Olive oil pungency and bitterness

Here are a couple of interesting details to add to the story of olive oil's back-of-the-throat, peppery pungency.

In 2003, a research group at Unilever reported on the sensory effects of several of the phenolic compounds found in olive oil, including the cough-inducing substance later named oleocanthal. Oleuropein, the phenolic compound that makes the fruit of the olive bitter, is water-soluble rather than fat-soluble, so it doesn't get transferred into the oil when the fruit is pressed. But a number of oleuropein relatives do end up in the oil, and the Unilever researchers tasted them in pure form. They found these compounds to contribute mainly bitterness and drying astringency, along with some numbing, cooling, sour, salty, and tingling sensations. Relatives of pungent oleocanthal also taste somewhat astringent and bitter. So an oil rich in phenolics can have a very complex taste and mouthfeel indeed.

Other studies have found that heating olive oil reduces the levels of most phenolic compounds. And pepperiness gradually fades as an oil ages. The balance of flavors in an excellent olive oil is thus temporary and so especially worth savoring when you find it. The deterioration of olive oil is slowed by keeping it cool and protected from light.

The balance of phenolics and of so of an oil's taste depends on the variety of olive, the way the oil is pressed and handled, and on the growing conditions for olive trees--especially the water supply. The olive tree is native to the arid Mediterranean and can get by with very little water. According to a number of recent reports, including one from the University of California at Davis, water stress produces very bitter and astringent olives, while copious irrigation produces olives and oils with very little bitterness, astringency, or fruity flavors. So the olive is another example of the many fruits and vegetables whose quality suffers when the plants are pampered.
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Andrewes, P. et al. Sensory properties of virgin olive oil polyphenols: identification of deacetoxy-ligstroside aglycon as a key contributor to pungency. J. Agric. Food Chem. 2003, 51, 1415-20.

Carrasco-Pancorbo, A. et al. Evaluation of the influence of thermal oxidation on the phenolic composition and on the antioxidant activity of extra-virgin olive oils. J. Agric. Food Chem. 2007, 55, 4771-80. http://dx.doi.org/10.1021/jf070186m

Berenguer, M.J. et al. Tree irrigation levels for optimum chemical and sensory properties of olive oil. HortScience 2006, 41, 427-32.

Curious Cook in the New York Times: Olive oil aromas and pungency

In today's Curious Cook column, I write about observing an olive oil competition, and about how one scientist's taste of a peppery oil led to the discovery of a natural version of ibuprofen.

One correction to the column: in the editing an important period got lost, and the result was to make Picual olive oil sound like a pretty miscellaneous collection of aromas. The printed paragraph is:

There were many different green notes pressed from the green fruit: of grass, celery, raw and cooked artichoke, green tea, seaweed. An oil from the Spanish picual variety smelled startlingly of tomato leaf, then green herbs: sage and rosemary and basil and mint and eucalyptus. From riper olives there were fruity and nutty aromas: citrus and almond and even banana.

It should read:

There were many different green notes pressed from the green fruit: of grass, celery, raw and cooked artichoke, green tea, seaweed. An oil from the Spanish picual variety smelled startlingly of tomato leaf. Then there were green herbs: sage and rosemary and basil and mint and eucalyptus. From riper olives there were fruity and nutty aromas: citrus and almond and even banana.

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The Los Angeles International Olive Oil Competition

Beauchamp, G. K. et al. Phytochemistry: ibuprofen-like activity in extra-virgin olive oil. Nature 2005, 437, 45–46.

Smith, A.B. et al. Synthesis and assignment of absolute configuration of (-)-oleocanthal: a potent, naturally occurring non-steroidal anti-inflammatory and anti-oxidant agent derived from extra virgin olive oils. Organic Letters 2005, 22, 5075-78.

In the dark: olive oil, milk, butter, and beer

In my last post I mentioned that olive oil is best stored in the dark. The same is true for milk and butter and beer. It's turning out that all these foods are sensitive to light for similar reasons.

