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Protect Your Vegetable Oil

Vegetable oils are one of the best sources of polyunsaturated fatty acids in the diet. The two essential fatty acids – linoleic acid and linolenic acid are found in varying amounts in different vegetable oils. Using vegetable oils on a salad is perhaps one of the healthiest ways to get more long chain polyunsaturated fatty acids (PUFAs), but we often use vegetable oils for frying and deep frying, where we have to be a little more careful if we want to maintain the nutritional value of the oil. Although we should be careful about the total amount of fat in our diet, we do need some fat, particularly PUFAs, and so we should make sure that, when we are cooking, the PUFAs are not destroyed.

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The PUFAs in vegetable oil give it its nutritional value but they also are the cause of its instability. From a chemical point of view, saturated fats are more stable than unsaturated fats, which are more unstable than polyunsaturated fats. The double bonds in polyunsaturated fatty acids between the carbon bonds are sensitive to oxidation. At the high temperatures that occur during frying and deep frying oxidation can become a problem. A chemical reaction can occur between oxygen in the air and the hot fat that causes the formation of very reactive compounds called radicals. The high temperature that is used in frying plus the metal of the frying pan can encourage the formation of radicals. These radicals can in turn attack other fatty acids producing an oxidized fat. Bottles of vegetable oil should be kept out of the sunlight as light can also cause vegetable oils to oxidize slowly. However, oils should not be kept in the refrigerator because the low temperature will cause a separation of the various types of fatty acids in the vegetable oil.

The oxidation of fatty acids changes the chemical properties of the fat; it reduces the nutritional value of the fat, darkens its colour and can cause off–flavours. The best way to avoid oxidized fatty acids from forming is to use fresh oil when frying. In some countries recommendations exist as to how many times an oil can be used in commercial deep frying before having to be replaced. However, surveys have shown that these recommendations are not always followed. It is generally considered that, in the home, using the same oil three times is prudent. Old oil is more likely to be oxidized. Most vegetable oils contain some antioxidants – either natural or added – that will help protect the oil from oxidation.

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