This arises by the oxidation of polyphenols to o-quinones by enzymes such as polyphenol oxidase (PPO) and peroxidase (Fig. 1.23). A day-to-day example
Fig. 1.23 Polyphenol oxidation.
Fig. 1.24 Some flavour compounds produced in caramélisation reactions.
would be the browning of sliced apple. In other foods, the reaction is wanted, for example, in the readying of prunes, dates and tea for the marketplace.
Whereas heating boosts non-enzymatic browning, the converse applies to enzymatic browning, as the heat inactivates enzymes. The alternative strategies to avoid the reaction are to lower the levels of polyphenols (the agent polyvinylpolypyrrolidone (PVPP) achieves this) or to exclude oxygen.
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