Fluid fats (oils) and solid fats (butters) extracted from oil-raw materials ( seeds, nuts, fruit pulps) are not always usable as crude. They have to undergo a purification which can assume different grades of accuracy depending on what use the fats are destined to.Usually, after mechanical pressing without use of any solvents and chemical reagents, oils undergo depuration and refining processes which enable to obtain a neutral, yellow oil, possibly colourless, odourless and tasteless starting from a crude oil originally with high acidity, coloured, odorous and sometime with unpleasant smell and containing many extra substances to remove.

It is now European and worldwide common within the lipids technology, the classification of the fats purification procedures according to two denominations related to two treatment phases, the depuration and the refining.”Conventional” term comes from this consolidated technology by which it is possible to obtain oils for cosmetics or pharma grade (recognized by the European and international standard of certification – Ph Eur, USP), top quality oils, also food grade depending on the industrial processes utilized (phisycal refining).