We explain olive oil's bitterness when mixed in a food processor.
Over the years, we’ve noticed that when we blend mayonnaise or vinaigrette with extra-virgin olive oil in a food processor or blender, they end up tasting bitter. Did we use over-the-hill olive oil in these cases, or was something else going on?
To verify our observations, we made two batches of aïoli using olive oil from the same freshly opened bottle. We emulsified one batch in a food processor, adding the oil in a slow stream to minced garlic, egg yolks, and lemon juice and processing until creamy. In the second batch, we whisked the olive oil into the other ingredients in a bowl by hand.
Tasters found the batch emulsified in the food processor to be markedly bitter.
Extra-virgin olive oil contains bitter-tasting compounds called polyphenols that are normally coated by fatty acids, which prevent them from dispersing in the presence of liquid. When olive oil is broken into droplets in an emulsion, the polyphenols get squeezed out and will disperse in any liquid in the mix, so that their flavor becomes evident. The blades of a food processor break olive oil into much smaller droplets than those created from whisking. The smaller the droplets, the more polyphenols that break free and disperse, and the more bitter an emulsion will taste. (Note: In recipes such as pesto, which contain lots of other robust flavors from herbs, nuts, and cheese, we found that any bitter taste went unnoticed.)
You can whisk your mayo or dressing entirely by hand—but it’s hard to whisk fast enough to form a stable emulsion. To capitalize on the convenience of a food processor, you have two options: Use pure, versus extra-virgin, olive oil, as it has fewer polyphenols (but you’ll lose some flavor). Or, process your aïoli or vinaigrette ingredients with vegetable oil just until an emulsion forms, then whisk in extra-virgin olive oil by hand.