When we cut the apple it turn brown in color because of the oxidation process. The peel of apple protect it from the oxidation. When the apple is peel off the oxygen and water molecules in air react with it and oxidation take place. The oxidation process is very efficient in ambient temperature.
For example, if the peal off apple is placed into the refrigerator it take a time to got oxidize and turn brown, but if it is placed in room temperature it quickly turn brown.
when oxygen is react with peel off apple , it trigger the polyphenol oxidase enzyme to oxidize the phenolic compound and quinones are formed which then react with amino acids and produced brown color.
