The evolution of low molecular weight polyphenols in Spanish oak heartwood of Quercus robur,Quercus petraea, Quercus pyrenaica, and Quercus faginea was studied by HPLC, in relation to the processing of wood in barrel cooperage. The polyphenolic composition of Spanish woods subjected to natural seasoning for 3 years and to the toasting process was studied in relation to those of French oak of Q. robur (Limousin) and Q. petraea (Allier) and American oak of Q. alba (Missouri), which are habitually used in cooperage. The concentrations of benzoic and cinnamic acids and aldehydes of Spanish woods increased during seasoning depending on the duration of this process and in the same way as those of French and American woods. The process having the main influence on the phenolic composition of wood was the toasting. It led to high increases in the concentration of phenolic aldehydes and acids, especially cinnamic aldehydes (sinapic and coniferylic aldehydes), followed by benzoic aldehydes (syringaldehyde and vanillin) and benzoic acids (syringic and vanillic acids). This polyphenolic composition in Spanish oak species evolved during toasting as in French and American oak, but quantitative differences were found, which were especially important in American species with respect to the others.

The evolution of tannins in Spanish oak heartwood of Quercus robur L., Quercus petraea Liebl.,Quercus pyrenaica Wild., and Quercus faginea Lam. was studied in relation to the processing of wood in barrel cooperage. Their evolution was compared with that of French oak of Q. robur (Limousin, France) and Q. petraea (Allier, France) and American oak of Quercus alba L. (Missouri), which are habitually used in cooperage. Two stages of process were researched: the seasoning of woods during 3 years in natural conditions and toasting. Total phenol and total ellagitannin contents and optical density at 420 nm of wood extracts were determined. The ellagitannins roburins A-E, grandinin, vescalagin, and castalagin were identified and quantified by HPLC, and the molecular weight distribution of ellagitannins was calculated by GPC. During the seasoning process the different ellagitannin concentrations decreased according to the duration of this process and in the same way as those in French and American woods. The toasting process also had an important influence on the ellagitannin composition of wood. Roburins A-E, grandinin, vescalagin, and castalagin decreased during this process in the Spanish wood species, in the same proportion as in the French and American ones. Also, the seasoning and toasting processes lead to qualitative variations in the structure of ellagitannins, especially in the molecular weight distribution, as was evidenced by GPC analysis of their acetylated derivatives.

Low molecular weight polyphenols and ellagitannins were analyzed by HPLC, and the molecular weight distribution of ellagitannins was calculated by GPC, in oak heartwood of Quercus robur L., Quercus petraea Liebl., Quercus pyrenaica Wild., and Quercus faginea Lam., grown in Spain, before and after 1 year of seasoning, in Bordeaux, France. During this process, the concentrations of low molecular weight polyphenols (acids and aldehydes, benzoic and cinnamic, and coumarins) increased, and those of ellagitannins (castalagin, vescalagin, and roburins A-E) decreased. A similar behavior for the A and B compounds in all species was not found. This modification in the chemical composition was similar in the four Spanish species of Quercus studied and allowed the differentiation between the unseasoned wood and the wood after the first year of seasoning.

Low molecular weight phenolics were analyzed by HPLC in oak heartwood of Quercus robur L., Quercus petraea Liebl., Quercus pyrenaica Wild., and Quercus faginea Lam. Data were processed with the STATGRAPHICS program. Gallic, ellagic, vanillic, syringic, and ferulic acids, vanillin, syringaldehyde, coniferaldehyde, sinapaldehyde, aesculetin, and scopoletin were found. Ellagic and gallic acid derivatives were also recognized according to their hydrolysis products and their UV spectra. Q. robur and Q. petraea differ from the others by the relative concentration of these compounds.

Low molecular weight polyphenols were studied by HPLC in samples of cork from different trees of Spanish Quercus suber and at different industrial processing stages. Gallic, protocatechuic, vanillic, caffeic, ferulic, and ellagic acids; protocatechuic, vanillic, coniferylic, and sinapic aldehydes, and aesculetin and scopoletin were identified and quantified. Ellagic acid was the main component in all of the samples, followed by the rest of the phenolic acids, which had very much lower concentrations. Four components, caffeic, ferulic, and protocatechuic acids and vanillin, were selected as those that provided the greatest differences among the trees studied. In relation to the four industrial processing stages considered, marked differences were observed between the first two stages, stripping and first rest, and the stages after boiling. In this case, the discriminant variables were coniferaldehyde, sinapaldehyde, and ellagic acid. Keywords: Quercus suber; cork; polyphenols; phenolic acids and aldehydes; coumarins; high-performance liquid chromatography