How Cooking Oil Goes Rancid

Have you ever forgotten an opened bag of potato chips in the pantry and by the time you remembered about it the chips were tasting “funny”? It’s because of the oils that the chips had been fried in have gone rancid? How did that happen?

Read on…

Before I talk about the ways cooking oils go rancid, I want to make an important distinction between fats and oils. Fats are stable at room temperature due to their higher saturated fatty acids content. Oils on the other hand are usually liquid at room temperature. The reason is they are higher in mono- and polyunsaturated fatty acids. Usually, fats are used to denote both fats and oils, but in this article I will use “fats” and “oils” interchangeably.


How does cooking oil become rancid?

Rancidity (or rancidification) refers to changing the chemical and physical structure of lipids (fats and oils). There are two different ways oils can become rancid: hydrolytic rancidity and oxidative rancidity.

Hydrolytic rancidity is when oils are partially catalyzed to free fatty acids (FFA), mono- and diglycerides. Usually the causes are enzymes (lipases) and microorganisms (lipases-producing, like Candida albicans). Lipases are enzymes, which break down fats into monoglycerides (digest fats).

All oils and fats are susceptible to hydrolytic rancidity, but the ones that can put out off-flavors are the fats with shorter chain fatty acids, like butyric, caproic, caprylic and capric acid. These oils include coconut oil and butter. Other highly-saturated fats, like cocoa oil, usually don’t smell offensive and it’s more difficult to find out if they’ve become rancid.

Side note: I’ve had coconut oil turn rancid on me before, and because I only considered oxidation as a way of oils to become rancid, I couldn’t understand how that could happen. So, coconut oil (90 percent saturated) can go rancid by way of hydrolytic rancidity.

Oxidative rancidity is the result of a chemical reaction between the fats in oil and oxygen. The rancidity from oxidation is always acompanied by off-flavors and odors due to the formation of peroxides, carbonyls, aldehydes, trienes (low-molecular-weight volatile compounds).

Oxidative rancidity occurs primarily in oils with monounsaturated (MUFA) and polyunsaturated fats (PUFA). These oils have one or more double bonds between carbon atoms (they have missing hydrogen atoms) that can be easily oxidized by oxygen from the air. Polyunsaturated fats have two or more double bonds and thus are more prone to oxidative rancidity than monounsaturated fats. In other words, the higher the level of unsaturation in oils, the higher the possibility and the speed of oxidative rancidity.

Apart from oxygen, other catalysts causing oxidative rancidity include metal ions (iron), heat and light.

Oxidation of oils starts even before the oils are extracted from fruits (olive, avocado), seeds (sunflower, hemp, flax), grains (corn) or legumes (soy). Some methods of extraction speed up oxidation and rancidity. For example, expeller pressed oils become rancid during the process of extraction because of the higher temperatures during the process. Cold-pressed oils are less rancid by the time they get to store shelves.

Common cooking oils that are most prone to rancidity from oxidation due to high PUFA content (and you should never cook with them) are the following in this order:

  • Safflower
  • Hemp
  • Sunflower
  • Flax
  • Corn
  • Soybean
  • Cottonseed

So, to answer the question “how does cooking oil become rancid”. It can become rancid:

  • if stored in light, clear bottles, in tins with a large air bubble at the top, at higher storage temperature or at direct sunlight
  • during cooking, especially if it is highly unsaturated. The higher the temperature (like in frying), the faster it oxidizes
  • during storage, if attacked by lipase-producing bacteria and microorganisms
  • if passed trough machines or vessels, made of or containing iron

Is rancid oil bad for your health?

Rats, fed rancid oil showed signs of liver, and in one case, kidney damage. In some of the studies the rats showed tendency to overeat (1, 2, 3, 4). Granted, the studied subjects were rats, but “dark-red patches, necrosis, and bleeding in the livers” doesn’t compel me to eat foods, cooked in reused (recovered) or otherwise rancid oils, which is how the oils are in commercial fryers (I used to work in a food place where the oils in the fryers were changed once a week!). You draw your own conclusions.

How to slow down rancidity in cooking oils?

To slow rancidity from hydrolysis and oxidation:

  • store cooking oils in dark or covered/wrapped bottles away from direct sunlight, or in metal tins with the oil nearing the top of the tin
  • store cooking oils in cool and dark places – best refrigerated (keep in mind that highly monounsaturated oils, like olive oil will become semi-solid at low temperatures)
  • use lower cooking temperatures (with the right cooking oils – below) and DO NOT reuse oils for cooking or frying!
  • in commercial applications, with the use of natural (polyphenols, ascorbic acid, mixed tocopherols) or synthetic antioxidants, added to oils

What oils should you use for cooking?

The best oils for cooking are those that are highly saturated or primarily monounsaturated in this order:

  • coconut oil (warning: low smoke point – use lower cooking temperatures)
  • butter, organic (or ghee – clarified butter)
  • palm oil
  • lard
  • extra-light olive oil
  • pomace oil
  • extra-virgin olive oil (lower smoke point)

In summary, buy oil for cooking in smaller cans or bottles, so you don’t have to store it for a long time, cook at lower temperatures WITH the right cooking oils, and DO NOT reuse oils.

Let me know if I’ve missed anything or you have comments.

2 Responses

  1. Excellent information. Thank you for such an in-depth explanation.

    My question is, how does cooking effect the potential (of saturated fats) to go rancid? I’m researching dehydrating eggs. To dehydrate scrambled eggs, it is recommended to use a non-stick pan because the cooking oil can go rancid in the dried product. I only cook with cast iron or stainless steel, and am wondering if I can cook the eggs in coconut or EVOO before dehydrating. Will these oils go rancid faster because they’ve been cooked. (I hope that makes sense).

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