Browning reactions, polyphenol oxidase

Ascorbic acid browning is also inhibited by the addition of sulfite (Wedzicha and McWeeny, 1974). The same holds for polyphenol oxidase-catalyzed oxidation of natural phenols in fruit. The mechanism of the inhibition is by reaction of oquinone intermediates with sulfite, which leads to nonreactive sulfocatechols (Wedzicha, 1995). 

All these reactions occur in the cell, but on the whole, the cell finds the second and third easier to perform, and, what is more, can perform either of them without the direct intervention of oxygen itself. Reaction (1) does have some importance though. There are enzymes, called oxidases, which catalyse the direct oxidation of their substrates by atmospheric oxygen. They are found in most cells, but are especially common in plants. The effects of one of them, polyphenol oxidase, are familiar to those who peel their apples before eating them. Apples contain traces of a compound called catechol which is oxidized, under the influence of poly phenol oxidase, to a complex, dark brown substance. When an apple is cut its surface is exposed to the air and polyphenol oxidase can begin to work, turning the surface of the apple gradually brown. 

Enzymes known as polyphenol oxidases cause enzymatic browning. Other names of the enzyme include phenolases and tyrosinases. The enzymes catalyze the conversion of monophenols and diphenols to quinones. The quinones can undergo a series of non-enzymatic reactions to produce brown, gray and black colored pigments, collectively known as melanins (11). Maillard reactions, caramelizations and ascorbic acid oxidations can produce similar types of colored compounds (12). For some food processing... 

Research projects in basic enzymology of polyphenol oxidase and the chemistry of non-enzymatic browning may lead to new browning inhibitors. Reviews on polyphenols oxidase (13) and on Maillard reactions (14) are available. The interest in enzymatic browning is aided by skin cancer research as melanins are the pigments involved in the coloring of human skin. 

Matheis and Whitaker (59) reviewed the chemistry of the modification of proteins by polyphenol oxidase and peroxidase. Monophenols such as tyrosine are first hydroxylated to dihydroxyphenylalanine (DOPA). The DOPA is then oxidized to an ortho-quinone. Ortho-quinones can undergo at least two types of reactions with thiols. Two molecules of a thiol can participate in an oxidation-reduction reaction with a quinone to form the corresponding hydroquinone and a disulfide, or the thiol anions may add to the conjugated system of a quinone to form a substituted hydroquinone. An excess of quinone in the reaction mixture usually oxidizes the monosubstituted quinone, which may then participate in another nucleophilic addition, and so on (Figure 13). The cysteine adduct(s) cannot further react with amino groups to form brown products. 

Enzymic browning reactions commonly occur when fruits and vegetables are bruised or peeled. They are due to the pol)rmeri-zation of phenolic compounds on exposure to air. The enzyme responsible for this browning is the polyphenol oxidase or pheno-lase enzyme which converts the phenol to its corresponding qui-... 

Phenolic compounds are substrates for polyphenol oxidases. These enzymes hydroxylate monophenols to o-diphenols and also oxidize o-diphenols to o-quinones (cf. 2.3.3.2). o-Quinones can enter into a number of other reactions, thus giving the undesired brown discoloration of fruits and fruit products. Protective measures against discoloration include inactivation of enzymes by heat treatment, use of reductive agents such as SO2 or ascorbic acid, or removal of available oxygen. 

Enzymes from the class of oxidoreductases catalysing enzymatic browning reactions are most often known trivially as polyphenol oxidases. Two groups of enzymes can be identified ... 

Merely reducing the temperature reduces the reaction rate, but the colour changes are quite rapid even at 0 °C. This means that sensitive products that have not been pre-treated should be frozen as quickly as possible, for example mushrooms and sliced peaches. Rapid intensive browning occurs during defrosting, when the activity of polyphenol oxidases increases due to disruption of cellular structures by ice crystals. 

Another option to reduce or inhibit the enzymatic browning reaction is the use of chemical reagents. Many substances capable of inhibiting polyphenol oxidases are known, but the mechanism of their action is often unknown. Complexing agents are able to bind copper ions, which the enzyme requires. Interaction with... 

The lag time effect probably results from the inhibition of copper-containing oxidases and other copper-catalyzed oxidative processes in apple by Sporix. These oxidative reactions normally would bring about the rapid loss of AA and permit browning to occur once the added AA was depleted (18). Sporix also would inhibit PPO directly by chelation of its copper (3), thereby decreasing the rate of polyphenol oxidation and subsequent browning. The ability of Sporix to exert its effect on enzymatic browning by these two independent mechanisms probably accounts for the apparent synergism obtained with Sporix-AA combinations. 

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