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Inhibitors of Browning |
Evidence thus indicates that the browning of fruits by polyphenol oxidase may be delayed or stopped by various inhibitors to the reactants, to the enzyme, or to product formation. These inhibitors may occur naturally in fruits or they may be applied. Two practical means of preventing discoloration in fruits are blanching to inactivate the polyphenol oxidases and removal of oxygen by preservation methods, such as vacuum packing or packing in a syrup. Enzymatic browning can also be retarded by chilling or by lowering the pH with acid.
Compounds containing sulfur act as inhibitors to discoloration, although their mechanism of action is not fully understood. Sulfur dioxide may act by reducing the available oxygen, it may react with quinones or other intermediates of oxidation, or it may inhibit the enzyme because of its reducing capacity (Ponting, 1960). Sulfite (Embs and Markakis, 1965) and thiol compounds (Walker, 1964) are thought to act by combining with the ortho-quinones, thus stopping their condensation to melanins. Other chemicals act through competitive inhibition of polyphenols for polyphenol oxidase or through non-competitive inhibition by complexing the enzyme or by replacing the prosthetic group of the enzyme.
Biochemical modification of substrate is being explored as means to prevent discoloration of fruits. For example, catechase is being used to oxidize and split the benzene ring of phenolic substrates (Corse, 1964). Corse also reported some success with O-methyltransferase used to methylate chlorogenic acid and other catechols to their corresponding 3-methyl esters.
Ascorbic acid is an inexpensive, harmless chemical used by processors of frozen fruit and by homemakers to prevent browning. Although Walker (1962) found no inverse relationship between the concentration of naturally-occurring ascorbic acid and browning of apples, Ponting and Joslyn (1948) found that no darkening occurred in this fruit until all of the ascorbic acid was oxidized. Ponting and Joslyn attributed both the oxidation of ascorbic acid and darkening to action of polyphenol oxidase. Baruah and Swain (1953) suggested that ascorbic acid affects the copper moiety of the polyphenol oxidase. They found that the inhibitory effect of ascorbic acid on potato polyphenol oxidase was directly proportional to the concentration of ascorbic acid. Weurman and Swain (1955) reported that ascorbic acid did not influence enzyme activity in apples. Pierpoint (1966) reported that ascorbic acid greatly decreased the initial rate at which tobacco-leaf ortho-diphenol oxidase oxidized chlorogenic acid, suggesting that ascorbic acid was acting on the enzyme. In addition, rate of oxygen uptake was increased by the addition of ascorbic acid. The latter observation suggested that ascorbic acid was being oxidized, thereby keeping the substrate reduced. Extra oxygen uptake and delay in browning were proportional to the amount of ascorbic acid added. The quinones, rapidly reduced by ascorbic acid, accumulated and condensed to brown products only after all the ascorbic acid had been oxidized. Krueger (1950) reported that addition of ascorbic acid increased oxygen uptake and acted as a reducing agent in the enzymatic oxidation of tyrosine and DOPA in the general reaction as given by Corse (1964):
In plant tissue, ascorbic acid is associated with cell walls (Jenson and Kavaljuan, 1956). Accumulation of ascorbic acid may precede and condition cell expansion by influencing the absorption and retention of substances at the surface of cells (Reid, 1941).
Updated: Wednesday, June 20, 2007.
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