Tomato peroxidase (POD) was inactivated by heat and ultrasound. Thermal inactivation of tomato POD was performed at temperatures of 63, 64, 65, 66 and 67°C. Thermal inactivation of POD in tomato showed apparent first-order kinetics. E(a) was calculated from the slopes of Arrhenius plot and found as 14.9×10(4) J/mol. The effect of ultrasound on tomato peroxidase inactivation was investigated at 15%, 25%, 40%, 50% and 75% ultrasonic powers for 20-150 s. It was observed that as the ultrasonic power increased, inactivation rate increased. 100% POD inactivation was observed at 50% power for 150 s and at 75% power for 90 s of ultrasonication. Regeneration of POD activity was investigated for the samples exposed to ultrasound at different ultrasonic powers. It was observed that at 15% and 25% ultrasonic powers, residual enzyme activity increased linearly, whereas at powers of 40%, 50% and 75% residual enzyme activity increased nonlinearly. There was no regeneration in the samples which 100% enzyme inactivation obtained by ultrasound. A significant decrease in vitamin C content of tomato extract was observed as a function of temperature and treatment time, whereas ultrasonic treatment had no significant effect on vitamin C content of tomato extract.

Ultrasound is versatile and innovative technology due to its wide range of application and increase in knowledge and research studies. It is used in food industry for many purposes including analysis methods and food processings such as freezing, cutting, drying, tempering, homogenization, degassing, antifoaming, filtration and extraction. Ultrasound can be used as a promoter or alternative to food processing. There may be numereous advantages of using ultrasound for food processing such as effective mixing, increased mass transfer, reduced energy, reduced temperature and increased production rate. Due to the elimination of microorganisms and enzymes without destroying nutrients of foods, ultrasound can be used as an alternative method to thermal treatments in the food preservation. Additionally, low power ultrasound is thought to be an attractive nonthermal method due to overcome problems which occur during heat treatments such as physical and chemical changes, nutritional loss and change in organoleptic properties. This review summarizes mechanism, operation and latest potential applications of ultrasound in the food preservation.

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The inhibitory effect of green tea extract on apple polyphenol oxidase (PPO) and on apple browning was investigated. PPO has been isolated from apples (cv. Golden Delicious), and extraction with polyvinylpolypyrrolidone and Triton X100 yielded a 6.45‐fold increase in the specific enzyme activity. The optimum pH and temperature of the enzyme was found as 5.5 and 25C, respectively. The kinetics of enzyme‐catalyzed reactions followed the Michaelis‐Menten model, and the PPO exhibited a Km value of 4.76 mM. Apple PPO activity was inhibited by green tea extract, and the inhibitory effect toward apple PPO increased with increasing extract concentration. Addition of 30 mg/mL green tea extract resulted in approximately 42% inhibition on apple PPO activity. To support the inhibition behavior of PPO, apple browning was also investigated by immersing apple slices into green tea extract, and it was revealed that the browning of apple slices was effectively retarded by the green tea extract. PRACTICAL APPLICATIONS The quality of fruits, especially apples, depends on controlling the enzymatic browning that affects its organoleptic and nutritional qualities. Addition of chemicals, pH adjustment, refrigeration, thermal treatments and exclusion of oxygen are among the most effective methods for the inhibition of polyphenol oxidases (PPO). Consumer demand for minimally processed and fresh‐like food products has undergone a continuous rise in recent years. Several authors have stated the necessity to evaluate the inhibitory effect of natural and nontoxic compounds on PPO activity and its potential applicability to the food industry. Experiments showed that green tea has an inhibitory effect on the activity of apple PPO and its browning. Our results suggest that green tea extract could potentially be used as a natural inhibitor of PPO, and its utilization is possible due to its wide occurrence, low cost and nontoxicity, but further feasibility studies are needed to increase its effect as an anti‐browning agent in its application in the food industry.

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.