Two processes have been developed for the refining of edible oils and fats, that is physical refining and chemical refining; the decision which process to use depends on the types and qualities of the crude oil to be processed. The names physical and chemical refining come from the process technology used to remove the free fatty acids (FFA) that are responsible for the oil acidity. Physical refining is a process making use of the lower boiling point of the FFA compared to the boiling point of the triglyceride oil. In chemical refining, an alkali is used to neutralise the FFA.
Chemical refining of edible oil is the traditional method used in past centuries. The main purpose of chemical refining is to saponify the FFA by an alkaline solution and dilute the resulting soaps in a water phase. These soaps are removed by separators. For small-scale batch processes static separation is used but for continuous processing and large-scale processes, centrifugal separation is used. The neutral oils are subsequently bleached and deodorised. This chemical refining can be used forreliably refining virtually all crude oils, including oils of low quality, with the exception of castor oil.
In addition to the removal of FFA other undesirable non-glyceride materials are also removed. These are mainly:
· Phospholipids (gums)
· Oxidized products
· Metal ions (e.g. iron, copper)
· Colour pigments (e.g. gossypol)
· Insoluble impurities (e.g. meal fines)
The process steps in chemical refining of edible oil can be adapted in accordance with the crude oil quality to give a good final oil quality with acceptable oil losses.
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