Abstract
The spectral profiles of edible burdock extract during browning reaction suggested that the oxidation of chlorogenic acid and its analogues mainly causes enzymatic browning in edible burdock. Polyphenol oxidase (PPO) was purified-16. 6-fold with a recovery rate of 21% using chlorogenic acid as substrate. The purified enzyme appeared as a single band on PAGE and SDS-PAGE. The molecular weight of the enzyme was estimated to be about 41, 000 and 40, 000 by gel filtration and SDS-PAGE, respectively. The purified enzyme quickly oxidized chlorogenic acid and (-) -epicatechin. The Km values of the enzyme were 0.4 mM for chlorogenic acid (pH 5. 0, 20°C) and 2.7 mM for (-) -epicatechin (pH 8.0, 20°C). The optimum pHs were 5.0 for chlorogenic acid oxidase (ChO) and 8.0 for (-) -epicatechin oxidase (EpO). In the pH range from 5 to 8, both ChO and EpO activities were quite stable at 4°C for 22 h. The optimum temperature of both activities was 20°C. Both activities were 50% inactivated after a heat treatment at 45°C for 30 min. Both activities were strongly inhibited by L-ascorbic acid and L-cysteine at 5 mM.
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