by Michael Hawkins
Short answer
The most common reason for pesto tasting bitter is that the olive oil is past its best and has started to turn rancid. If the pesto has been made in a food processor or blender, there's also the possibility that it has turned bitter from the crude, sheering action of the blades.
Long answer
First things first, there’s no reason why shop-bought pesto should ever taste bitter. It may taste slightly sour or acidic (because that is one of the main ways producers can achieve such a long shelf-life), but bitter is a very different taste sensation. Adding a little sugar can mask some of the bitterness, but that really shouldn't be necessary if you've bought a top-quality product.
When it comes to homemade pesto, there are a few potential culprits:
Choice of oil
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. Just like fine wine, the taste is affected by a whole range of factors, including the variety and ripeness of the olives, the climate, the soil conditions, and how it has been stored. Everyone has different flavour tolerances, so while an oil may taste bitter to you, it may not to someone else.
The simplest solution is to taste a bit of the oil before you start making your pesto and check that you are happy with its flavour profile. Quite simply, a poor-quality oil is going to produce a poor-quality pesto.
If your olive oil has been open for more than 12 months, then you may start to detect hints of must, vinegar, bitterness, or a metallic aftertaste. Once oil has turned rancid, there's nothing you can do to fix it, and you'll simply have to throw the whole lot away.
Other ingredients
If you're making a traditional basil pesto, then don't assume all basil plants are created equal. Some leaves may have notes of anise, lemon, and even cinnamon, so you could just have found yourself with a variety that you haven't tasted before. Thai basil, for example, is worlds apart from sweet Italian basil.
Some nuts, walnuts in particular, 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.
Production method
Once you're 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.
