Characterization of enological oak tannin extracts by a multi-analytical approach

Complex ellagitannin extracts from oak are commonly used to improve wine properties, but their compositions are not well known. The composition of eight commercial oak tannin extracts was characterised using a multi-analytical approach. The concentrations of phenolic compounds evaluated by gravimetric analysis of the fraction adsorbed on polyvinylpolypyrrolidone and by UV spectrophotometry varied from 473 to 776 mg/g and from 334 to 576 mg/g, respectively. HPSEC-UV-dRI analysis showed differences in molecular size distribution. The concentration of sugars and polyols (quercitol), determined as alditol acetates by GC-FID, ranged from 59 to 259 mg/g. Nitrogen compounds determined by Kjeldahl analysis were present in very low amounts. Untargeted metabolomics data highlighted 159 features, including ellagitannins and their derivatives (e.g., formed upon toasting) and other molecules such as lignans and triterpenosides, showing large differences between samples. This work demonstrates the variability in the composition of commercial oak tannin extracts, likely to impact their properties.