Sliced Apples

You open your lunch box to [blank] that the lovely apple you sliced this morning now appears unsightly and brown. Why does this happen? This unappetizing 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 [blank] to oxygen. This triggers an enzyme known as polyphenol oxidase (PPO) to oxidize polyphenols in the apple's flesh. This results in new chemicals (o-quinones), [blank] 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 [blank] 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 [blank] to genetically engineer apples that do not produce the PPO enzyme, so perhaps brown apples will someday be a thing of the past.