Are All Dark Glass Bottles used for Storing Extra Virgin Olive Oil the Same?
admin | April 25, 2010EVOO oxidises in two ways. One is called auto-oxidation and the other, and arguably the most destructive sort, is known as photo-oxidation. Photo-oxidation as its name suggests in initiated by light.
The good old fashioned oxygen that you find in air and that is dissolved in EVOO isn’t by itself very destructive. However, if it is hit by light which is packed with enough of an energetic punch, then it can push the generally harmless oxygen to a new and dangerous psychotic state. This energised oxygen, called triplet oxygen starts a destructive chain reaction, oxidising fat molecules left right and centre. The ultimate result of this reaction is a rancid oil that lacks fresh olive flavours, and worst still, fills your food with aweful flavours of old fish and chip shop or suburban corner grill.
Photo-oxidation can be simply avoided by ensuring that the oil is not exposed to light. To this end, good EVOO producers pack their oils in dark glass bottles or cans, and they implore their customers to store their oils in a dark place. While tin cans are completely impervious to light, dark glass is not. Different coloured glass of different levels of darkness are used to store EVOO. Look carefully: some are a classic green, others more of a “sea green” colour. Others are more olive in colour and yet others are amber.
Many producers think that the darker the glass the better, and that is all there is to it. Well they would be wrong.
Oxygen can only be energized if it is hit by light with a short enough wavelength – particularly light in the ultraviolet part of the spectrum. So, coloured glass that absorbs UV light will best protect the oil from oxidation. Those that let UV light through will not afford any protection to the oil, regardless of how dark or light the bottle appears to the eye.
So how do we measure the effectiveness of different glasses and material in blocking UV light? Scientists use a machine called a spectrophotometer or spectro for this gig. It sends a beam of continuously changing wavelength light through the glass and measures the amount of that light that doesn’t make it through (called the absorbance).
Each type of coloured bottle has a distinct ‘absorbance’ fingerprint. Some colours absorb light better at different wavelengths than others. Indeed, that is the exact reason why they are perceived by our eye as having different colours in the first place. The figure below shows the absorbance fingerprints of a variety of commonly used wine bottle colours (from Skouroumounis et al. 2006).
You’ll notice that there are a number of different types of green represented – classic green, antique green, emerald green and the like. They’re all green but they clearly absorb different wavelength light to different extents. The glass labeled ‘flint’ is better known to us as clear glass. It doesn’t absorb light at any wavelength. All the light just passes right through. Hence it looks clear and colourless.
But not all wavelengths are born equal when it comes to destroying EVOO. The ‘light’ with wavelength of 400 nanometers (nm) or less is known as ultraviolet radiation. Our eyes can’t see it but the spectro can. Below 315nm it is known as UV-B, and above 315 UV-A. (the dark lines on both figures represent these parts of the spectrum) .Both types have enough energy to excite oxygen and cause eventually cause rancidity. The absorbance at the other longer (visible) wavelengths gives the bottles their distinct colour.
So think about what the consequences would be of storing olive oil in cobalt blue bottles. They’ll look pretty funky (I guess), but cobalt blue glass doesn’t absorb very well in the UV part of the spectrum. So that means that any UV that hits the bottle will pass straight through the glass and attack the oil. And the glass will provide little protection regardless of how dark the blue. Darker blue glass will absorb more in the parts of the spectrum that don’t influence oxidation but will still let pretty well all the damaging UV light through.
So what about olive oil bottles? This morning I intentionally smashed two common types of square bottles that are used to store EVOO. I bought the oils at a local specialty store. One bottle was common dark green and the other an equally common dark olive green/brown colour. I also cut a bit out of a clear PET bottle used to store olive oil that I purchased at a supermarket and measured the absorbance from the visible through to the UV part of the spectrum. Here’s what the glass looked like and below that are the results.
(Yellow=PET, Aqua=green glass, Brown=olive green/brown)
Clearly the olive/brown version is better than green when it comes to blocking UV. But neither is perfect. So store those oils in a dark place regardless of the fact that they are in dark glass. The PET was a bit of surprise. It absorbed very well in the UV-B, but was pretty well a clear window for UV-A. Some PET contains inbuilt UV blockers. This was clearly one of them. Not that you could tell by looking at it!