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I read somewhere -- maybe a James Peterson cookbook? -- that putting olive oil in the blender for more than a few seconds will make it bitter. However, many recipes for hummus, aioli, and other things call for blending olive oil. At times I've felt that making hummus without the oil, and then stirring it in at the end gives a better flavor, but I can't say for sure.

Does anyone have any advice about this?

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Well, this is a common confusion between "sour" and "bitter".

The better your olive oil is, the lower its acid value - extra virgin olive oil has the lowest acid value, it is pressed in ways that reduce contact with atmospheric oxygen. Contact with oxygen increases the acid value of olive oil.

Whizzing olive oil in a blender aerates it, and the resultant oxidation increases the acid value, ie it gets more acidic and tastes "sourer".

Lots of people can discern the change, but they don't use the right word to express it ...

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  • Thank you, good explanation. Interesting that both the recipes I mentioned also typically include lemon juice, so a little sour flavor from the olive oil shouldn't be a problem.
    – handsofaten
    Jun 8, 2011 at 1:32
  • @handsofaten - I think I have oversimplified in the answer - the things that oxidise to provide fatty acid products are aldehydic and other flavour components, so it is the removal of these that gives the flavour change. Whatever, to get a bitter flavour would require the production of basic products from oxidation, rather than acidic products, so to refer to the oil as becoming bitter is a misnomer in any case.
    – klypos
    Jun 8, 2011 at 21:31
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Hammering extra virgin olive oil in a blender or food processor allows astringent ("bitter") tasting polyphenol compounds to be detectable by the tongue.

Cook’s Illustrated explained it in their March & April 2009 issue, page 30:

Extra-virgin olive oil contains bitter tasting polyphenols coated by fatty acids, which prevent them from dispersing. If the oil is emulsified in a food processor, these polyphenols get squeezed out and the liquid mix turns bitter.

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  • And people pay good money for the bitter tasting compounds...weird
    – TFD
    Apr 23, 2015 at 1:04
  • This is misinformation. Serious Eats even debunks it here: seriouseats.com/2018/06/…
    – Allison
    Jun 16, 2020 at 1:52
  • @Allison thanks for the link. Interesting that it's by the same website that convinced me the bitterness was due to the blender. I'm not convinced their latest findings are definitive, but it does cast doubt on my explanation. I know I personally have made the same mayo recipe (without garlic) where the only difference (that I knew of) was canola vs extra virgin olive oil. The canola mayo was excellent. The EVOO mayo was absolutely inedible. The only thing clear to me is that nobody has done a rigorous experiment the determine the real mechanism.
    – mpoisot
    Jun 16, 2020 at 17:13
  • I've never had olive oil go bitter from blending (and I make an emulsified olive oil based pesto in a vitamix, the hammery-est of blenders, with cups and cups of polyphenol-filled greens at least once a week), but I always use fresh, high-quality olive oils and can smell rancid or near-rancid oil from a mile away. Sounds like rancid oil to me or, as another commenter pointed out, the inability to distinguish between bitterness and the increase in acidity from aeration. Another thing to consider is you weren't using real olive oil -- 70-80% of the olive oil market is saturated with cheap fakes.
    – Allison
    Jun 16, 2020 at 19:05
  • @Allison my oil definitely wasn't rancid, and I can tell the difference between bitter and sour (acid). But maybe it was counterfeit oil? That would help explain why the phenomenon is so hard to pin down.
    – mpoisot
    Jun 17, 2020 at 1:26
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The Olive Centre suggests that; other factors can also affect the quality of your oil which include air, light, heat, water and too much sediment. Light speeds up the oxidation process which shortens the shelf life of the oil.

Also, there was research carried out which confirmed that dark glass (preferably browner) Antique Green was one of the best ways in which to miminise oxidation compared to other types of packaging.

For further reading here are come suggested links:

http://www.theolivecentre.com/Olive-Equipment-Menu/LA-OLIVE-OIL-STORAGE-SETTLING/ARTICLE-Oil-Storage-Settlling-for-Oil-Quality

Effect of Storage Containers on Olive Oil Quality: https://rirdc.infoservices.com.au/items/09-160

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