I've written it dozens of times: Processing olive oil in a blender can make it unpleasantly bitter. I've crafted recipes to ensure it doesn't happen, instructing readers to make mayo by blending in a neutral oil first and then whisking in some olive oil by hand afterward. Then, while working on an article about the textural differences between hand-whisked and machine-blended mayo, I noted a flavor divide as well—the blended mayo was more bitter than the hand-whisked one. Exactly as expected.
I wrote my article, and I explained the bitterness as the consequence of blending olive oil at high speed. But I also casually floated a second explanation: that the bitterness may have been caused by the more aggressive puréeing of garlic in the blender (the hand-whisked one contained garlic that I'd minced with a knife). After all, I know for a fact that how thoroughly you mince garlic can have a dramatic effect on its flavor, with a more intense bitterness emerging the more finely the garlic is broken down.
I also know that sometimes at home I break my own rule and blend olive oil at high speed, and most of the time, it doesn't bother me. I'm not alone: While some of the cooks I've spoken to about this over the years seem to notice this bitter blended olive oil phenomenon, others don't. Nancy Harmon Jenkins, in her book Virgin Territory: Exploring the World of Olive Oil, writes, "I've also seen mutterings on the Internet that...olive oil is made bitter by vigorous beating.... I must confess, honestly, that I've not had that experience, nor have any of several other cooks or chefs I've queried." There's clearly a lot to unpack here.
I've since run a variety of tests, written an earlier version of this article, then run more tests and consulted scientific and olive oil industry experts, and now written this updated article. The good news is I now have clearer answers that help explain not only why blending can make olive oil bitter, but also why not everyone experiences this effect.
For those who want to get straight to the answer, here it is: natural bitter compounds in the oil called phenols are water-soluble, so high-speed blending of oil with any water-based ingredient like lemon juice, vinegar, and egg whites can draw them out; emulsions like mayo are among the most effective at transferring those phenols from the fat to the water phase. But the bitterness isn't always strong or noticeable, since different olive oils have different phenol levels (among other variables). Here’s a look at how I arrived at these answers.
What do we know about olive oil and bitterness? Well, we know that its bitterness (which should not be confused with that back-of-throat spiciness common to many olive oils) is caused by phenols, a large family of antioxidant molecules found in both plants and animals. It's thought that secoiridoids, a class of phenols, are most directly responsible for any perceived bitterness and pungency in olive oil.
The idea that the bitterness of olive oil is exacerbated by high-speed blending or food-processing has been around for a while. The food writer James Peterson mentioned it in his book Cooking, and the topic has popped up on food message boards for years. Peterson gives no explanation for the underlying mechanism that causes this bitterness, but almost all of the internet chatter I've seen points back to this 2009 Cook's Illustrated article on the topic. According to that article, the bitterness is increased when the spinning blades of a food processor or blender disperse the oil and its fatty acid–coated polyphenols into tiny droplets in an emulsion, making them more easily detected by the taster. The problem, they say, is worse with higher-quality extra-virgin olive oil and less of an issue with pure-grade olive oil, due to a lower phenol content in the latter.
Reading the Cook's Illustrated story, though, got me wondering: What's the best way to test this? According to Cook's Illustrated, the phenomenon is specific to emulsions, like mayonnaise, that contain olive oil. To test it fairly, you'd have to make two identical emulsions, one by hand and the other with a blender or food processor, and then have blind-tasters evaluate the results. But, as my article on whisking mayo by hand shows, the results have such obviously different textures that there'd be no way to blind-taste the two side by side without knowing which was which.
Putting that blind-tasting challenge aside, there's the question of garlic. According to the Cook's Illustrated article, they made two batches of mayo, one blended and the other hand-whisked, and compared the results. But both those batches also contained minced garlic, which is a tricky variable to throw into the mix.
Garlic, as I know from my mincing tests, can have a wide range of flavors depending on how (and how thoroughly) you mince it. That's because, as I wrote, "When [garlic's] cells are damaged,...two molecules, one called alliin and an enzyme named alliinase, come into contact with each other, and together produce a new compound called allicin, which is responsible for the pungent, vampire-repelling smell we associate with garlic. The more cell damage that occurs, the more allicin is produced, and the stinkier the garlic becomes." And not just stinkier, but more intensely bitter and spicy as well.
Consider what happens when you process minced garlic in a food processor or blender while drizzling in oil to make mayo: You're mincing the garlic even more finely. Compare that with what happens to the garlic when you're making a mayo entirely by hand—since the whisk can't cut the garlic, you have to mince it by hand first, then stir it in. So the garlic in a blender mayo ends up with a lot more cell damage (and thus pungency) than the garlic in a hand-whisked one.
