Made from a blend of basil, garlic, pine nuts, Parmesan and olive oil, pesto provides a rich sauce for pasta, meats and vegetables, or for use as a spread. Lackluster ingredients can result in a bitter pesto, but a few simple precautions paired with some quick remedies can right the flavor and save the meal.
Before You Start
Several standard ingredients in pesto can develop a bitter flavor, so taste-testing everything before you begin mixing eliminates the bitterness before it becomes a problem. The oils in old pine nuts and walnuts can go rancid as they age, as can olive oil. Basil from plants that have already flowered develops a bitter flavor as well, so even fresh basil can harbor bitter compounds. Don't trust the freshness of your ingredients until you have given them a taste test yourself.
Mechanical blending, such as with a food processor or immersion blender, can cause extra virgin olive oil to become bitter. Bitter flavor compounds that are normally coated in fatty acids and diluted are freed from their coating during hard blending. Keep out bitterness by blending your pesto by hand with a mortar and pestle whenever possible. If you must use a processor or blender, blend all the ingredients except for the olive oil, then stir the olive oil into the pesto paste by hand.
Ramp Up the Flavor
Strong flavors can help overcome slight bitterness in pesto. Add more cheese, garlic or extra nuts to help counteract the bitterness. If the basil is to blame, try mixing it with an equal part of non-bitter greens, including more basil or some spinach. If the pesto is already mixed and the olive oil is the cause of bitterness, making a fresh batch without mechanical mixing and then blending the two batches may lessen the bitterness enough so you can salvage the pesto. Pairing the pesto with other strong flavors, such as using it with a tomato-based dish instead of on plain bread or pasta, may also camouflage bitterness.
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Extra salt can counteract the bitterness in most foods, including pesto. When adding more salt, taste frequently because overly salty pesto isn't any better than a bitter sauce. Ingredients that contain salt have the same effect, such as salty Parmesan or Romano cheeses. You can also pair the pesto with salty ingredients in the final dish. For example, add a salty sausage to the pasta if you are using pesto as a sauce, or present it with salty deli meats if you are serving it as a spread.