Degumming is necessary because crude oils contain associated substances that favor hydrolytic and oxidative lipolysis during storage. During degumming, proteins and phospholipids (such as lecithin) are precipitated by hydrolysis.
The process is based on the flocculation or coagulation of dissolved or suspended mucilaginous substances, which are subsequently separated from the oil. The mucilagal swells, increasing the specific gravity. Subsequently these components can be removed by filtration or centrifugation.
There is always a risk of lipolysis when the seed and the crude oil are stored. An enzymatic, microbial, hydrolytic-chemical and auto-oxidative triglyceride decomposition leads to the formation of free fatty acids which, depending on the quantity involved and the composition, give the oil undesired characteristics. The most extensive process currently used is the neutralization with alkali hydroxides, in the formation of soaps and water.
R-COOH + NaOH = R-COONa + H2O
Under normal pressure, from 60 to 85 ° C, the reaction proceeds fully to the right. The soaps formed in this way can be neutralized under pressure. Therefore, it is more usual to use acids to neutralize the soaps.
The neutralization reaction can be carried out in batches, semi-continuously or continuously. In each case,
the hot oil comes in contact with the alkaline solution (almost always sodium hydroxide). Subsequently, the aqueous soap solution is separated, mixed and dried. The composition of oils derived from fatty acids of different molecular weights varies from one oil to another. This means that the amount of sodium hydroxide required for neutralization will also vary.
To ensure the most complete neutralization possible, an excess of alkali should be added. This helps if there is particularly close contact between oil and alkali (this can be achieved by stirring the mixture) since this keeps any excess alkali to a minimum, but there is a risk that emulsions may form. It is difficult to quantify this risk, which can lead to serious problems in the workplace. Therefore, it often makes more sense to increase the amount of alkaline solution used. Once the soap has separated, the preparation is washed in soap and water and dried under vacuum.
With reference to their nature, oils and fats contain pigments, mainly carotenoids or, less commonly, chlorophyll. A substantial proportion of these pigments is eliminated during degumming and deacidification. Substances associated with low volatility (chlorophyll and high molecular weight hydrocarbons) can only be eliminated by bleaching with a fuller’s earth or by the absorption of active charcoal. The oil is heated to 90-120 ° C and the necessary amount of fuller’s earth is stirred inside. Once you have had time to act (depending on the oil, it can vary from a few minutes to half an hour) the bleaching agent is separated from the oil. The standard method to do this is filtration (filter press); A new method involves continuous filtration. The fuller’s earth contains up to 30% oil, which is recovered by extraction.
To a certain extent, most oils have a characteristic smell and taste. In addition, during the storage of the seed or oil, undesired decomposition products may be formed in edible oils. Even after the whitening procedure, the oil still contains these substances associated with high volatility, which may be accompanied by pesticides and the residue of the extraction agent. These can be eliminated by a process of vaporization (deodorization) which, in principle, means a vacuum distillation.
The oil is heated to 180-250ºC in an empty container (pressure 5-15 Torr) and the steam is expelled through some jets. Solid fats need to be steamed for about 2 hours, and other kinds of oil for 3-4 hours. Under these conditions, all volatile substances are transported back to the vapor. The oil is then rapidly cooled, pumped into a storage tank and stored under inert gas.
As an alternative to the chemical refining process described above, the product can be physically refined. The free fatty acid is not neutralized with alkali, but it is distilled. The vaporization process is carried out under harder conditions (270-300ºC, pressure <5 Torr). However, steam can not be very long, as it can lead to the breakdown of many of the polyunsaturated fatty acids.