Inactivation kinetics and photoreactivation of vegetable oxidative enzymes after combined UV-C and thermal processing

Abstract

Abstract The inactivation kinetics of lipoxygenase (LOX), peroxidase (POD) and polyphenoloxidase (PPO) in phosphate buffer (pH 4.0 and 7.0) treated by combined thermal (25–65 °C) and UV-C (1–10 min) processes were fitted using a traditional first-order kinetics model and the Weibull distribution function. For complete inactivation, a treatment at 65 °C for 7.5–10 min for LOX, POD and PPO at pH 7.0 and 45 °C for 5–7.5 min for POD and PPO at pH 4.0 was necessary. Deviations from the log-linear behavior were observed by the appearance of shoulders, tails or both (sigmoidal). The traditional log-linear model failed to characterize the UV treatment effectively due to the under- and overestimation of enzyme inactivation. The Weibull model was better able to explain the nature of the UV treatment. The extent of enzyme inactivation was less in orange juice due to the greater absorbance of the juice in the UV-C range. In general, activities of residual enzymes after UV-C treatment did not recover after storage for 24 h at refrigeration conditions with or without light exposure. The proposed combination of thermal and UV-C processing was able to improve the stability of the treated samples. Industrial relevance UV irradiation has demonstrated to be an effective technology to decontaminate surfaces and reduce microbial load of liquid food in a low cost, simple and chemical-free manner. However, its application in the pasteurization of liquid food needs to be validated against achieving an adequate enzymatic stability. A kinetic study on the inactivation of quality-related enzymes (peroxidase, lipoxygenase and polyphenoloxidase) after the combined thermal and UV-C processing (25–65 °C for 1–10 min) was conducted. The combined treatment (45–65 °C for 5 min) was able to achieve a complete reduction of enzymatic activity. The extent of enzyme inactivation was less in orange juice due to the greater absorbance of the matrix. Enzymes were irreversibly inactivated and enzyme activities did not recover after storage.