What Are the Dangers of Rancid Oil?

Written by maria ciubotaru
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What Are the Dangers of Rancid Oil?
Polyunsaturated oils are very unstable and highly reactive to oxidation. (Jupiterimages/liquidlibrary/Getty Images)

Rancid oils are a major source of destructive free radicals in the human diet. Oils oxidise through exposure to air, heat and light and become unstable and highly toxic to your body. Polyunsaturated fats such as sunflower, soy and sesame oil are the most unstable and prone to oil rancidity, because their chemical structures are missing several pairs of hydrogen atoms.

Reasons for Dangers

Manufacturers extract most cooking oils using heat, which makes the oils rancid and toxic even before they reach the store shelves. Manufacturers then refine and deodorise the oils through various chemicals to remove the bad smell and give the oils a nice colour. This process makes it virtually impossible to tell if an oil is rancid or not. Once bought, storing the oil in a cupboard at room temperature and in subdued light will not stop the oxidation process. Cooking greatly accelerates the oxidation rate and allows free radicals to form, harming body cells and using up antioxidants.

Health Hazards

Eating rancid oil will expose you to accelerated ageing, raised cholesterol levels, obesity and weight gain. Daily consumption increases the risk of degenerating diseases such as cancer; diabetes; Alzheimer's disease; and atherosclerosis, a condition in which artery walls thicken due to a build-up of fatty materials. According to a study from the University of Basque Country, the breakdown rate and total formation of toxic compounds depends on the type of oil and temperature. Initially, the oil decomposes into hydroperoxids, then into aldehydes.


Among the most highly refined polyunsaturated oils are canola, safflower and corn. Another example is flaxseed oil, rich in omega-3, an essential fatty acid. Although flaxseed oil sells well in the health food stores, it is highly sensitive and unstable. Monounsaturated fats like virgin olive oil, have only a pair of missing hydrogen atoms, which makes them less vulnerable to oxidation. You can use virgin olive oil for low temperature cooking and for salad dressings. Once open, store it in the refrigerator and use it within a month.


Buy only vegetable oils that are cold pressed, unrefined and organic. Make sure the oils are stored in dark bottles and keep them away from heat. Get small bottles so you use them up before the oil is exposed to air for too long. For high temperature cooking, use only saturated fats like butter and lard, which are stable and highly resistant to oxidation. Coconut oil is also high in saturated fat and an all-purpose cooking oil. It is stable under heat, but has a relatively low smoking point.

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