Enzymatic synthesis, purification and in vitro antioxidant capacity of polyphenolic oxidation products from apple juice

5-O-Caffeoylquinic acid (CQA) and (−)-epicatechin (EC) commonly present in apple were oxidized with a polyphenoloxidase extract (PPO) from apple to produce oxidation products formed in the course of apple processing into juice. The oxidation products were characterized by RP-HPLC-ESI-MS. CQA homodimers and EC–CQA heterodimers were the main compounds produced as shown on the extracted ion LC-MS chromatograms of the corresponding deprotonated molecules at m/z 705 and 641, respectively. Thirteen fractions purified by semi preparative HPLC were obtained as pure compounds according to the MS spectrum. In vitro DPPH˙ radical scavenging activity of those purified oxidation products was evaluated and compared to the native parent molecules and Trolox. The results were discussed on the basis of structure–activity relationship. On a molar basis, heterodimers exhibited higher antioxidant potential than homodimers. However, they showed a lower activity than an equimolar mixture of the parent molecules. These results provide new information about antioxidant activities of polyphenolic oxidation products which have an impact on the nutritional value of apple juices and other apple-derived foods.