When your peel, bruise or cut fruits or
vegetables, the plant tissue releases some
enzymes; there is one called Polyphenol oxidase
(PPO, phenolase) which in the presence of oxygen
from the air, goes into chemical reactions of
plant compounds to give brown pigments known as
melanins. This reaction is known as enzymatic
An enzyme is a protein that
accelerates the rate of chemical reactions.
Enzymatic browning can be a significant
problem, because it limits the shelf life of many
fruits and vegetables. However, enzymatic browning
is not always unwanted. The browning reaction
contributes to the desirable color and flavor of
raisins, prunes, coffee, tea, and cocoa.
Although enzymatic browning causes changes
in flavor and taste (bitter, astringent), and may
reduce quality, the melanins formed are not toxic.
Brown fruits are safe to eat for some hours after