Enzymatic browning of foods

The mechanism of enzymatic browning of foods which in most cases affects adversely the quality of foods is briefly described. Enzymatic browning is in most cases connected with the action of polyphenol oxidase (PPO), which in the presence of oxygen hydroxylates colorless monophenols to o-diphenols which are subsequently oxidized to colored o-quinones. Through a series of nonenzymatic reactions, o-quinones are often converted irreversibly to brown polymeric pigments. In order to prevent the deteriorating effect of enzymatic browning, various inhibitors are applied during processing and storage of foods. The inhibition of enzymatic browning generally proceeds via the following three mechanisms: 1. Direct inhibition of polyphenol oxidase, 2. Nonenzymatic reduction of o-quinones which are formed by enzymatic oxidation of o-phenols to the derivatives of o-diphenols, and 3. Chemical modification or removal of phenolic substrates of polyphenol oxidase. The most efficient multifunctional agents in control of enzymatic browning of foods are various sulfites. However, their use is in many cases limited by sanitary regulations. For this reason a very intensive research in the development of sulfite alternatives is underway. The best known alternative to sulfite in control of browning of foods is L-ascorbic acid and its cheaper stereoisomer D-ascorbic acid commercially known as erythorbic acid. The other well known substitute to sulfites is 4-hexylresorcinol, a part of EverFresh formulation. Very active inhibitors of enzymatic browning are various chelating agents, which either directly form complexes with PPO, or react with its substrates. Example of an inhibitor which reacts with the copper containing prosthetic group of PPO is b-cyclodextrin. Other inhibitors of enzymatic browning which are commercially used are: sulfhydryl containing amino acids, specially cysteine and reduced glutation, inorganic salts, like sodium and zinc chloride, and honey which inhibits the browning of white grapes. As part of our own work, the effect of erythorbic acid applied under pressure or vacuum to the discoloration of fruit and vegetables and the importance of proper selection of cultivar are briefly described in this review.