Why Add Starchy Pasta Water To Pesto?

Short answer
Starchy pasta water is used to help thicken and emulsify your pesto, resulting in a silky, restaurant-quality finish.

Long answer
If you've ever wondered why the water you cook your pasta in turns a murky grey, it's because some of the starch from the pasta has leached out. We know it looks rather unappetising, but this water's starch content (which will vary depending on what your pasta-to-water ratio was) helps to thicken and emulsify sauces, giving them a beautifully glossy finish in the process.

This water can be used in other novel ways, too. Think about substituting plain old tap water for starchy pasta water when making your next sourdough or focaccia. It can also be used in place of water to make homemade stock or soup. The possibilities are endless!