Why to store olive oil in opaque bottles?
Nowadays it is common to store olive oil in opaque (dark green/brown) bottles to preserve its condition.
- Does the olive oil get bad if I would use clear glass bottles instead?
- Does the light "damage" the oil in general, or would this only appear in case of strong sunlight exposure directly towards the bottles (but not in case of placing within shaded areas)?
Best Answer
With a lot of commercial oils, the industry distinguishes auto-oxidation from photo-oxidation (also called photosensitised oxidation), where the former is in the absence of the light, and the latter with light:
[W]hen olive oils are exposed to light, photo-oxidation occurs through the action of natural photosensitizers (i.e. chlorophyll), which react with triplet oxygen to form the excited state singlet oxygen. Thus, protection from direct light is required for commercial edible oils.
Note how the mechanisms are slightly different, even though both involve the C=C bond of monounsaturated and polyunsaturated fatty acids. The issue with photo-oxidation is that it is through the action of chlorophyll and other chemical pigments that absorb wavelengths of light.
Hence the reason that extra virgin olive oil requires darkened glass is more to do with the cause of its colour, i.e. the fact that it has a relatively large amount of chlorophyll. As the 2006 study states:
Light is much more important than temperature in 1O2 oxidation. Light of shorter wavelengths had more detrimental effects on the oils than longer wavelengths.
...and interestingly about the role of chlorophyll:
Although chlorophylls are strong prooxidants under light via acting as a sensitizer to produce 1O2, they act as antioxidants in the dark possibly by donating hydrogen to free radicals.
The concentration of the antioxidant tocopherol on the other hand (well known in its role as Vitamin E), known for slowing oxidation, varies greatly across different olive oils:
Tocopherol content of virgin olive oils (VOOs) varies from 97 to 785 mg/kg. Despite differences in the concentration of total tocopherols that can be attributed to agronomical, geographical, and technological factors, ?-tocopherol is the dominant homologue in virgin olive oils making more than 90% of total tocopherols.
Also note that these oxidised products, the hydroxyperoxides, generally aren't detectable on their own - it is their breakdown products that are directly detected by our nose as "rancid".
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Quick Answer about "Why to store olive oil in opaque bottles?"
[W]hen olive oils are exposed to light, photo-oxidation occurs through the action of natural photosensitizers (i.e. chlorophyll), which react with triplet oxygen to form the excited state singlet oxygen. Thus, protection from direct light is required for commercial edible oils.Should olive oil be stored in a clear bottle?
Store your olive oil in a dark-colored glass bottle to help keep out the light, or in a stainless-steel container. This will protect the oil from sunlight damage.Why does olive oil have to go in a dark bottle?
Store olive oil in a dark-colored glass bottle, which helps to keep out the light, or in a stainless steel container. This will protect the oil from exposure to sunlight.Does olive oil need to be in a sealed container?
Olive oil should be stored in a tightly sealed container to keep it from rapidly oxidizing. If olive oil is stored in a bottle, always replace the cap on the bottle and keep it tightly sealed. Never put olive oil in a container that does not have a tight cap or some other method of sealing the container.Should olive oil be stored in dark container?
Tinted glass that keeps air and light out is an ideal container; avoid plastic, as the oil may leach harmful substances from it. Keep the oil in a dark place, away from the stove and other heat producers.5 Tips for Storing Extra Virgin Olive Oil at Home
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Answer 2
"Nowadays" means - "since the moment we learned to make clear glass and noticed how bad it is to store oil in it."
Light damages oil in general. Among the many reactions it speeds up or makes possible, the most noticeable is the oil going rancid. My ex mother-in-law thought she'd gotten a great deal on bottles of olive oil in Italy. It took two days of transportation in transparent glass and one day of storage on a shelf for it to go rancid (due to oxidation).
When we're talking about direct sunlight, what is meant is (mainly) "temperature rise." You start to give so much energy in a short period of time that you might start chemical reactions within the product (think "over low heat").
Just "light" (even a shaded area, which is why we usually say "store in a dark place") gives oil a dose of UV light. Saturated fats take that pretty well. Unsaturated, not as well.
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Suzy Hazelwood, RODNAE Productions, RODNAE Productions, Jacob Moseholt