MECHANISM OF BROWNING IN FRESH HIGHBUSH BLUEBERRY FRUIT (VACCINIUM-CORYMBOSUM L) - PARTIAL-PURIFICATION AND CHARACTERIZATION OF BLUEBERRY POLYPHENOL OXIDASE

The most efficient method for polyphenol oxidase (PPO) extraction from Highbush blueberry fruits was the preparation of an acetone powder. N o activity was detected after direct extraction with phosphate buffer (pH 6.5) and detergents such as Triton X-100. PPO has been purified 19 -fold, by ultrafiltration, ammonium sulphate precipitation and hydroph obic chromatography. Native-PAGE of the purified fraction revealed the presence of two isoforms. PPO has an observed optimum pH at 4.0, foll owed by a shoulder at pH 5.0. Caffeic acid is the best substrate (100% ), followed by chlorogenic acid (60%) and pyrocatechol (32.5%). No act ivity was detected towards catechin, protocatechuic acid, resorcinol a nd monophenols.