Enzymatic browning of fruit and vegetables: A review

Abstract

Enzymatic browning in fruits and vegetables occurs by exposure to the air after cutting and slicing and in pulped states, mechanical damage during transportation, and thawing of frozen or cold stored foods. Polyphenol oxidase (PPO) and peroxidase (POD) are the main enzymes responsible for browning. PPO is classified as an oxidoreductase enzyme with four atoms of copper as a prosthetic group. It catalyzes the oxidation of functional OH group attached to the carbon atom of the benzene ring of monohydroxy phenols (phenol, tyrosine, p-cresol) to o-dihydroxy phenols (catechol, dopamine, adrenalin) and dehydrogenation of odihydroxy phenols to o-quinones. The oxidation of phenolic compounds to quinones and production of melanin give rise to a dark color in the foods. The POD is thermostable enzyme that belongs to a group of oxidases that use H2O2 as a catalyst for oxidation of phenolic compounds. The POD is related to undesirable changes in flavor, texture, color, and the nutritional quality of foods. The level of PPO and POD varies in fruits and vegetables and their content changes with maturity and senescence depending upon the ratio of bounded and soluble enzymes. Change in color of fruits and vegetables by enzymatic reactions is a major problem during harvesting, transportation, storage, and processing. Color deterioration, off-flavor, and loss of nutritive value in foods are unacceptable to the consumers. The purpose of this chapter is to provide information available in the literature on PPO and POD in different fruits and vegetables, their role in browning/color changes, and available prevention methods.