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Refining of edible oils and fatsRefining removes undesirable compounds such as gums, free fatty acids, pigments and undesirable flavour and odour compounds. In general, there are two ways to refine seed oils: physical refining and the more conventional chemical refining:
Conventional chemical refining, as applied to crude oils, for instance includes degumming (removal of phospholipids), neutralisation (removal of free fatty acids), bleaching (decolourisation) and deodorisation. Degumming involves the addition of water to hydrate any gums present, followed by centrifugal separation. Non-hydratable gums are removed by converting them first to a hydratable form using phosphoric or citric acid, followed by the addition of water and centrifugation. The following step is neutralisation in which an aqueous alkali, typically caustic soda or sodium carbonate, is sparged into the oil which has been preheated to around 75–95°C. The alkali reacts with free fatty acids in the oil to form soaps, which are separated by settling or centrifugation. A drying step may be incorporated after the neutralisation to ensure the complete removal of the added water.
The neutralised oil is bleached to remove colouring matter (such as carotenoids) and other minor constituents, such as oxidative degradation products or traces of transition metals.
Bleaching uses activated fuller’s earth with treatments typically in the 90–130°C range for 10-60 minutes. The earth is sucked into the oil under vacuum and is removed by filtration. The bleached oil is steam distilled at low pressure to remove volatile impurities including undesirable odours and flavours. This process, known as deodorisation, takes place in the temperature range 180-270°C and may last from 15 minutes to five hours depending upon the nature of the oil, the quantity, and the type of equipment used. For example, if a batch deodoriser is used it would take between 4.5–5 hours depending on the oil type and quantity.
However, if the same oils were processed in a semi-continuous deodoriser it would take about 15 minutes.
Olive oil and husk (pomace) oil refining is similar to oilseed oil refining.
Physical refining is a simpler process in which the crude oil is degummed and bleached, followed by steam stripping which removes free fatty acids, odours and volatiles in one step. The oil is heated to temperatures of up to 270°C and then refined by flowing it over a series of trays countercurrent to the flow of the stripping steam. The advantages of physical refining are higher yield, lower cost and fewer chemicals used. One of the disavantages compared to chemical neutralisation can be a lower quality of the end product.
There are three other processes which may be carried out during oil refining, depending upon the oil source.
These are ‘winterisation’, which essentially is the removal of wax which would otherwise cause cloudiness; this is carried out on oils such as sunflower. The process involves cooling the oil followed by filtration, using a filter aid to remove wax.
The second process is fractionation; typically this may be carried out on crude or refined oil. This involves completely melting the solid oil, followed by cooling to yield solid and liquid fractions which have different functional properties.
The third process is interesterification; this involves the separation of triglycerides into fatty acids and glycerol followed by recombination. The reaction is carried out using citric or phosphoric acid plus a catalyst, typically sodium methoxide. Interesterification modifies the functional properties of the treated oil and may be carried out after neutralisation or deodorisation.