Pesto is a common Italian sauce made with basil, garlic, Parmesan cheese, pine nuts, salt, black pepper and extra virgin olive oil. Sometimes pesto can have a bitter flavour. People have theorised that the basil leaves can be bitter, or the pine nuts, but according to an article in "Cook's Illustrated" magazine: "Extra-virgin olive oil contains bitter tasting polyphenols coated by fatty acids, which prevent them from dispersing. If the oil is emulsified in a food processor, these polyphenols get squeezed out and the liquid mix turns bitter."
Substitute Grapeseed Oil for Extra Virgin Olive Oil in your pesto recipe. Use the same quantity of oil and do not change the steps in the recipe you're using.
Add 1 tsp.of sugar to your recipe if you do not wish to substitute Grapeseed Oil for Extra Virgin Olive Oil. Add the sugar while the pesto is in the food processor. You will want to taste the pesto and add more sugar if the sauce is still bitter.
Put all of the ingredients except for the Extra Virgin Olive Oil and Parmesan cheese in a food processor. Remove the blended paste, add the olive oil, Parmesan cheese and a 1/4 cup of water. Stir well and serve.
Blanche your basil first before using it in the Pesto sauce. This may remove some of the bitter flavour of the finished sauce.