The phenolic chemistry and spectrochemistry of red sweet wine-making and oak-aging

•Colour evolved towards yellow–brown colour with aging red sweet wines.•Vitisins increased and exhibit orange tones that play an important role during aging.•Hydroxybenzoic and hydroxycinnamic acids increased during the aging period.•Anthocyanin, flavan-3-ol monomer, oligomer and polymer levels decreased with aging.•The characteristics of aged red sweet wines produced and marketed in Spain were defined. A natural sweet wine (NSW) was made with dried grapes from Vitis vinifera L. cv Garnacha Tintorera. A fortified sweet wine (FSW) was also obtained: the maceration-alcoholic fermentation of Garnacha Tintorera must was stopped by addition of ethanol 96% (v/v). UV/Vis spectrophotometry and HPLC/DAD-ESI/MS were applied to determine, respectively, the evolution of colour and phenolic compounds in Garnacha Tintorera based-sweet wines during aging. In sweet wines, aging decreased a∗ (red/green), colour saturation and lightness and increased b∗ (yellow/blue), and hue angle. Most of the phenolic compounds determined, such as anthocyanins, esters of hydroxycinnamic acids, flavan-3-ols monomers, oligomers and polymers decreased in both sweet wines during aging. On the contrary, hydroxybenzoic and hydroxycinnamic acids and vitisins increased after one year of aging. Despite that both terminal and extension subunit compositions show very small changes, mean degree of polymerisation of proanthocyanidins decline slightly as aging progressed in both sweet wines.