Why Add Starchy Pasta Water To Pesto?

Short answer
Starchy pasta water helps 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 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 your pasta-to-water ratio) helps to thicken and emulsify sauces, giving them a beautifully glossy finish in the process.

You can use this water in a variety of novel ways. When making your next sourdough or focaccia, consider substituting plain old tap water for starchy pasta water. You can even use it in place of water to make homemade stock or soup. The possibilities are endless!