There are plenty of tips out there that claim to help you hang onto the color of pesto. From blanching the basil leaves to adding a bit of lemon juice, chefs all have their own tricks for pesto. One trick you probably haven’t heard of: an ice cube.
Pesto has come a long way from the traditional mortar and pestle to modern food processors. A step you can skip is the blanching of basil. This process involves tossing the leaves into a pot of boiling water then quickly transferring them to an ice bath and squeezing the leaves dry before blending the pesto. Instead, drop an ice cube into the bowl of the food processor when pulsing the basil. The ice will shock the herbs and prevent them from browning.
You can do this with any type of pesto with other herbs and greens such as parsley, mint or arugula. The ice will rattle in the processor at first, but will melt and break down as you pulse the pesto. Any leftover ice bits will dissolve when you blend in the olive oil. You’ll end up with a vibrant green pesto without any complicated techniques or extra steps.
Source: the Kitchn