Given this, it'd be hard for tasters to distinguish between bitterness generated by the blending of the olive oil and bitterness generated by the blending of the garlic. My task, then, was to find other ways to structure the tests to avoid these complicating factors.
Olive Oil Bitterness Testing
To see if I could get more clarity on just what was responsible for the perceived bitterness—the olive oil or the garlic—I ran a couple of initial tests.
Blended Versus Unblended Olive Oil
I started with the most basic test: Blending olive oil all by itself. I gathered five different bottles of extra-virgin olive oil from four different sources (Australian, Italian, Californian, and Spanish), and blended each one by itself with an immersion blender for one full minute. I let all the samples sit until any cloudiness from air bubbles introduced during the blending had dissipated, so that there was no visual indication of the blending. I then asked blind-tasters, who didn't know what I was testing in even the most general sense, to taste each blended sample against its unblended counterpart (varying the order of which sample came first), and to give feedback on the differences, if any, they noted between the two.
The responses were all over the place: Several tasters said the unblended samples were more bitter or pungent or astringent or spicy, a couple said the blended samples were more harsh, and one couldn't detect much of a difference at all.
Since I was conducting the test, I wasn't able to do the tastings blind, but I tasted all the samples as well and found no obvious differences between the blended and unblended oils. The biggest factor, I found, was the order in which the oils were tasted, since the intensity of olive oil can be cumulative—the second spoonful can hit you harder than the first one.
I did a second round of testing using these same samples of previously blended and un-blended oil to make hand-whisked mayo with no garlic, just to see if they might register differently to tasters in an emulsified form. And once again, tasters had no clear ability to distinguish among them.
These tests suggested that the act of blending olive oil by itself at high speed is not responsible for any potential increase in bitterness. There had to be some other factor at play, possibly more than one.
To determine whether the garlic in the mayonnaise test was the primary contributing factor to perceived bitterness, I next made two batches of mayo using an immersion blender. I added a clove of garlic to one batch at the beginning, allowing it to be fully processed during the making of the mayo. For the other batch, I minced the garlic by hand, but withheld it until after the mayo was blended together, after which I stirred in the garlic.
In this test, four of my tasters identified the blended-garlic mayo as being more bitter and intense, while two thought the batch with minced garlic was more bitter. "Bitterness" was the word of the day in this test, with only one taster not remarking on it, and most people thought the mayo with garlic that had been blended was more bitter.
This test does add weight to the idea that the garlic can be a factor in increased bitterness when making mayonnaise in a blender, but it wasn't decisive enough to fully answer the question.
If you read the Cook's Illustrated article, it seems to suggest that the phenomenon is particular to olive oil emulsions made specifically in a blender. Thus far, in the interest of making my samples as indistinguishable as possible, I had blended the olive oil first, then made the mayo by hand (except for in the garlic test, for which both mayos were immersion-blended). Perhaps they were right—maybe this spike in bitterness happens only when olive oil is emulsified into a liquid at high speed.
Since there's no way to make two emulsions, one with a blender and one by hand, that are indistinguishable from each other, I decided to try an even simpler test: I would make a mayonnaise emulsion of olive oil worked into a whole egg using an immersion blender, with no other ingredients at all—no vinegar, no salt, no garlic—and then I would compare it with the same oil in its liquid state. I figured that the mayo would either be noticeably more bitter than the oil it was made from, or it wouldn't. And, to try to account for variations in the olive oil itself, I again used multiple olive oil brands.
Once more, I couldn't detect any obvious differences in bitterness.
This left me with a bit of a puzzle. Clearly, many people have experienced this bitterness when making mayonnaise with olive oil in a blender, and yet I was failing to reproduce it. Was it possible that I and others who don't seem to experience this problem are unable to taste the bitterness through some genetic deficiency? Or could it be that there was something specific to the olive oils I was using that could explain it?
As it happened, shortly after my early tests, I had a meeting with Leandro Ravetti, Technical Director of Cobram Estate, a major Australian olive oil producer, who set me on the path to unlocking the mystery.
As soon as I described my testing to Ravetti, he told me that this bitterness is indeed caused by phenols in the oil and is a well-known phenomenon in the olive oil industry. It's tied, he said, to a very specific characteristic of the phenols: their higher water solubility. According to Ravetti, he sees it most with freshly-made olive oil that has just recently been expressed from the crushed olives.
That's because olives, while high in fat, also have some natural water content, and that water gets temporarily mixed into the oil during crushing. And since the phenols have a higher affinity to water than oil, their bitter flavor is detectable for as long as the water remains dispersed in the oil. Only after it's been centrifuged out and the oil is given a chance to settle does the bitter flavor decrease.
