Enzymatic browning in foods and its prevention
Özet
Phenolic compounds are widely distributed in the plant kingdom and are considered to be secondary metabolites. Structurally they contain an aromatic ring bearing one or more hydroxyl groups, together with a number of other substituents. The polyphenolic composition of fruits and vegetables varies in accordance with species, cultivar, degree of ripening and environmental conditions of growth and storage. Phenolics also contribute to colour, astringency, bitterness and flavour. Enzymatic browning of fruits and vegetables is caused mainly by the conversion of native phenolic compounds to quinones, which are polymerized, in turn, to brown, red or black pigments. Browning occurs when the phenolic substrate, the enzyme, and oxygen are brought together under appropriate conditions of pH, temperature and water activity. Bruising, cutting, peelings or otherwise disrupting cells promotes enzymatic browning of many fruits and vegetables. One of the most important causes of color alterations is due to either formation or degradation of pigmented compounds usually present in the produce. This phenomenon is mediated by endogenous enzymatic activities such as polyphenol oxidase (PPO) and peroxidase (POD). In the presence of oxygen, polyphenol oxidase catalyzes the hydroxylation of monophenols to o-diphenols (cresolase activity) and the oxidation of o-diphenols to their corresponding o-quinones (catecholase activity). Peroxidases (donor: H(2)O(2) oxidoreductase; EC 1.11.1.7) constitute a group of glycoproteins the main function of which is the oxidation of different substrates at the expense of H(2)O(2). For fresh or processed fruits and vegetables, the enzymatic browning produces undesirable colors and off-flavors. In addition to the loss of aesthetic quality of fruits and vegetables, enzymatic browning also reduces nutritional quality through the destruction of nutrients such as ascorbic acid. These reactions are not generally desiderable for the food industry because of decreasing in market value and the concomitant economic losses. Some enzymatic browning reactions are however very beneficial to the overall acceptability of foods such as tea, coffee and cocoa. The control of browning in fruits and vegetables depends on an understanding of the mechanism(s) responsible for browning in fruits and vegetables, the properties of polyphenol oxidase and peroxidase enzymes, their substrates and inhibitors, and the chemical, biological and physical factors which affect each of these parameters. Many studies have focused on either inhibiting or preventing polyphenol oxidase and peroxidase activities in foods. Various techniques and mechanisms have been developed over the years for the control of these undesirable enzyme activities. These techniques attempt to eliminate one or more of the essential components (oxygen, enzyme, copper, or substrate) from the reaction. The control ways of enzymatic browning such as processing, inhibitors, thermal treatment, the use of antibrowning agents, anti oxidants/reducing agents has also been discussed. The studies related to control of the enzymatic browning have been continued. The choice of a particular anti-browning agent will depend on the basis of effectiveness, cost, and regulatory status, as well as the method of treatment. Only a limited number of browning inhibitors are considered acceptable with respect to consumer safety and/or cost and act as potential alternatives to sulphites.
Whether chemical treatments of food products are achievable or not will depend on their effectivness and cost relative to that of alternative approaches and on the regulatory status of their use as food additives.
