Like guacamole, pesto can turn brownish when it’s exposed to air. To prevent this, pour a thin layer of olive oil on top of the pesto before covering and refrigerating. Spoon extra pesto into an ice cube tray and freeze until solid, then pop out the frozen cubes and freeze in a resealable bag. They’ll keep for months, and you can thaw as little or as much as you need. If you’re making pesto just for freezing, leave out the cheese and stir in after thawing.