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Since rancid oil produces free radicals, would that rancid oil if mixed with fresh oil cause the fresh oil to go rancid as well?

  • Hi Ricky and welcome! Just wanted to say that the best way to thank someone for a good answer is to upvote their answer. – Cindy Jan 4 '19 at 15:35
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Short answer: No.

Long answer: No - because the free radicals are very short lived and highly reactive - each free radical will generally only interact with one other molecule before being destroyed. This could cause degradation of reactive nutrients such as vitamins and other nutrients in the oil, but is unlikely to cause much problem with other foods (I would guess). They also have a very limited interaction with the food which the oil is covering (surface area only), so they have a limited chance of causing damage. You are much much more likely to have bacterial or fungal degradation of the food than free radical.

I would have thought that you are more likely to throw the other food out because it is coated in unpleasant tasting oil rather than be worried about the degradation of the food.

Note that rancidification is the production of smelly compounds, mostly aldehydes, from the long-chain fatty acids found in fats and oils. This is a property peculiar to fats and oils and is generally not a problem for other food types as they do not contain large amounts of the fatty acids, so can't get "rancid" as such, though they can be considered spoiled if the fats within them (e.g. fat in meat) go rancid.

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  • Thank you for the information. So if rancid oil mixed with fresh oil, it wouldn't make the 'fresh' oil rancid, I would guess. – Ricky Jan 4 '19 at 15:24
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    It would not cause the fresh oil molecules to become rancid - however, you would not be able to distinguish the fresh oil from the rancid if fully mixed. What you would have is a diluted solution of rancid oil. It would still taste and smell bad. – bob1 Jan 4 '19 at 15:28
  • Thanks again! It's appreciated. – Ricky Jan 4 '19 at 15:30
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I doubt about the needing of an answer as it is self-contained in the question but it seems worth a chemistry clarification.

Oxidative rancidification goes indeed via radicalic pathways. In "fresh" oil, the process will be eased by, e.g., peroxides already formed in the already rancid oil that you would eventually mix.

Peroxides are relatively unstable and the homolitic breaking of their O-O bonds leads to two radicals that re-initiate the process.

Image from https://en.m.wikipedia.org/wiki/Rancidification

This is how the chemistry goes.

However I can't judge how faster will be the rancidification of the fresh oil as compared to that of the same let alone. There should be a difference but perhaps indecteable from an organoleptic stand point. Except for the following...

Beside the fact that new rancid products form or not, a small amount of deteriorated rancid oil can certainly ruin the fresh oil depending on how it will be used in the kitchen. Some mixing could be done to fry probably but should be strictly avoided to season a salad, isn't? This is the only part in which I do agree with another answer.

https://en.m.wikipedia.org/wiki/Rancidification

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  • That's interesting. Thank you. I assume if one were to introduce a tiny amount of rancidity into a large volume of 'good' oil then the antioxidants within the good oil would stop the rancification process? – Ricky Jan 6 '19 at 18:39
  • Correct. That is quite possible.Still you ease the rancidification of the new oil as for antioxidants get consumed upon mixing. It will really depend on the antioxidants conc as well as the mixing ratio as well as the rancidity level of the spoiled oil. – Alchimista Jan 6 '19 at 21:00

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