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Studies on Thermodynamics and Kinetics of Thermo- Inactivation of Some Quality-Related Enzymes in White Yam (Dioscorea rotundata)

The effect of heat treatment on the activities of three quality related enzymes:- peroxidase (POD), polyphenol oxidase (PPO), and lipoxygenase (LOX), from edible white yam (Dioscorea rotundata) was studied over a temperature range of 50 to 80°C using mathematical analysis of the kinetic and thermodynamic parameters for the thermoinactivation of the enzymes. Denaturation of these enzymes, measured by loss in activity, could be described by a simple first-order reaction that was resolved into biphasic inactivation curves. This indicates the existence of two isoforms of different thermal stabilities with k-values between 0.032 and 0.525. D-values decreased with increasing temperature, indicating faster inactivation of the enzymes at higher temperatures. Results suggested that peroxidase is relatively more thermostable than polyphenol oxidase and lipoxygenase with a Z-value of 4.11, Ea of 2510kJ mol-1 for the first phase of the biphasic inactivation reaction. The Gibbs free energy (ΔG), values range from -552.95 to 279.01kJ/mol for the three enzymes. The results indicate that the oxidation reactions were: (1) not spontaneous (ΔG > 0) for peroxidase at low temperatures, (2) spontaneous (ΔG 0) for lipoxygenase and (4) reversible (ΔS < 0) for all the three enzymes at all temperature. The high z-value obtained for peroxidase in the first phase of inactivation indicates that a high amount of energy was required to initiate its denaturation, and supports why it is used as a marker for inactivation of quality-related enzymes.