently in aroma evolution, and five such processes, affecting 37 compounds, are linked to the oak cask. These are extraction from the wood, oxidation of wine alcohols and amino acids, microbiological formation of ethyl phenols, sorption processes, and condensation of acetaldehyde with polyphenols. The wood can release linear gamma- and delta-lactones, beta-damascenone, and ionones. Some compounds are released very fast from wood, which suggests they lie in the external part of the wood. Some extraction profiles are too complex to be explained by physical processes. Finally, the levels of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and 2-ethyl-5-methyl-4-hydroxy-3(2H)-furanone increase even in the reference wine, which suggests the presence of a precursor.

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A model based on physicochemical parameters has been developed and applied to the study of the kinetics of extraction of aroma compounds from oak casks having been used a variable number of times. Two major deviations from the model can be observed: a strong seasonal dependence of extraction (unexpected in a thermostated cellar); and higher yields with a smaller number of longer extractions. 4-Propylguaiacol follows both behaviors, and its levels have been found to be closely related to those of 4-ethylguaiacol and to the number of uses of the barrel, which suggests that this compound may be formed by the same microorganisms forming ethylphenols from a precursor found in wood. Although the extraction kinetics of all aroma compounds appear to be more or less mediated by the action of microorganisms, those of whiskeylactones, eugenol, and 4-allyl-2,6-dimethoxyphenol are more in accordance with the physicochemical model, whereas those of vanillin, syringaldehyde, furfuryl alcohol, and, of course, 4-propylguaiacol are much more coherent with a microbiological extraction/formation.