Enzymatic Browning In Fruits

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Enzymatic browning is one of the most important reactions that occur in fruits and vegetables, usually resulting in negative effects on color, taste, flavor, and nutritional value. The reaction is a consequence of phenolic compounds' oxidation by polyphenol oxidase (PPO), which triggers the generation of dark pigments. This is particularly relevant for apples, which are rich in polyphenols and highly susceptible to enzymatic browning. Phenolic compounds are partially responsible for color, astringency, bitterness, flavor, and nutritional qualities in fruits and vegetables. In fruits, they are the most important group of secondary metabolites, varying widely in quantity and quality among different species. In plant cells, phenolic compounds …show more content…

Several studies have shown that the enzymatic browning in apple pulp is associated with polyphenol content and/or PPO activity. The enzymatic browning is a consequence of the oxidation of polyphenols to their corresponding quinones by …show more content…

The enzyme catalyses the o-hydroxylation of monophenol molecules in which the benzene ring contains a single hydroxyl substituent to o-diphenols (phenol molecules containing two hydroxyl substituents). It has an optimum pH of 6.7. It can also further catalyse the oxidation of o-diphenols to produce o-quinones. PPO causes the rapid polymerization of o-quinones to produce black, brown or red pigments (polyphenols) that cause fruit browning. The amino acid tyrosine contains a single phenolic ring that may be oxidised by the action of PPOs to form o-quinone. Hence, PPOs may also be referred to as tyrosinases. PPO catalyzes the conversion of phenolic compounds to quinones and assists their products’ polymerization. There are also other enzymes such as peroxidase that are also commonly found in studies to be responsible for enzymatic browning. However as it pertain to this investigation, PPO is the most common one found in apples. Two kinds of reactions generated by PPO are the hydroxylation of monophenols to o-diphenol and the oxidation of o-diphenol to o-quinone. A generalized schematic reaction catalyzed by PPO is as

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