KINETICS AND THERMAL ACTIVATION/INACTIVATION OF STARKING APPLE POLYPHENOL OXIDASE

Abstract

ABSTRACT The objective of this study was to evaluate the activity, kinetic behavior and thermal inactivation kinetics of apple polyphenol oxidase (PPO). PPO was extracted from an apple (cv. Starking Delicious), and enzyme activity was measured with 4-methyl catechol and pyrogallol. Optimum pH of the enzyme was found as 5.5 with 4-methyl catechol and 7.2 with pyrogallol. The kinetics of PPO catalyzed oxidation obeyed Michaelis–Menten model with both substrates. Apple PPO had a higher affinity to 4-methyl catechol than pyrogallol having Km values of 2.24 mM and 8.04 mM, respectively. Thermal inactivation of apple PPO was studied at temperatures from 45C to 75C with different exposure times. When remaining enzyme activity was measured with pyrogallol, PPO activity initially increased and then decreased which shows activation of apple PPO at initial stages of heat treatment at 45, 55 and 65C. In apple extract there were both soluble and latent PPO with different substrate affinities and thermal stabilities. Activation energies of soluble and latent PPO were found as 12.85 kcal/mol and 7.89 kcal/mol, respectively (T < 75C). Soluble PPO was more heat stable than latent PPO. Heat treatment at 75C for 10 min resulted in 46% inactivation of soluble PPO where latent PPO was 62% inactivated at the same condition. PRACTICAL APPLICATIONS Color is an important quality attribute of food. PPO catalyzes the oxidation of phenols to colored quinones. These quinones are further polymerized to form brown pigments which are generally considered to be detrimental to food quality from both organoleptic and nutritional points of view. The most effective method to inactivate PPO is heat treatment. Thermal stability of apple PPO has been investigated by many researchers and it was found that each variety has different characteristics and thermal stabilities. This research demonstrates that the characterization of the Starking PPO is necessary to control enzymatic browning of this apple. This study shows that Starking apple has both soluble and latent PPO with different thermal stabilities and the enzyme can be activated with mild heating. To control enzymatic browning of this apple variety, heat treatment must be applied with temperature greater than 75C.