Why do some vegetables and fruits discolour after being cut ?

Freshly harvested vegetables from your garden always taste better, than the one purchased from a shop. Once a vegetable is harvested it begins to change (except hibernating plant parts like onions and potatoes). One a vegetable is harvested, it is cut off from its source of nutrients. So it starts to consume itself and accumulates waste products, hence the flavour and texture starts to change. Some vegetables lose half their sugar in a few hours after harvest and they either convert it to starch or use the energy to stay alive. Some vegetables lose the water pressure in their cells and become limp and chewy. Hence freshly harvested vegetables are full-flavoured than store bought produce, which is usually days to weeks from field.

Many fruits and vegetables, like banana, potatoes, apples and brinjal quickly develop a brown or red discoloration when cut or bruised. This is part of the plants defence mechanism, to protect itself from microbes when it is cut. This discolouration is caused by three chemical ingredients: 1 and 2 ringed phenolic compounds, certain plant enzymes and oxygen. When the intact fruit is cut, the phenolic compounds from the damaged cells vacuole (a storage bag, present in every plant cell) mixes with enzymes of the cells and oxygen in air and form light absorbing clusters. In nature when insects or microbes damage the cell, these phenolic compounds attack the invaders own enzymes and membranes. The brown pigments we see are essentially spent weapons. Though the browning is only on surface and does not affect flavour, for visual appeal you can reduce it by following methods. 1. Coat the surface with lemon juice, the browning enzyme works very slowly in acidic condition 2. cool the cut pieces below 4c, its slows down the reaction time. 3. Immerse it in cold water. The water limits the availability of oxygen and the cold temperature reduces reaction time. 

The enzyme that causes this fruit discolouration has a special use- it can convert stinky chemicals in our breath to odourless molecules. So eating an apple after a meal, can not only keep the doctor away, but also bad breath away.

Trivia: Methanethiol is the molecule that causes garlic breath- it is related to the same chemical that gives odour to skunk spray.