Degumming is necessary because crude oils contain associated substances that encourage hydrolytic and oxidative lipolysis.
During degumming, proteins and phospholipids (such as lecithin) are precipitated by hydrolysis. The process is based upon flocculation or coagulation of dissolved or suspended mucilaginous substances, which are subsequently separated from the oil. The mucilage swells, increasing the specific gravity; these components can then be removed by filtration or centrifugation.
Deacidification (neutralisation with alkali)
There is always a risk of lipolysis where seed and crude oil are kept in storage. Enzymatic, microbial, hydrolytic-chemical and auto-oxidative triglyceride decomposition leads to the formation of free fatty acids which, depending on the amount involved and on composition, give the oil unwanted properties. The process employed most widely nowadays is neutralisation with alkali hydroxides, with formation of soaps and water.
R-COOH + NaOH → R-COONa + H2O
Under normal pressure, at 60 to 85°C, the reaction proceeds completely to the right. The soaps thus formed can be broken down further under pressure. It is, however, more usual to use acids to break soaps down.
The neutralisation reaction may be performed in batches, semi-continuously or continuously. In each case, warm oil is brought into contact with alkaline solution (nearly always sodium hydroxide). Aqueous soap solution is then separated, re-washed and dried. The composition of oils derived from fatty acids of differing molecular weights varies from one oil to the next. This means that the amount of sodium hydroxide required for de-acidification will also vary.
In order to ensure the fullest possible neutralisation, a surfeit of alkali must be added. It helps if there is particularly close contact between oil and alkali (this can be achieved by stirring the mixture) as this keeps any surplus of alkali down to a minimum, but there is the attendant risk that emulsions may form. It is difficult to quantify this risk, which can result in serious problems in the workplace. Therefore, it often makes more sense to increase the amount of alkaline solution used. Once the soap has been separated out, the preparation is washed in soap-free water and dried under vacuum conditions.
By their very nature, oils and fats contain pigments, chiefly carotenes or, less commonly, chlorophylls. A substantial proportion of these pigments is removed during degumming and de-acidification. Low-volatility associated substances (chlorophylls and high-molecular weight hydrocarbons) can only be removed by bleaching with fuller's earth or by absorption on activated charcoal. The oil is heated to 90°-120°C and the necessary quantity of fuller's earth is stirred in. Once it has had time to act (depending on the oil, this may take from a few minutes to half an hour) the bleaching agent is separated from the oil. The standard method for doing this is filtration (filter plate presses); a new method involves continuous filtration. The fuller's earth contains up to 30% oil, which is recovered by extraction.
To some extent, most oils have a characteristic odour and taste. In addition, decomposition products can also form whilst the seed or oil is being stored and these are not welcome in edible oils. Even after the bleaching process, the oil still contains these highly-volatile associated substances, which may be accompanied by pesticides and extraction agent residue. These can be removed by steaming (deodorisation) which, in principle, amounts to steam distillation in vacuo.
The oil is heated to 180°-250°C in a vacuum vessel (pressure 5-15 Torr) and steam is blown in through jets. Solid fats need to be steamed for approx. 2 hours, other kinds of oil for approx. 3-4 hours. Under these conditions, all volatile substances are borne away in the steam. The oil is then cooled rapidly, pumped into a storage tank and stored under an inert gas.
As an alternative to the chemical refining process described above, the product may be refined physically. Free fatty acid is not neutralised with alkali, but distilled off. Steaming is conducted under even harsher conditions (270°-300°C, pressure < 5 Torr). However, steaming should not be too prolonged, as this may, first and foremost, result in decomposition of many of the polyunsaturated fatty acids.