Polyphenol oxidase (PPO) -catalyzed browning reactions are of significant importance in the fruit and vegetable industry. These reactions proceeding in many foods of plant origin cause deterioration and loss of food quality. A better knowledge of the factors that influence the action of PPO is imperative in order to control and manipulate its detrimental activity in plant products. This paper presents an overview of the current understanding of the reaction properties, biochemical characteristics and potential physiological roles of PPO in plants. Reaction properties will include general PPO reactions, specificities and molecular mechanisms of these reactions, and methods available to assess PPO activity. Physicochemical properties will evaluate substrate specificity, environmental influences such as pH and temperature, multiplicity, latency, and activators and inhibitors of PPO. The discussion will conclude with potential physiological roles of PPO in plants. 362 R. YORUK and M.R. MARSHALL PROPERTIES AND ROLE OF PPO 363will evaluate substrate specificity, environmental influences such as pH and temperature, multiplicity, latency, and activators and inhibitors of PPO. IMPORTANCE OF PPO IN THE FOOD INDUSTRY ProblemThe catalytic action of PPO has an enormous impact on the quality of several fruit and vegetable crops and results in alteration of color, flavor, texture, and nutritional value (Vamos-Vigyazo 1981). It is a limiting factor in the handling and technological processing of crops as peeled, sliced, bruised or diseased tissues rapidly undergo browning. Some commercially important edible plant products susceptible to adverse browning reactions include fruits such as apple (Harel er al.

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Using high performance liquid chromatography, tomato cultivars which contain the Crimson gene (og)were usually found to have higher lycopene content (5086 to 5786 mg/100 g fresh weight) than those cultivars lacking the gene (2622 to 4318 mg/100 g fresh weight). A comparison of the color readings taken from tomatoes at the equatorial region with those of the homogenate prepared from the same region showed that the hue of tomato homogenate was a better indicator of lycopene content than tomato surface hue. The tomatoes' lycopene content was not affected by ethylene treatment or cooking for 4, 8, and 16 min at 100 °C.

Orange juice (OJ) was treated with a continuous high‐pressure carbon dioxide (HPCD) system, and the quality parameters pH, Brix, titratable acidity (TA), cloud, pectinesterase (PE) activity, color, and sensory attributes were determined. Treatment pressures were 38, 72, and 107 MPa, and CO2/juice (w/w) ratios were from 0.40 to 1.18. Residence time was kept at 10 min. Cloud increased between 446% and 846% after treatments. There were no significant changes in pH and Brix (α= 0.05). TA of the treated juice was found to be significantly different from that of control (α= 0.01). PE was not completely inactivated. There was a small, but significant increase in juice L‐value (α= 0.01) and a decrease in a‐value for color. There was no significant change in b‐values (α= 0.05). Statistical analysis for the sensory data showed no difference between fresh, frozen control juice and treated juice after 2 wk of refrigerated storage at 1.7 °C.