Effects of microwave heating on pigment composition and colour of fruit purees

Microwave energy was applied to inactivate the oxydoreductases peroxidase ( POD, EC 1.11.1.7) and polyphenol oxidase (PPO, EC 1.14.18.1) in processed f ruit products. Microwave blanching of papaya, strawberry and kiwi purees at various conditions of power and time produced inactivation of PPO and POD activities depending on the fruit and the heating conditions. Treatment at 850 W/60 s produced about 60% of POD inactivation for papaya and kiwi fruit . POD activity in strawberry, however, seemed to be more resistant to micro wave inactivation; treatment at 850 W/60s only achieved a loss of POD activ ity near 8%. Papaya oxidoreductases showed lower stability in the microwave treatments tested. Microwave blanching at 475 W/45 s produced about 75% in activation of POD activity and nearly complete PPO inactivation. Kiwi fruit and strawberry purees exhibited similar inactivation of PPO - 32% at 475 W /30 s and 70% at 475 W/60 s. The decrease of PPO activity in both products was almost linear at constant power. This thermal treatment, however, direc tly affects the colour of the fruit pulps. Papaya, kiwi and strawberry pure es suffered slight colour (CIE L*a*b*) changes. Carotenoid, chlorophyll and anthocyanin changes were evaluated by HPLC and related to objective colour . Microwave treatments produced small modifications of the quantitative and qualitative composition of carotenoids (in papaya) and anthocyanins (in st rawberry). Chlorophylls (kiwi) showed significant degradation as a conseque nce of microwave heating. (C) 1999 Society of Chemical Industry.