The effect of heat treatment on edible yam (Dioscorea cayenensis-rotundata cv Longbô) polyphenol oxidase activity was studied over a range of 35 to 75°C. Denaturation of this enzyme, measured by loss in activity, could be described as a first-order reaction with k-values between 0.004 and 0.108 min-1. D- and k-values decreased and increased, respectively, with increasing temperature, indicating faster polyphenol oxidase inactivation at higher temperatures. Results suggested that polyphenol oxidase is a relatively thermostable enzyme with a Z-value of 29.41°C and Ea of 67.67 kJ mol-1. The Gibbs free energy ΔG values range from 86.524 to 88.938 kJ/mol at 308-348°K. The results of the thermodynamic investigations indicated that the oxidation reactions were: (1) not spontaneous (∆G > 0), (2) slightly endothermic (∆H > 0) and (3) reversible (∆S < 0). The high value obtained for the variation in enthalpy indicated that a high amount of energy was required to initiate denaturation, probably due to the molecular conformation of polyphenol oxidase.

Key words 
Yam, Dioscorea cayenensis-rotundata, polyphenol oxidase, kinetic and thermodynamic analysis

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