Fruit is a rich source of polyphenols. Polphenols are chemicals that may give fruit some of its health promoting qualities in humans, where they are thought to function as antioxidants in plasma. Polyphenols are a diverse group of chemicals and include flavonoids, hydroxybenzoic acids, hydroxycinnamic acids, chlorogenic acid, stilbenes and lignans. Fruit such as apples and bananas, if bruised, turn brown and this is a result of the polyphenols contained within the cells. This process can be explained by the action of an enzyme called polyphenol oxidase catalysing a reaction that converts polyphenols to quinones, through addition of molecular oxygen. The quinones produced are then converted to brown pigments called melanins, through a non-enzymatic process. It is the brown melanin pigments that are the cause of the browning of fruits. Different fruits contain various concentrations of polyphenols and polyphenol oxidase which determines their propensity to turn brown.
In healthy plant cells, the enzyme polyphenol oxidase is compartmentalised away from the polyphenols, so as to protect the polyphenol chemicals from reaction. However, upon damage such as cutting, freezing, thawing or bruising the enzyme is released from its sub-cellular location and brought into contact with the substrate. This allows the reaction to proceed at a rate dictated by the concentration of the enzyme and the substrate. The role of polyphenol oxidase is not fully understood, but a number of functions have been hypothesised. One theory claims that plants produce melanins to protect seeds from predators and pathogens, because melanins cause the seed coats to become more impermeable following damage. As the fruit ripens, the need for polyphenol oxidase falls as there is a lower requirement to protect the seeds. Alternatively polyphenol oxidase may play a role in oxygen chemistry and photosynthetic reaction.
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