When milk is exposed to light, especially sunlight or to the fluorescent lights in a market, it develops an unpleasant, sulfurous "sunlight" or "lightstruck" flavor. It's been known for a long time that the vitamin riboflavin is involved in this reaction, and a recent report by David Min and colleagues at Ohio State summarizes the current understanding of what happens. It turns out that the off flavor signals significant nutritional losses. When riboflavin absorbs certain frequencies of light, it catalyzes the conversion of ordinary oxygen to an especially reactive "singlet" form. Singlet oxygen in turn attacks the milk fat, producing fragments with grassy aromas, and it attacks the amino acid methionine, producing a compound with an overcooked-vegetable aroma (dimethyl disulfide). It also attacks both the riboflavin that made it, and vitamin D, which we need to absorb the calcium in milk efficiently.

Exposure to light also damages the flavor of beer, which accumulates a characteristic "skunky" sulfur compound known as MBT (3-methyl-2-butene-1-thiol). Earlier studies had shown that MBT is produced when flavor compounds from hops, the hop acids, react with sulfur-containing compounds. But the hop acids themselves don't absorb the wavelengths of light that cause skunkiness. It appeared that that the energy for the reaction was supplied indirectly, and probably by the same molecule that damages milk-- riboflavin! Richard Pozdrik and colleagues in Melbourne, Australia have strengthened the case against riboflavin by showing that light absorption by riboflavin in beer correlates well with the development of skunkiness.

According to a new study of butter done in Norway and Denmark, riboflavin isn't the only "photosensitizer" in dairy products. J.P. Wold and colleagues found that traces of chlorophyll and related substances in butter also absorb light energy and transfer it to other butter components, thus causing oxidation reactions and unpleasant flavor changes. This makes sense, because absorbing and transferring light energy is exactly what chlorophyll is designed to do in the leaf of a living plant. And it's the lovely green chlorophyll and related molecules that are the major photosensitizers in olive oil.


So it's a good idea to buy and keep all these foods in opaque or at least dark containers. If they're in clear glass or plastic, or the butter is wrapped in light wax paper, then keep them in the dark as much as possible.

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D.G. Bradley et al. Effects, quenching mechanisms, and kinetics of water-soluble compounds in riboflavin photosensitized oxidation of milk. J. Agric. Food Chemistry 2006, 54, 6016-20.

R. Pozdrik et al. Spectrophotometric method for exploring MBT formation in lager. J. Agric. Food Chemistry 2006, 54, 6123-29.

J.P. Wold et al. Active photosensitizers in butter detected by fluorescence spectroscopy and multivariate curve resolution. J. Agric. Food Chemistry 2006, 54, 10197-10204.

Keeping olive oil cool

Refrigeration usually slows the deterioration of our foods by slowing the chemical reactions that cause it. But extra-virgin olive oil turns out to go rancid at about the same rate in the cold as it does at room temperature. Chemists at the University of Udine in Italy found that the benefits of slowed reactions at low temperature are counterbalanced by the gradual crystallization of the more saturated oil molecules. This process leaves the remaining liquid oil with a higher proportion of vulnerable unsaturated fats, and with a smaller proportion of antioxidant substances, which tend to get trapped in the crystals. So oxidation continues at about the same pace despite the low temperature.

There's no need, then, to crowd the fridge with bottles of special olive oil. Just keep them in a cool, dark place. And this is especially true for unopened bottles. Another study from the Universities of Milan and Castilla-La Mancha reports that freshly bottled Italian and Spanish oils high in antioxidants retained much of their antioxidant capacity after as much as 240 days storage at 40 degrees Centigrade, or 104 degrees F. One oil made from the Picual olive even remained fully within the specifications for extra-virgin olive oil.

One other useful fact to be gleaned from the first report: the viscosity of olive oil nearly triples as it cools from room to refrigerator temperature. As many cooks know, you can make a plain vinaigrette very thick and creamy simply by serving it and the salad ice-cold.


Calligaris, S. et al. Influence of crystallization on the oxidative stability of extra virgin olive oil. J. Agric. Food Chem. 2006, 54, 529-35.

Lavelli, V. et al. Effect of storage on secoiridoid and tocopherol contents and antioxidant activity of monovarietal extra virgin olive oils. J. Agric. Food Chem. 2006, 54, 3002-07.