"When you're doing the mayo," says Ravetti, "it's the same. All those polyphenols have a higher affinity to water than oil, but the blender makes it worse: It keeps those polyphenols in contact with the water phase much more efficiently, taking them out of the oil phase and putting them into the water phase."
It's an explanation that's backed up by Serious Eatscolumnist (and former molecular biologist) Nik Sharma, who independently arrived at a similar conclusion while working on his newest cookbook, The Flavor Equation. "When you're making the mayo, you're increasing interaction between water, fat, and the phenols," he told me recently. "You're forcing them to interact quite vigorously."
The presence of emulsifiers like the lecithin in the egg yolk used to make mayonnaise likely increases the effect even more. "The phospholipids [like lecithin] in the egg yolk are forming a structure to hold the water and fat phases together," Sharma said. "Because the lecithin in the yolk has a hydrophobic tail that loves fat and a water-loving head, one side binds the water and on the other it binds the fat molecules. When this is happening, the phenols are coming out of the olive oil and going into the water phase."
So, if this is what's happening, why was I not tasting it? Sharma suggested I take a test to check my sensitivity to bitter foods. I ordered a testing kit online, and had my wife administer the tasting strips to me blind (the kit included four samples on paper strips: phenylthiourea (PTC), sodium benzoate, thiourea, and a control). The bitterness was instantly obvious to me in all cases except the control, which, at least according to the kit instructions, qualified me as a "supertaster"—presumably, that is, someone who should be able to pick up on the bitterness in an olive oil emulsion. (It's worth noting beyond this test that I have never had any noticeable problem detecting bitterness in any other foods in my entire life and cooking career.)
Still stumped, I kicked the question to Ravetti. His answer pointed to the olive oils themselves. While he couldn't predict the phenol content of most of the olive oils I'd been using in my tests without analyzing them, he did point to the "Destination Series" oil from California Olive Ranch that had been a mainstay in my testing. "Those oils are particularly low on the secoiridoids [phenols] that give the bitterness and astringent feel on the side of the tongue," he told me, which might be why the bitterness wasn't registering for me.
I asked what kinds of olive oil I should buy to get a higher amount of phenols, thinking maybe I needed to splurge on fancier oils with higher price tags. "The phenols are very important," he said. "But they're not telling you about the quality of the oil. Phenols are in the fruit in response to stress." Whether the olive orchards are irrigated, the kinds of soil they're grown in, and other environmental factors help determine how high the phenol levels are in the olives, and while that can have a direct effect on the characteristics of an oil, it doesn't necessarily correlate with quality. There can be good high-phenol oils and bad ones, good low-phenol oils and bad ones.
After talking to Ravetti, I walked back into my kitchen. On the counter was a container of that low-phenol California olive oil I'd been messing around with, blending it with water for minutes on end, then tasting it to see if I could pick up on any uptick in bitterness that might have resulted from the phenols moving into the water phase. I hadn't noticed it earlier, when the water was still somewhat suspended in the oil, but now it had fully settled on the bottom. I strained the oil off the top and tasted the water below. With the oil gone, the water was undeniably quite bitter.
Does making mayonnaise in a blender with olive oil lead to bitter results? Yes, it can, but it's not guaranteed to be unpleasantly so. What we know for sure is that bitter polyphenols that occur naturally in olive oil are water-soluble, and, given enough of an opportunity, can migrate from the oil into any water present (say, in the form of lemon juice, egg white, or vinegar in a mayonnaise). This can make the mayonnaise taste more bitter.
But whether you actually notice this bitterness will depend on several factors. The amount of phenols in any given oil is likely a big variable, but it's a difficult one to predict unless you have fancy scientific equipment that can measure such a thing. The power of your blender and the heat generated can also matter, as Ravetti pointed out when he told me he's noticed a correlation between bitterness and both the olive mill speed "intensity and temperature," with bitterness going up as crushing speeds and temperatures rise. This is likely why hand-whisking is less of an offender than high-speed blending, and it's conceivable that a more powerful blender and/or longer blending times may exacerbate the bitterness issue.
Beyond that, I can't discount the possibility that some people are more sensitive to this bitterness than others, despite my own (somewhat questionable) "supertaster" status. And in mayo recipes with garlic, there's the added variable of how it's incorporated into the mixture, with powerful blending likely increasing bitterness even more.
So should you avoid using a blender when incorporating olive oil into emulsions like mayonnaise? I'd say it depends. How sensitive to bitter flavors are you? And has bitter mayo been an issue for you before? If you've used a familiar brand of olive oil to make mayo with a blender and it's never led to trouble for you, I'd say don't worry about it too much. But if it's a recurring problem, it's probably better to stick to the old advice of whisking olive oil in by hand, or switch olive oils until you find one that is less problematic.
Editor's note: This article has been updated with additional testing and expert interviews.