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Around half of the households in the UK use olive oil regularly in cooking and food preparation. This demonstrates a big change in our use of cooking oils in recent years. The switch to olive oil may be because olive oil is linked to the healthy Mediterranean diet which is know to reduce the risk of an early death. However, there has been concern recently about the damaging effects frying food in vegetable oil can have on health. So is it safe to fry food in olive oil?
What happens when you fry food in very hot vegetable oil?
Heat causes chemical changes in all oils and this alters their aroma, flavour and nutritional content.
When vegetable oil is heated to high a temperature toxic chemicals called aldehydes are produced. Beneficial nutrients and phytochemicals found in many unrefined oils are also destroyed when the oil is overheated.
How do aldehydes affect health?
Aldehydes are simple organic compounds that contain carbon and oxygen and are common in nature. We even produce a small amount in our own bodies when fructose and alcohol are broken down.
When we eat fried food our intake of aldehydes rises. Consumption of dietary aldehydes has been linked to the development of several diseases including type 2 diabetes, cancer and heart disease.
Do all vegetable oils produce the same amount of aldehydes when they are heated to high temperatures?
Researchers from universities in Spain and England have analysed olive, sunflower and flaxseed oils for their aldehyde content after the oils have been heated to 190℃ (standard frying temperature).
The researchers found heating oils high in polysaturates produced greater quantities of aldehydes more quickly, compared to heating oils high in monounsaturated which created fewer aldehydes and much later in the heating process.
So, what oils should I be cooking with?
If you want to reduce aldehyde production when you fry food, research suggests you should go for an oil which is high in monounsaturates and low in polyunsaturates.
Olive oil is a good compromise because it is about 76% monounsaturates, 14% saturates and only 10% polyunsaturates. It is more stable than most other vegetable oils at high temperatures. It can be used to shallow fry food and roast vegetables and it is unlikely that your body would be exposed to greater concentrations of aldehydes than it normally would as a result of normal metabolic processes.
Any other tips on cooking with oil?
The researchers advise when cooking with olive oil, it is better to use high quality oil and keep the oil below its smoke point*; it will also make your food taste nicer.
Try to minimise the amount of oil you use when frying and soak up excess oil with kitchen paper.
Also be careful with how you store your oil. Fats are susceptible to oxidation when they are exposed to light, heat and oxygen so try and keep your oils tightly sealed in a cool, dry cupboard.
While there are clearly healthier ways to cook foods, frying food with olive oil is unlikely to be significantly bad for your health.