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If I leave some olive oil uncovered and fully exposed to indoors air for enough time (though away from UV exposure or any other environmental factors), what will happen to the oil?

I read here that some of its constituents ('lighter' molecules) will start to evaporate but that heavier ones will be left behind, which will 'not quite' evaporate.

If that is true (it may not be), would the non-evaporative part of the oil eventually harden and assume a cake-like consistency, almost like wax or hardened animal lard stored at refridgerator temperatures?

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Some of the lighter constituents could evaporate. There are a lot of minor components in olive oil, and they are largely responsible for the fragrance and flavor. As these evaporate, the fragrance and flavor would change. It is unlikely that they would all evaporate though, as many of these components likely have high boiling points/low vapor pressures.

The oil itself would not evaporate, but it would not solidify either. At most you might notice a thickening of the oil.

Over long periods of time the oil may react with oxygen to form various oxidation products. This process is so slow, however, that it is definitely not a concern for olive oil you might leave out in your kitchen. The oil will start tasting and smelling stale long before significant oxidation occurs.

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  • $\begingroup$ Just to be clear, the oil will taste and smell stale (then rancid) due to oxidation. I realize that you were referring to a more thorough version of the process. :) $\endgroup$ – Jason Patterson Jan 11 '15 at 16:57
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    $\begingroup$ The funny thing is, I was wondering if it may actually be useful for something (as a material) - rather than worrying about it - and upon googling 'epoxidized' I discover: 'Epoxidized soybean oil, better known by its acronym, ESBO, is a plasticizer used in polyvinyl chloride (PVC) plastics' - so there you go... $\endgroup$ – user1469 Jan 11 '15 at 20:38
  • $\begingroup$ >epoxidized olive oil || that's fantasy. Oxidation does happen, but via radical route, leading to hydroperoxides and products of their decay. $\endgroup$ – permeakra Jan 11 '15 at 22:21
  • $\begingroup$ @permeakra, you are correct. I think I just got distracted by the prevalence ESBO and didn't think through the actual mechanisms of oxidation applicable here. $\endgroup$ – Kyle Jan 11 '15 at 22:37
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Vegetable oils do lose some lighter constituents through evaporation, particularly substances responsible for aroma, but the primary degradation pathways, making it rancid, are through oxidation, hydrolization and microorganism metabolism; see http://www.ivannikolov.com/how-cooking-oil-goes-rancid/ and http://en.wikipedia.org/wiki/Rancidification.

Some tree-nut oil, in particular, quickly develops a noticeable off-taste. BHT, BHA and other additives (often applied to packaging rather than to the food itself) help decrease oxidative rancidity.

That said, vegetable oil used for lubrication in old machine parts certainly does thicken to the consistency of wax, though I do not know how much of that is due to evaporation vs. degradation.

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The key to the answer is what kind of oil do you have, there are differences. In general, oils with big amount of double bonds MAY oxidize by oxygen from air. The process is radical, UV-induced, and very slow. Some other oils, with no or too little double bonds do not oxidize. and remain mostly same.

The process of oxidation (drying) initially produces hydroperoxides, later able to serve as a source of more free radicals, leading eventually to inter-molecular linkages. I found contradicting notions for what these linkages consist of (it is either oxygen bridges or direct links similar to polymers), but the end process if well known and used in varnishes. However, varnishes has extra components: they are usually diluted by some solvent to reduce viscosity and always contain a metal catalyst to accelerate drying of the oil, that also is usually heated for prolonged period of time, again, to increase the speed of drying.

The considerations above ignore obvious process of evaporation of light impurities and possibility of bio-degradation (I'm not aware of bio-degradation processes for pure fatty oils, but there are bacteria that live in almost undiluted toluene and there are bacteria that thrive on petrol, so I'm open to this idea).

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