How extra virgin olive oil myths are born
admin | November 11, 2012A great deal of mysticism surrounds extra virgin olive oil – what it is, how it is made, and (in some cases) the miraculous things that it can do to your health. Leaving aside the alleged positive effects of consumption on erectile dysfunction (Esposito et al. 2006), acne (Skrosa et al. 2012) and just about every other ailment of man – a more recent trend is trying to validate whether you have purchased real extra virgin in the comfort of your own home using readily available household implements.
One of the myths that have been circulating for some time is that you can tell an extra virgin olive oil by whether or not it solidifies in the fridge. So armed with only a bottle of oil and a working fridge, you too can foil those sophisticated adulterers of extra virgin olive oil. Like a lot of enduring myths this one is based on a tiny bit of science, and a lot of misguided logic.
Recently the social media has been ablaze with a new DIY test for extra virgin olive oil – and all you need this time is a green laser pointer.
Here is how it supposed to work
Shine the green laser through a clear glass bottle containing the oil. If the laser light appears orange or red as it passes through the oil, then it is extra virgin olive oil. If the laser light remains green as it passes through the oil then it is a fake.
Here is an image showing how it works:
Source: Gordon (2012)
First a disclaimer (of sorts). I LOVE lasers!. When I studied physics at university in the late 70’s one of those (then big) things cost about 2 years of a scientists salary. Now you can pick one up from your local disposal store for about the price of packet of crisps. And you can shine them on things from really far away!
But back to the point (forgive the pun). The ‘green laser test’, like the fridge test has a modicum of science behind it, which of course makes it believable, but also dangerous in that it leads people to believe that detecting fraud in extra virgin olive oil is a simple task.
So here are the reasons why both green laser light goes red in extra virgin olive oil, and also why it isn’t a good test for authenticity of extra virgin olive oil.
All extra virgin olive oils naturally contain the green coloured plant pigment chlorophyll. Some oils contain a lot, others very little. But as EVOO is simply the mechanically extracted oily juice from crushed olives, if any of the olives were green (which some always are), or there were some leaves mixed in with the olives (ditto), then the resultant oil will contain chlorophyll. It’s not a matter of if, but only how much.
Laser light is different from ordinary light. Light is a mixture of various wavelengths while lasers emit light of a single wavelength (in the case of green, the wavelength is 532 nanometres). Here is the important bit. Light of a specific wavelength emits a specific amount of energy.
In the case of green laser light, this is EXACTLY the right amount of energy to excite chlorophyll molecules into a higher energy state (wait for it)…. But everything that is excited in nature, must return to a more relaxed state sooner or later. When the green laser excited chlorophyll relaxes, it releases a little less energy than it took in, in the form of longer wavelength light at 620 nanometres – or to you and I – reddish-orange light. This phenomenon is called fluorescence, and the it has known to occur in chlorophyll since 1838!
But unlike humans who relax in lots of different ways, molecules like chlorophyll are restricted by the laws of nature to relax in this very specific way – Green in, Red-orange out. So that is why you see that distinctive reddish-orange fluorescence you see when you shine green laser light through an EVOO. It’s just chlorophyll.
For the skeptical. Figure 1 shows the fluorescent spectra of chlorophyll– I’ve added the colour bar so you can relate wavelength to colour. Figure 2 shows the actual fluorescence spectra for an extra virgin olive oil, and for two refined olive oils. You can see that the maximum of the spectra for chlorophyll and for EVOO correspond exactly.
Figure 1: Adapted from Krause and Weis (1984)
Figure 2: Source – Fawaz et al. (2009)
Here’s why fluorescence in extra virgin olive oil is not a particularly shining example of a valid EVOO adulteration detection methodology.
If I blended some legitimate extra virgin olive oil which will fluoresce, with a refined oil (olive or seed) that won’t fluoresce, the resultant (non-extra virgin) blend WILL fluoresce. Remember that not all EVOO’s contain a lot of chlorophyll. Some are green, but many are not, so different extra virgin olive oils will fluoresce to different extents. So if you add some refined oil to an EVOO rich in chlorophyll it will fluoresce exactly the same as a low chlorophyll extra virgin olive oil. The former is not legit – the latter is. But it’s orange time for them both.
Lastly, it would be pretty easy to make a refined olive oil fluoresce even without adding any EVOO at all. Add a few olive leaves, or industrially available natural chlorophyll, or (possibly*) synthetic chlorophyll (E411) and your refined olive oil will fluoresce brighter than a Christmas tree in the eyes of a 4 year old. Adding chlorophyll or its synthetic analogues to refined olive oil has been one of the tools of the trade of adulteraters for years. Want to make a refined olive oil look Tuscan and increase its value 10 fold? Add chlorophyll. The green laser will swear that it’s a legitimate extra virgin.
And that is how myths are born.
References:
Esposito et al. (2006) doi:10.1038/sj.ijir.3901447
Gore (2012) Phys Teach. 50: 377-378.
Krause and Weis (1984) Photosynthesis Research, 5: 139-157.
Skrosa et al. (2012) doi: 10.1177/1403494812454235
Fawaz et al. (2009) doi: 10.1063/1.3250101
* synthetic chlorophyll is a compound with a copper core, rather than a magnesium core like real chlorophyll, something that would most likely would affect fluorescence.