Oils are edible fatty substances in liquid form. They can be derived from a number of plants, seeds, cereals, fruit and nuts. All oils are an extremely concentrated form of energy (kilocalories) and they contain different degrees of saturated, mono-unsaturated and poly-unsaturated fatty acids.
Oils are extracted either by simple crushing or pressing, or by further processing, usually with heat treatment. Virgin oils are those obtained from cold, usually the first, pressing, and are sold unrefined. The more pressings an oil goes through, the more the oil needs to be refined to deodorise, clean and filter it.
A great deal of oil on sale is intensely refined and processed with chemicals, heat and filtration to remove impurities and stabilise the oil. The result is often cheap, flavourless oil with many of the beneficial qualities stripped out.
Try not to buy too much oil at a time. Most oils, apart from highly refined ones, do not keep well beyond the sell-by date. Once opened and exposed to heat and light, they will deteriorate and soon turn rancid. Store oil in a cool, dark, dry place. However, do not keep oil in the fridge, as it is likely to solidify if it gets too cold. Never freeze oil.
Different oils suit different uses. On the whole, mildly flavoured oils with a high smoking point are suitable for frying and cooking, but more strongly flavoured oils such as extra virgin olive oil are better used for flavouring or in dressings. However, it’s also a matter of taste: experiment to find your favourites. Cooking oils are stable at high temperatures, although their burning point will vary depending on the oil. However, oil should never be re-used more than two or three times because of potential adverse chemical reactions. Nor should it be re-used for cooking different types of food as it will transfer off-flavours to the dish.