Why Is My Pesto Bitter?

The most common reason pesto tastes bitter is that the olive oil is past its best and has started to turn rancid. If you have made your pesto in a food processor or blender, the oil may also have turned bitter from the crude, sheering action of the blades.

Top-quality pesto should not taste bitter

Shop-bought pesto may taste slightly sour or acidic (because that is one of the main ways producers can achieve a long shelf-life), but it should never taste bitter. Adding a little sugar can mask some bitterness, but that shouldn't be necessary if you've bought a top-quality product.

When it comes to homemade pesto, there are a few potential culprits:

Your olive oil is naturally bitter

Whenever someone asks us why their homemade pesto tastes bitter, our first question is, "What olive oil are you using, and is it still in date?"

Olive oil can vary from mild, floral, and neutral to fruity, sour, acidic, peppery, grassy, or slightly bitter. Like fine wine, the taste is affected by various factors, including the variety and ripeness of the olives, the climate, the soil conditions, and how you store it. Everyone has different flavour tolerances, so while an oil may taste bitter to you, it may not taste bitter to someone else.

The simplest solution is to taste some of the oil before making your pesto and check that you are happy with its flavour profile. Quite simply, poor-quality oil will produce a poor-quality pesto.

If your olive oil has been open for more than 12 months, you may start to detect hints of must, vinegar, bitterness, or a metallic aftertaste. Once oil has turned rancid, you cannot fix it, and you'll simply have to throw the whole lot away.

Herbs, nuts, and garlic can also taste bitter

If you're making a traditional basil pesto, don't assume all basil plants are equal. Some leaves may have notes of anise, lemon, and even cinnamon, so you could have found yourself with a variety you haven't tasted before.

Some nuts, particularly walnuts, are notorious for leaving a bitter aftertaste, especially if they have been poorly stored. If your garlic is past its best, it may still be salvageable, but you should take extra care to remove the green "germ" from the middle of each clove, as these can be very bitter.

Your production method is responsible for it

Once satisfied that you're not starting with bitter ingredients, the next question is, "Are you using a food processor or blender to make your pesto?"

These appliances are like using a sledgehammer to break a nut. They use a very crude method of breaking down foods by essentially just slicing ingredients thinner and thinner at high speed. That's not a problem for most foods, but for olive oil it very much is.

You see, when you sheer olive oil with a fast-moving blade, you're effectively slicing the oil into microscopic droplets. This aids the release of compounds called polyphenols, which are highly bitter. Blending at a very high speed compounds the problem further.

There are two solutions. First, use a pestle and mortar instead. We guarantee you will end up with a superior product. The second is to use your food processor to break down the dry ingredients, and then simply mix in the olive oil by hand right at the end.

Age and storage
The colour, texture, smell, and flavour of all food change over time, so our final question is, "How long has your pesto been open and how has it been stored?"

The very moment you twist the cap on a jar of pesto, you are starting its inevitable journey of degradation. Sure, most open pestos will last in the fridge for up to five days, but that doesn't mean they'll taste as good as they did on day one.

If your pesto tastes more bitter than it did a day or two earlier, the most likely explanation is that the herbs have started to oxidise. Prolonged exposure to air turns herbs bitter and may be accompanied by the telltale sign that the colour of the sauce looks more brown or grey than it did before.

You can reduce the risk of oxidation by making sure that your half-eaten jar of pesto goes back in the fridge with a layer of oil on top. This creates a barrier between the herbs and the air and slows down their journey towards bitterness.

There's no perfect way to fix bitter pesto, although a little sugar or honey can help take the edge off if you find yourself in that predicament.