Why do fruits turn brown?

Fruits turn brown due to exposure to oxygen. This causes the oxidation of phenolic compounds that are naturally present in fruit tissue. This reaction takes place in the chloroplasts by means of an enzyme called polyphenol oxidase.

All plant tissue contains the enzyme polyphenol oxidase. The concentrations vary, which causes some fruit to brown faster than others. Citric juices contain high levels of antioxidants which, when applied on the skin of fruits, help slow the amount of oxygen that reaches the flesh of the fruit. Brown spots can be cut away to reveal healthy-looking flesh. This happens because it takes time for oxygen to reach the surrounding cells.