Polyphenoloxidases in Jerusalem artichoke (Helianthus tuberosus L.)

Purpose - The purpose of this study was to investigate the polyphenoloxidases (PPO) activities in Jerusalem artichoke tubers as well as their inactivation. The following are important for the processing of this food: treatment and conversion used in preserving processes. Design/methodology/approach - The PPO of whole tubers and the separated tuber skins as determined using a modified PPO essay with 0.01 M dopamine.HCl as reactant and photometry (Delta A(470) plotted against reaction time up to 3 min). Nine varieties used in German tuber production were in the test. Findings - The highest enzyme activity of PPO was in the skin, range from 1,274 up to 3,026 nkat. In the pulps of the tubers nearly traces of PPO could be detected (range: 2 up to 5 nkat). The inhibition of PPO activity in tuber homogenates was investigated in simulated processes which were used in the food industry: heat (drying, pasteurisation) and oxidase inhibitors such as lemon juice, ascorbic and/or citric acid. The optimum temperature of PPO was 60 degrees C and the inactivation occurred at 85 degrees C. The range of the PPO activity was between pH 5 and 10, with the optimum at pH 7.0. Lemon juice is a natural inhibitor of the PPO activity in fruit and vegetable juices containing Jerusalem artichoke as well as in canning the tubers. Lemon juice combined with citric acid and ascorbic acid is recommended against enzymatic browning reaction. Originality/value - The characterisation of the parameters which influence enzymatic browning of Jerusalem artichoke tubers during food processing: heat and pH, have not been described before.