The main acid that oxidises after cutting fresh fruit/veg is
Ascorbic Acid, better known as
Vitamin C. This is pretty damn good for you, and hence
fresh fruit and
vegetables should only be cut just before they are cooked/served, especially if they are being served
raw. The whole
TV Chef thing of having pre-prepared bowls of chopped fruit and veg is actually
bad practice, unless cooking for a large number of people or in other
speed critical situations, in which case
blanched or
acidiculated fruit/veg is recommended.
(thanks to Jimnyo for that)
However, if you're going to
boil the broccolli yellow or otherwise
overcook the food, this is irrelevant. Overcooking, especially
boiling, tends to create Ascorbic Acid Oxide which is subsequently lost into the water or other cooking medium.