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You open your lunch box to discover that the lovely apple you sliced this morning now appears unsightly and brown. Why does this happen? This phenomenon is actually due to a chain of biochemical reactions known as “enzymatic browning.” When an apple is injured (or cut into pieces), the plant tissue is exposed to oxygen. This an enzyme known as polyphenol oxidase (PPO) to—wait for it—oxidize polyphenols in the apple’s flesh. This results in new chemicals (o-quinones), which then react with amino acids to produce brown-colored melanins. Different apple varieties contain different amounts of both the initial enzyme and the polyphenols, and thus they brown at different rates.
Enzymatic browning is not to apples. Pears, bananas, and eggplants also turn brown fairly quickly when cut. Enzymatic browning is also responsible for the desirable dark color of prunes, coffee, black tea, and cocoa. Scientists are working to genetically apples that do not produce the PPO enzyme, so perhaps brown apples will someday be a thing of the past.