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.

Polyphenol oxidase and peroxidase in fruits and vegetables.

Assay of catechol oxidase—a critical comparison of methods

Polyphenoloxidase (PPO) activity and polyphenol concentrations decreased during maturation and remained relatively constant during cold storage. However, there was no set pattern of changes in degree of browning during maturation and cold storage. Not all cultivars showed the same relationship between degree of browning and enzyme activity. In some cultivars (Classic Delicious, RI Greening, McIntosh and Cortland), PPO activity was directly related to degree of browning while in others (Empire, Rome and Golden Delicious), degree of browning was related more to phenolic concentration. High performance liquid chromatography analysis of the phenolics in apples showed that the types of phenolic compounds in all cultivars were similar and that no one particular compound could account for the differences observed in browning.

Changes in Apple Polyphenoloxidase and Polyphenol Concentrations in Relation to Degree of Browning

The seed coat of Pisum elatius is normally impermeable to water. When seeds are dried in the absence of oxygen their coats are totally permeable to water. Structural differences are observed between permeable and impermeable seed coats. In the genus Pisum, species with normally impermeable seed coats have a high content of phenolics and of catechol oxidase, while seed coats of P. sativum contain very little catechol oxidase and have a very low content of phenolics. Such differences are not noted in the cotyledons. We hypothesized that during dehydration of seeds, oxidation of phenolic compounds in seed coats through catalysis of catechol oxidase in presence of O2 might render the seed coats impermeable to water.

Permeability of seed coats to water as related to drying conditions and metabolism of phenolics.

The response surface methodology was applied to investigate the way in which variables, such as time of treatment, sodium or calcium chloride concentrations and lactic acid concentration, affect the blanching of potato slices in boiling solutions Two 3 factor-5 level, second order central composite designs were developed to analyse the considered variables Results showed that all the considered blanching treatments allowed polyphenoloxidase inactivation With regard to colour, the use of calcium chloride, already at low concentrations, would seem better than the use of sodium chloride Under the applied operative conditions, the best results were obtained with short times of treatment and low lactic acid concentrations

Prevention of enzymatic browning in sliced potatoes by blanching in boiling saline solutions

The effect of variety, climatic factors and nutritional factors on the enzymic browning of cut potatoes has been investigated. The major factor determining the rate of enzymic browning appears to be the concentration of tyrosine in the tuber. There was no direct correlation between the activity of the phenolase enzyme or the amount of chlorogenic acid and the rate of browning. The level of tyrosine in a potato was shown to be determined by genetic, climatic and, to a lesser extent, by nutritional factors. Of the climatic factors, rainfall, or the moisture available to the plant, appeared to be the most important.

Influence of variety, cultural conditions and temperature of storage on enzymic browning of potato tubers

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Influence of variety, cultural conditions and temperature of storage on enzymic browning of potato tubers