Fried foods not only add kilojoules to the diet, but can contain toxic compounds – especially if old oil has been used during the frying process. What should you know?
Dangerous chemical changes
Experts agree that it's generally safe to use high-quality, fresh oil when frying. However, care should be taken not to use already used or overused oils. Vegetable oils are reactive substances that can undergo dangerous chemical changes.
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"When cooking oils and fats are used over and over in frying processes, they start to break down to eventually form unhealthy products," says Prof Lodewyk Kock from the Department of Microbial, Biochemical and Food Biotechnology at the University of the Free State, South Africa. In some instances, varnish-like compounds can be formed.
According to Kock, consumption of re-used oil has been linked to growth problems, diarrhoea, heart disease, liver damage, enlargement of the liver, spleen and adrenal glands, and cancer. A link between degraded oil consumption and the progression of HIV to Aids has also been suggested.
A recent study by the University of Minnesota also shows that in vegetable-based cooking oils that are heated or reheated for hours at a time, there is a build-up of a toxin linked to heart disease and neurological disorders.
The oxidation process is one of the most important factors in the chemical breakdown of oil. Food particles and the presence of metals such as copper, iron and zinc can also contribute to oil degradation.
Some oils better for frying
Certain oils seem to be more suitable for frying than others.
According to the European Food Information Council (EUFIC), oils containing mostly unsaturated fatty acids degrade more rapidly than more saturated oils when heated. The higher the unsaturated fatty acid content of the oil (that is, oils that are poly-unsaturated), the less suitable it is for cooking at high temperatures.
But olive oil, for example, is fairly stable at high temperatures, thanks to its high mono-unsaturated fatty acid content. "Olive oil can be used for frying purposes as long as it is well refined. If it isn't, it can easily form unpleasant fumes during the frying process," Kock says.
Canola oil is another good, safe mono-unsaturated oil.
While the type of oil can make a difference to the formation of breakdown products, proper oil management remains the key to preventing degradation, according to the Food Advisory Consumer Service of South Africa. Even the more stable oils can become dangerous when used repeatedly.
Tips for safe oil use
A simple indicator that oil may have been abused is its colour. Fresh oil has a clear, light yellowish colour; old, used oil has a dark brownish colour; and overused oil, besides being blackish, also has a rancid taste.
In order to safeguard yourself against abused oils, keep the following in mind:
buy the best quality oil you can afford and only buy from reputable suppliers;
use only stainless-steel frying vessels;
maintain a frying temperature of 180 degrees Celsius or below;
soak raw chips in water and then dry them before frying to remove excess moisture, thus delaying the oxidation of oil;
filter oil after use to remove small pieces of food;
store oil in a closed container in a cool, dry place;
throw away oil if it is dark brown, tastes or smells unpleasant, or is stringy.
(Carine van Rooyen, Health24, updated May 2008)
Sources:
European Food Information Council (www.eufic.org)
Fact Sheet compiled by Nando's South Africa
Food Advisory Consumer Service of South Africa (FACS)
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