Characterization and Role of Polyphenol Oxidase and Peroxidase in Browning of Fresh-Cut Melon

摘要

Polyphenol oxidase(PPO)and peroxidase(POD)were extracted from two different varieties of melon(Cucumis meloL cantalupensis cv.Charentais and C.melo L.inodoruscv.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°C in both'Charentais'and'Amarillo'cultivars,whereas POD maximum activity was found at 45°C in'Charentais'and at 25°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°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°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°C was highly correlated with changes of parameters a*,if,chroma,and hue angle(r2 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.