First, let’s understand vegetable oils. These are oils that are called vegetable, but are in fact obtained from an array of plants, such as seeds, nuts and fruit. Examples are corn, canola, olive, palm, coconut, peanut, sunflower, sesame, soybean, avocado and coconut oil.
For years, the general perception was that vegetable oils such as sunflower or corn oil were healthier when used for cooking, than saturated animal fats such as butter and lard. Now the results of a study [the only one study so far] have prompted UK scientists to recommend the opposite.
As part of the BBC TV series Trust Me I’m a Doctor, lead study author Professor Martin Grootfeld of the De Montfort University Leicester (DMU) in the UK was asked to put the oils to the cooking test to find out which ones were healthier when heated.
How the test was done
They asked a bunch of volunteers to cook using sunflower, corn, cold-pressed rapeseed (known as Canola oil in South Africa) and olive oil as well as butter, goose fat and lard (pig fat) and to send the leftover oil to the lab for analysis.
After analysis they found that sunflower and corn oil produced aldehydes (chemical compounds linked to an increased risk of heart disease and cancer) at levels 20 times higher than recommended by the World Health Organisation. ?
“A typical meal of fish and chips”, fried in vegetable oil, contained as much as 100 to 200 times more toxic aldehydes than the safe daily limit set by the World Health Organisation", he said.
Olive oil, rapeseed oil, butter and goose fat, on the other hand, produced far fewer aldehydes. The reason for this, says Grootveld, is these fats (including lard) are richer in monounsaturated and saturated fats and are more stable when heated. Sunflower and other polyunsaturated oils, they found, generate high levels of damaging aldehydes.
When fats and oils are heated at or close to 180 degrees C (356 degrees F), their molecular structure changes through a process of oxidation, producing these chemicals called aldehydes. Vegetable oils, such as sunflower oil, release great quantities of aldehydes at high temperatures. Even inhaling the fumes when these oils are heated means you are ingesting aldehydes.
The study also shows that the longer oils such as sunflower and corn were heated, the more aldehydes they produced.
Grootveld told the BBC: “It's a simple chemical fact that something which is thought to be healthy for us is converted into something that is very unhealthy at standard frying temperatures."
So what must we cook with?
“Olive oil for frying and cooking – even butter and lard are better than vegetable oil when used at high temperatures."
To reduce aldehyde production, he recommends you go for an oil or fat that is high in monounsaturated or saturated fats (preferably greater than 60% for one or the other, and more than 80% for the two combined), and low in polyunsaturates (less than 20%).
Michael Mosely, who presented the experiment in Trust Me I’m A Doctor says the ideal "compromise" oil for cooking purposes is olive oil, "because it is about 76% monounsaturates, 14% saturates and only 10% polyunsaturates - monounsaturates and saturates are much more resistant to oxidation than polyunsaturates".
Since Canola oil (rapeseed oil) also scored well - it consists mostly of monounsaturated fats (61 percent, almost as much as olive oil) and is generally less expensive than olive oil, it is a good choice for many of us watching our pockets.
In fact, DietDoc had this to say to a user question about Canola: "Canola oil is rich in monounsaturated fatty acids and vitamin E, which makes it superior to ordinary cooking and salad oils which are made with sunflower oils."
She also reminds us that certain oils such as avocado oil remain stable with no formation of trans-fats up to much higher temperatures than sunflower oil. It is also known that reheating oil repeatedly to high temperatures can increase the trans-fat content and that this practice should be avoided.
"Heating plant oils ONCE will not cause problems, but if you should store the used oil and repeatedly use it at high temperatures, then the fatty acids may break down and form so-called trans-fatty acids which are extremely harmful to health and the heart.
So only use your oil at high temps once. Red palm oil and avocado oil have the highest smoke points of all the oils, so they are more resistant to breaking down.
Finally, Grootveld finally says that we should all try to fry less food and if we use sunflower oil, to use it - unheated - in salad dressings.