CHARACTERIZATION AND ROLE OF POLYPHENOL OXIDASE AND PEROXIDASE IN BROWNING OF FRESH-CUT MELON

Polyphenol oxidase (PPO) and peroxiclase (POD) were extracted from two different varieties of melon (Cucumis melo L. cantalupensis cv. Charentais and C. melo L. inodorus cv. Amarillo) and characterized using reliable spectrophotometric methods. In both cases the enzymes followed Michaelis-Menten kinetics, showing different values of kinetics parameters between the two cultivars: K(m) = 7.18 +/- 0.70 mM ('Charentais') and 6.66 +/- 0.20 mM ('Amarillo') mM; V(max) = 7.93 +/- 0.35 units/min ('Charentais') and 13.82 +/- 0.37 units/min ('Amarillo'), relative to PPO; K(m) = 24.0 +/- 2.10 mM ('Charentais') and 5.05 +/- 0.19 mM ('Amarillo') mM; V(max) = 344.83 +/- 10.32 units/min ('Charentais') and 80.64 +/- 2.01 units/min ('Amarillo'), relative to POD. Optimum pH for PPO was 7.0 for 'Charentais' and 7.5 for 'Amarillo, whereas it was 4.5 for both cultivars relative to POD. Melon PPO had maximum activity at 60 degrees C in both 'Charentais' and 'Amarillo' cultivars, whereas POD maximum activity was found at 45 degrees C in 'Charentais' and at 25 degrees C in 'Amarillo'. POD from both cultivars showed higher thermolability compared with PPO, losing > 90% of relative activity after only 5 min of incubation at 70 degrees C. POD's activation energy was much higher than that of PPO (Delta E(#) = 86.3 and 160.6 kJ mol(-1) for 'Charentais' and 'Amarillo', respectively). PPO and POD activities from both cultivars showed a decreasing pattern as sugar concentration in the assay medium increased, except in POD extract from 'Charentais', which maintained its activity in the presence of high D-glucose concentration (up to 5 M). Changes in L(*), a(*), b(*), chroma, and hue angle values were chosen to describe the browning development in the samples during storage at 5 degrees C. A slight decrease in L* value and a more marked reduction of a* value were noted in both cultivars above all at the end of storage period. POD activity during storage at 5 degrees C was highly correlated with changes of parameters a*, b*, chroma, and hue angle (r(2) from 0.82 to 0.97) for cultivar 'Charentais'. According to these results, only POD activity seemed to be involved in browning of minimally processed melon.