The most important thing to remember about vegetable oils is that they are unstable and far more prone to turning rancid when exposed to oxygen, light – far more so than saturated animal fats. For this reason liquid vegetable oils should always be stored in dark bottles, away from sunlight in a cool spot and are best consumed unheated in salad dressings or mayonnaise-style sauces as they traditionally always were. Vegetable oils are best used for gentle sautéing rather than high-heat frying because of their propensity to turn rancid.
One key distinguishing feature between traditional vegetable oils and fake, new vegetable oils is that the former derive from the fruit of the plant (such as the olive or the coconut) whilst the latter derives from the seeds of the plant (such as sunflower-seed, rape-seed, seeds of the soya-bean plant, cotton-seed, grape-seed). Most seed oils (though not all) are fake foods and are best avoided. (For a complete list of real food and the fakes see below).
Vegetable oils are referred to either as polyunsaturated or monounsaturated for the simple reason that they contain, as a percentage, more polyunsaturated and monounsaturated fats than animal fats, with the exception of coconut and palm oil both of which are highly saturated. Polyunsaturated and monounsaturated fats are liquid at room temperature. Saturated vegetable oils are solid at room temperature.
Most seed and nut oils, being hard to express, would rarely have formed a major part of the traditional European diet. The first expressed seed oil to become wide-spread in northern, eastern and central Europe was sunflower oil in the mid-nineteenth century following industrialisation. It is interesting to note that although sun-flowers derive from Mexico there was no tradition among the Mayan or Inca populations of expressing sunflower seeds to extract the oil for cooking.
The practice of expressing sun-flower oil became common-place following the invention of high-speed mills powered by fossil fuels and which were capable of expressing enough oil from the sunflower seeds. From there the practice of expressing sunflower oil spread to other parts of the world most notably Europe where by the mid-twentieth century it was used extensively in domestic kitchens and by the industrial cook as a cheap alternative to butter, lard or drippings for high-heat frying.
In the mid-twentieth century industry went one step further and began altering the genetic make-up of various plants the oils of which were hitherto considered too toxic for human consumption. Most notable amongst these are rape-seed oils (known as Canola in North America) and soya bean oils. In their natural state these plants contain toxins that the human body is unable to tolerate, which is probably why they never formed a part of traditional food and cooking. Although some of the toxic elements of these plants will have been removed thanks to contrived bio-engineering, the consumption of these new fake oils, as we will see, leads to other challenges for the digestive system.
In the case of rape-seed oil/canola this oil was never consumed in most European or North American countries for the simple reason people knew it to be toxic. In its natural state rape-seed oil contains large doses of erucic acid – a long fatty chain humans are unable to digest. In the 1960’s Canadian scientists managed to genetically modify the rape-seed plant so that modern varieties – although they look the same – do not contain the toxic erucic acid. The rape-seed which is now grown and harvested is thus a genetically modified plant that dates back no further than the 1960’s. Although modern plant varieties of rape no longer contain erucic acid they do have a disproportionately high level of omega-6 to omega-3 fats, are prone to turning rancid when extracted with high speed mills and are highly processed in order to remove bitter, unpalatable flavours.