What kind of chemical change do you think occurs when a banana peel turns brown in the open air?
Enzyme catalysed oxidation and then polymerisation the resulting product.......
Bananas contain the enzyme polyphenol oxidase. One of the actions of this enzyme is to promote oxidation of the amino acid tyrosine (naturally occurring in the banana) into o-quinone. The enzyme then causes rapid polymerisation of the single ringed o-quinone into one of the polycylic compounds known as melanins. These are dark coloured compounds, which is why the peel turns brown.