Polyphenol oxidase and peroxidase in fruits and vegetables

Polyphenol oxidases and peroxidases are among the most studied enzymes in fruits and vegetables. Owing to the deleterious effects of discoloration and off-flavor formation induced by their actions, these enzymes have not ceased to be a matter of concern to food technologists, while their versatility as catalyst and their diversity as protein present a challenge to the biochemist. This article gives an account on the present state of knowledge in this field. The occurrence of polyphenol oxidases and peroxidases in food and food raw materials, and their role and importance in food processing are briefly outlined. Results of biochemical research including catalytic properties, substrate specificity, susceptibility towards pH and temperature, action of inhibitors, isolation, purification, and characteristics of the enzymes are given, with special emphasis on recent achievements based on high resolution separation and isoenzyme techniques. Finally, the behavior of polyphenol oxidase and peroxidase in selected major groups of fruits and vegetables is discussed. Some contradictions found in the literature are pointed out and some questions that have not been given the necessary attention by researchers so far are mentioned.

[Flavonols and flavones of vegetables. VI. On the changes of the flavonols of onions (authors transl)]

The epidermis of onion scales exclusively contains glucosides of quercetin as flavonols, while the dry outer skins contain quercetin in the free state mainly. First spiraeoside (quercetin-4'-glucoside) is built; the formation of diglucosides follows during storage and increases continously. The mesophyll appears to be free of flavonols. The flavonol concentration decreases from the outer to the inner scales, with higher levels in the outer than in the inner epidermis. In the green leaves the flavonol synthesis is light-dependent and also kaempferol glucosides are formed beside quercetin glucosides, but not spiraeoside and the known diglucosides of the scales. During drying on the field an accumulation of flavonols takes place in the drying leaves. This production is associated with the formation of free quercetin and spiraeoside, but not of free kaempferol.