It is essential to use only first cold pressed oil, decanted and filtered by physical processes and non-chemical and contained in glass bottles, dark if possible. It is also important that the temperature at the time of pressing does not exceed 30°. Only with this type of extraction the “purity” of the oil guaranteed and the presence of more or less variable amounts of polyunsaturated fatty acids, also referred to as “vitamin F”.
It is also important that these oils are derived from seeds, stored in dark glass bottles and refrigerated after opening. It is imperative at this point to make a digression on vegetable fats and the method used for oils extraction to understand why it is essential to use only oil from the first pressing.
In recent decades, to increase the yield, “hot” oils have started to be extracted (using water vapor at 160-200°) With this method, the oils are colored and have an unpleasant odor and therefore there is need for further refining intervention and deodorization. In addition, often, there is another cold pressing, also using solvents (chrome), to obtain a maximum productivity.
With this process the oils have more ingestible substances (waxes for example) for which further refining is needed – and there is no assurance of the absence of solvent in the finished product. Additionally heating the oil destroys vitamin E, a vitamin that is naturally present in oils with a first cold pressing; a vitamin that is essential for its antioxidant action against polyunsaturated fatty acids, which are therefore more susceptible to peroxidation reactions.
To overcome when has been stated above, the oils then must undergo other processes: addition of synthetic vitamin E and a hydrogenation process. With the latter process, there is the transformation of a portion of molecules from the cis-cis form into the cis-trans form (as already seen for the preparation of margarine), a form more stable but biologically inactive.
At the end of all these processes the oil turns out to be “dead” biologically. On the contrary the first cold pressed oil is “alive” biologically, rich in polyunsaturated fatty acids, or vitamin F. Additionally to avoid easy oxidation reactions, facilitated by light, and thus resulting in the ranciding of the oil itself, it is useful to keep the oil in “dark” containers and once opened they should be stored preferably in the refrigerator.
Olive Oil is more stable at high temperatures and therefore can be used for cooking, frying as well as raw, while seed oils can only be used raw or in addition to baked goods such as bread and cakes.
Proper nutrition should provide an equal amount of saturated fat, monounsaturated and polyunsaturated.
However, olive oil is deficient in polyunsaturated fats (it contains a quantity of monounsaturated fats 7 times higher than polyunsaturated fats) and consequently also of essential fats. Those who use only olive oil should supplement their diet with oily fish and nuts (especially walnuts), otherwise they cannot hope to intake an adequate amount of essential fats without the simultaneous use of seed oils. Same thing regarding vitamin E, contained in much greater amounts in many seed oils.
The problem is that the availability of unrefined and cold-pressed seed oils is very limited, nearly all seed oils on the market are refined, while there is an equal division between the two products regarding olive oil, this is why the use of olive oil is preferred, but the great advantage of the latter is only for frying purposes.