DISAPPEARANCE OF ANTHOCYANINS AS GRAPE JUICE IS PREPARED AND OXIDIZEDWITH PPO AND PPO SUBSTRATES

Red pigments were isolated from Muscat Bailey A and Cabernet Sauvignon grape skins and analyzed by high performance liquid chromatography. C abernet Sauvignon skin extract contained only anthocyanidin-3-monogluc osides, while that from Muscat Bailey A skins contained about 75% 3-mo noglucosides and 25% 3,5-diglucosides. Polyphenoloxidase (PPO) was pre pared 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 t oward various phenols, especially vicinal diphenols such as caftaric a cid, catechol, and caffeic acid. The insoluble fraction had stronger a ctivities 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 o xidize anthocyanins directly (either as flavylium or carbinol forms), but they were decolorized in coupled oxidation of diphenols such as ca techol and caftaric acid. The addition of benzenesulfinic acid inhibit ed decolorization completely and very rapidly, while glutathione parti ally suppressed the oxidative reaction, showing that quinones produced from oxidizable phenols are the active agents in decolorization of re d pigment. The degree of decolorization or degradation of individual a nthocyanins was influenced by their structures. The mechanisms involve d and the significance of these results are briefly discussed.