Interferences during analysis of polyphenols in fruit juices

Abstract: One of the most commonly used methods for analysis of polyphenols is the Folin-Ciocalteu (FC) method, where the FC reagent has been reported to interfere with some substances in fruit juices. The interfering substance that is present in the highest amount in most fruit juices is ascorbic acid. In this study, the total phenolic content (TPC) of three fruit juices was analysed by the FC method as well as with an enzymatic method with hydrogen peroxide oxidoreductase type II (HRP) for comparison. To investigate how the interfering ascorbic acid affects the TPC, juice samples were analysed after removal and addition, respectively, of ascorbic acid. The samples were analysed with HPLC both before and after the extraction phase of the FC method. The results show a decrease in ascorbic acid of approximately 21 % after extraction compared to before. This study showed that the FC method and the HRP method differ in sensitivity regarding various groups of polyphenols. The analysis concerning cyanidin-3-O-β-glucoside and tannins yielded higher values with the FC method than the HRP method whereas procyanidin B2 instead had a lower value. Furthermore, it was shown that heat treatment of fruit juice could result in degradation of polyphenols. Since fruit juices have very complex matrices, TPC values should be compared for each type of fruit juice separately.