Characterization of the medium infrared spectra of polyphenols of red and white wines by integrating FT IR and UV–Vis spectral data

Infrared spectroscopy is widely applied in the wine sector. The medium infrared (MIR) spectral region between 1800 and 900 cm-1 is very informative for polyphenols. Aim of this work was to perform an in-depth analysis of this spectral region of polyphenolic wine extracts. To this goal evolution of the polyphenolic fractions of white (Vermentino) and red (Cannonau) typical Sardinian wines was studied during wine-making. Wine polyphenolic extracts, obtained by solid phase extraction, were analyzed by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) and UV–Vis spectroscopy. Spectral data from the two techniques were integrated and results depicted as Heat Map of the correlation coefficient values allowing a more accurate characterization of the MIR fingerprint region. For Cannonau, correlations between absorptions in the 420–650 and 380 nm visible spectral ranges allowed assignment of MIR peaks to anthocyanins (1016 and 1198 cm-1) and flavonols (1159 and 1277 cm-1), respectively. In Vermentino we identified MIR bands due to non-flavonols and others arising from flavonols (1376 and 1407 cm-1), present in this white wine where vinification allows short time contact with grape skins. During time, intensities of MIR bands at lower frequencies assigned to the sugar moiety of glycosylated phenols decreased in both wines.