Keeping avocado green

Microwaving avocado helps to keep the lovely green color for hours due to the fact that the heat from the microwave denatures the "PPO Protein" (also called the enzyme Polyphenol oxidase). This protein, when exposed to oxygen, is responsible for turning the avocado pulp brown via oxidation. The microwave alters the structure of this protein, disabling it and thus slowing oxidation.

Separately, the hormone that causes green bananas and unripe avocados to ripen is not PPO protein, but ethylene. When unripe items are put into a brown bag and microwaved, the concentration of PPO  increases and speeds up the ripening process.