Why do bruised or cut fruits (bananas, apples) get brown ?
When fruits or vegetables are peeled, bruised or cut, enzymes contained in the plant tissue are released. In the presence of oxygen from the air, the enzyme polyphenol oxidase (PPO, phenolase) catalyzes one step in the biochemical conversion of plant phenolic compounds (such as the colours in apple peel) to brown pigments known as melanins. As the reaction is catalysed by the PPO enzyme, this reaction is known as enzymatic browning. (Source) Enzymatic browning can be a significant problem, limiting the shelf life of many fruits and vegetables which have had little heat applied during processing. However, enzymatic browning is not always unwanted. The browning reaction contributes to the desirable colour and flavour of raisins, prunes, coffee, tea, and cocoa. For tea and cocoa the browning process is wrongly named fermentation, but in fermentation reactions micro-organisms are involved, which does not happen with enzymatic browning. Although enzymatic browning causes changes in flavour
