Fresh Pesto -- that delicious basil, garlic, pine nut, Parmesan or pecorino cheese, and olive-oil sauce -- quickly turns from green to brown if it is not stored properly. The flavor is unaffected, but the color is less appetizing. To prevent this discoloration, which results from contact with air, pour a thin layer of olive oil over the top, and refrigerate in an airtight container.
Keeping Pesto Green
Martha Stewart Living, June 2002