Abstract: The effect of different concentrations of citric acid treatment on the relative activity, kinetic parameters (Km and Vmax) and browning parameters (Hue, Chroma and BI) of polyphenol oxidase (PPO) from mushroom. The results showed that the relative activity of PPO decreased gradually with increasing citric acid concentration and was only 1% of its original activity at 70 mmol/L. In the presence of citric acid, the reaction rate of PPO showed a gradual increase with increasing concentration of catechol, the substrate and reached its maximum level at a substrate concentration of 15 mmol/L. However, the Km remained basically unchanged despite an increase in citric acid concentration, while the Vmax gradually decreased. Lineweaver-Burk plots demonstrated that the Vmax values of PPO in the presence of citric acid at concentrations of 0, 20 mmol/L and 40 mmol/L were 578.4, 437.8 U/min and 111.6 U/min, respectively, suggesting that citric acid is a non-competitive inhibitor for the reaction between PPO and catechol. Increasing citric acid concentration resulted in small changes in L* and a* and a sharp decrease in b*. The b* values from browning catalyzed by PPO in the presence of 40 mmol/L and 70 mmol/L citric acid were reduced to 57.5% to 13.7% of its origin value observed when citric acid was absent. In addition, Hue, Chroma and BI (browning index) presented a dramatic decline as well. These consistent changes implied that relative activity and Vmax had direct correlations with b*, Hue, Chroma and BI.

Key words: citric acid, polyphenol oxidase, relative activity, kinetic parameters, browning parameters