Effect of heat and thermosonication treatments on peroxidase inactivation kinetics in watercress (Nasturtium officinale)

The effect of heat and the combined heat/ultrasound (thermosonication) treatment on the inactivation kinetics of peroxidase in watercress (Nasturtium officinale) was studied in the temperature range of 40-92.5 °C. In the heat blanching processes, the enzyme kinetics showed a first-order biphasic inactivation model. The activation energies and the rates of the reaction at a reference temperature for both the heat-labile and heat-resistant fractions were, respectively, E_a1 = 421 ± 115 kJ mol^-1 and E_a2 = 352 ± 81 kJ mol^-1, k184.6°C= 18±14min-1 and k284.6°C=0.24±0.14min-1. The initial relative specific activity for both isoenzyme fractions were also estimated, being C ₀₁ = 0.5 ± 0.08 μmol min^-1 mg protein ^-1 and C₀₂ = 0.5 ± 0.06 μmol min^-1 mg protein^-1, respectively. The application of thermosonication was studied to enable less severe thermal treatments and, therefore, improving the quality of the blanched product. In this treatment the enzyme kinetics showed a first-order model. The activation energy, the rate of reaction at a reference temperature and the initial relative specific activity were, respectively, E_a3 = 496 ± 65 kJ mol^-1, k387.5°C= 10±2min-1 and C₀₃ = 1 ± 0.05 μmol min^-1 mg protein^-1, proving that the enzyme became more heat labile. The present findings will help to design the blanching conditions for the production of a new and healthy frozen product, watercress (Nasturtium officinale), with minimized colour or flavour changes along its shelf life.