Flavor – Why to store olive oil in opaque bottles

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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, α-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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