Pesto is perfectly good to freeze. If you've made a lot of pesto, it's a good idea to freeze it in small portions so that you can remove and defrost small amounts as and when needed. The easiest way to do this is to use an ice cube tray. Pack each tray with some of the pesto mixture, and freeze so that you have little pesto cubes. Then. remove the frozen cubes and store in an airtight container/bag in your freezer, and defrost as needed.