Disappearance of Anthocyanins as Grape Juice Is Prepared and Oxidized with PPO and PPO Substrates

Red pigments were isolated from Muscat Bailey A and Cabernet Sauvignon grape skins and analyzed by high performance liquid chromatography. Cabernet Sauvignon skin extract contained only anthocyanidin-3-monoglucosides, while that from Muscat Bailey A skins contained about 75% 3-monoglucosides and 25% 3,5-diglucosides. Polyphenoloxidase (PPO) was prepared from Muscat Bailey A juice. Soluble and insoluble PPO fractions were obtained by centrifugation. The PPO fractions had two activities, catecholase (much higher) and cresolase, and showed high activities toward various phenols, especially vicinal diphenols such as caftaric acid, catechol, and caffeic acid. The insoluble fraction had stronger activities than the soluble fraction and was washed free of associated phenols. Enzymatic decolorization and degradation of anthocyanins were studied by measuring the absorbance at 520 nm and by determining the residual anthocyanins by HPLC after the reaction. The enzyme did not oxidize anthocyanins directly (either as flavylium or carbinol forms), but they were decolorized in coupled oxidation of diphenols such as catechol and caftaric acid. The addition of benzenesulfinic acid inhibited decolorization completely and very rapidly, while glutathione partially suppressed the oxidative reaction, showing that quinones produced from oxidizable phenols are the active agents in decolorization of red pigment. The degree of decolorization or degradation of individual anthocyanins was influenced by their structures. The mechanisms involved and the significance of these results are briefly discussed.