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?
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.
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:
So, to answer the question “how does cooking oil become rancid”. It can become rancid:
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.
To slow rancidity from hydrolysis and oxidation:
The best oils for cooking are those that are highly saturated or primarily monounsaturated in this order:
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.