Extraction of phenolics in liquid model matrices containing oak chips: Kinetics, liquid chromatography–mass spectroscopy characterisation and association with in vitro antiradical activity

Four different liquid model matrices were utilised to study the leaching of polyphenols from oak chips. The matrices included distilled water, 12% (v/v) ethanol, 12% (v/v) ethanol adjusted to pH 3.4, and 55% (v/v) ethanol. Extraction of phenolics into the liquid systems was monitored by the estimation of the total polyphenol concentration, using the Folin–Ciocalteu method. The in vitro antiradical activity was also recorded using the stable DPPH radical, to ascertain enrichment of the solutions with potentially antioxidant compounds. As a final step, the polyphenolic composition of each matrix was characterised by means of liquid chromatography–electrospray ionisation mass spectrometry. The kinetics of polyphenol leaching into the liquid phase was found to obey a 2nd parameter power equation of the type y = axb, which produced a good fit of the data (p < 0.0001). Kinetics was faster in distilled water up to a point, where after polyphenol extraction occurred at higher rate in the 55% ethanolic solution. The antiradical activity in all cases was highly correlated with total polyphenol concentration (p < 0.001), providing that the amount of polyphenols extracted into the liquid media exerted a proportional antioxidant effect. The analytical examination by liquid chromatography–mass spectrometry revealed that the compounds implicated are hydrolysable tannins and hydrolysis products thereof.