A comparison of methods used to define the phenolic content and antioxidant activity of Croatian wines

Concentrations of phenolic antioxidants and antioxidant activities were determined for three different vintages of red varietal Plavac mali wines (Grgich), white varietal Posip wines (Grgich) and white varietal Zlahtina wines (Grskovic). All three mentioned cultivars (Vitis vinifera L.) are well exploited in vineyards along the Croatian coast. Two different tests, the spectrophotometric Folin-Ciocalteau test and redox derivative potentiometric titration with electrogenerated chlorine, were used to quantify phenolic antioxidants and express them in gallic acid equivalents. The sequence of wines obtained by the two methods, ranked according to the increasing phenolic content, was comparable. Among all the tested wines, Plavac mali of the vintage 2003 showed the highest phenol content of similar to 5 g/L. As expected, due to the lack of anthocyanins and other pigments present in red wines, all six white wines showed approximately ten times lower phenolic levels in comparison with red wines, averaging between 190-380 mg/L. This study demonstrates the utilization of quick and reliable analytical techniques, spectrophotometry and derivative potentiometric titration, in quantification of wine phenolics. The change in free radical scavenging ability of the same set of wines was evaluated according to the Brand-Williams assay. The results show, on average, eight times higher free radical scavenging ability of red wines. Also, a slight decrease in the free radical scavenging ability of the older vintage white wines was observed, while the antioxidant activities of the older vintage red wines (Plavac mali) were slightly higher, due to formation of condensed tannins with time.