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Reused oil predisposes you to cancer

As Maama Ruth, a fish vendor in Makindye, briskly rolls her food this way and that way in her kalayi (pan), her hurried manner is striking.

She drops small tilapia fish into the almost black oil and as soon as it is ready, she drops it on a newspaper sitting on the side. She drops more fish into the oil, all the while selling the fish that is ready. She is a jolly woman and her jolliness is so infectious that her customers leave her stall with smiles.

Unknown to Maama Ruth’s smiling customers, the rancid dark brown oil she deep fries her fish in is predisposing them to cancer. In another village in Mannya, Rakai district, residents admit that they even buy this used, black oil from kiosks at Shs 300 a scoop, and many of them have no idea what fresh cooking oil even looks like!

Angella Nalukonge, a food scientist, defines rancid oil as that oil that has disintegrated owing to being boiled at temperatures where it reaches smoking points.

“When I say a cooking oil has been boiled to its smoking point, I mean that it has been boiled to a point where smoke – the fumes are usually black – comes out of it,” Nalukonge says.

Cooking oils also become rancid on being exposed to light or oxygen for periods ranging between two months and a year. Sunflower and sesame (simsim) oil have a shelf life of three months and two to four months respectively. This is according to www.cavemandoctor.com.

Soybean and safflower oil both have a shelf life of six months, while almond oil has a shelf life of six to 12 months. Palm, corn, coconut and avocado oils have a shelf life of 12 months each. Following the end of their shelf lives, these oils begin to go rancid.

What happens when oil goes rancid? This question is pertinent because as Nalunkonge observes, a number of foodstuffs in Uganda are deep fried in rancid oil, because food vendors reuse cooking oil to maximize profit. Cassava chips, sweet potatoes, pancakes, chips and sausages are some of these foods.

When oil goes rancid, free radicals are released and these could result in cancer, Nalukonge says.

“Reusing cooking oil, even if it wasn’t boiled to smoking temperatures, is not advised,” Nalukonge says.
“Every time an oil is reused, its smoke point temperature is lowered, because it has already undergone some break-down from previous uses,” information online reads.


When cooking oil is boiled to smoking temperatures and it releases fumes, a cook and those that inhale the fumes are further predisposed to cancer. www.cavemandoctor.com says when cooking oil smokes, it disintegrates into glycerol and fatty acids, with glycerol disintegrating into acrolein.

Acrolein, which is released with tobacco smoke and cooking oil heated to smoking points, has been linked to lung cancer.

Researchers for the study, Acrolein is a major cigarette-related lung cancer agent: Preferential binding at p53 mutational hotspots and inhibition of DNA repair, concluded: “Acr[olein] is a major etiological agent for CS-related lung cancer … it contributes to lung carcinogenesis through two detrimental effects: DNA damage and inhibition of DNA repair.”

Acrolein was also implicated in lung cancer in Asian women. The study, Lung cancer in Asian women-the environment and genes, concluded that “Cooking oil vapours associated with high temperature wok cooking and indoor coal burning for heating and cooking in unvented homes, particularly in rural areas, [were] risk factors [to lung cancer] for Chinese women.”

Though these studies have linked Acrolein to lung cancer, the American Centres for Disease Control and Prevention in 2005 said Acrolein, which is used in pesticides, had not been classified as a carcinogen then.


Different cooking oils have different smoking points: sunflower oil and corn oil smoke at 232 degrees Celsius, while palm oil’s smoking point is 235 degrees Celsius.

Unrefined oils such as extra virgin oil have lower smoking points than refined oils. Cooking oil that is being reused also has a lower smoking point. Nalukonge advocates for shallow frying, “where foods are only halfway submerged into cooking oil; the foods will have to be turned when they are browned.”

Should one deep-fry one’s food, one should not reuse the oil. The cooking oil can be used to make soap or be added to animal feed, to avoid wastage.
Alternatively, cook with butter or ghee; when taken in moderation, these animal fats are healthy.


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