The discoloration of the exposed flesh is largely caused by oxidation, and ascorbic acid in the lemon juice slows down that process. Another technique is to cook the food, because heat inactivates the enzymes in the food that would otherwise bring about discoloration in the presence of oxygen. However, don't cook avocados, because when heated, avocados undergo a chemical reaction that produces unwanted, bitter-tasting compounds. That's why you seldom see hot avocado dishes on menus or canned avocado products on supermarket shelves.
** Asian Recipes