If we cut apples into pieces and keep the pieces in the open for a while, why do they become brown in colour?
Coimbatore, Tamil Nadu.
The cells of apples and other produce (e.g., pears, bananas, peaches, potatoes) contain an enzyme (called polyphenol oxidase or tyrosinase) that, when in contact with oxygen, catalyzes one step of the biochemical conversion of plant phenolic compounds to brown pigments known as melanins.
You see the browning when the fruit is cut or bruised because these actions damage the cells in the fruit, allowing oxygen in the air to react with the enzyme and other chemicals. This reaction is known as enzymatic browning and occurs at warm temperatures when the pH of the plant material is between 5.0 and 7.0.
This reaction is also sped up by the presence of iron (such as an iron or rusted knife) or copper (such as a copper bowl). The reaction can be slowed or prevented by inactivating the enzyme with heat (cooking), reducing the pH on the surface of the fruit (by adding lemon juice or another acids. The acid in the lemon juice stops the polyphenol oxidase from working), reducing the reaction rate by storing the fruit in the refrigerator, reducing the amount of available oxygen (by putting the cut fruit under water but this may leach out the vitamins into water or vacuum packing it), or by adding certain preservative chemicals (like sulfur dioxide).
If you cut a browned apple into two again, you'll notice that the insides are still white. That's because the cells inside were intact, and did not let oxygen enter right inside.
MSc (Chemical Sciences),
Pondicherry University, Pondicherry
Writing in Scientific American, Lynne McLandsborough, a professor of food science at the University of Massachusetts Amherst, U.S., says: When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colourless precursors to brown-coloured secondary products. O-quinones then produce the well documented brown colour by reacting to form compounds with amino acids or proteins, or they self-assemble to make polymers.