Crude oil contains desirable and undesirable compounds. Desirable compounds include triacylglycerides (TAGs) (neutral lipids) and health beneficial compounds such as tocopherols and phytosterols. Undesirable compounds or impurities consist of non-triacylglycerol components such as free fatty acids (FFA), partial acylglycerols, phosphatides, metallic compounds, pigments, oxidation products, pesticides, glycolipids, hydrocarbons, sterols, waxes, etc.
Impurities can negatively influence the taste and smell of the oil as well as its appearance, thus reducing consumer acceptance and marketability. They can also limit the use of the oil as the objectionable components interfere with further processing and cause the oil to darken, foam, smoke, precipitate, and develop off-flavours. In addition, impurities such as pesticides, herbicides and heavy metals are health hazards.
Hence these must be partially or totally removed in order to become acceptable for human consumption. Crude oils are therefore submitted to several treatments, the objective being to remove the objectionable minor components with the least possible damage to the oil fraction (trans fatty acids, polymeric and oxidized triacylglycerols, etc.) and minimal losses of desirable constituents. Hence refining produces edible oil with characteristics that consumers desire such as bland flavour and odour, clear appearance, light colour, stability to oxidation and suitability for frying
Physical Refining and Chemical Refining
The two major purification-processing methods are chemical refining and physical refining. A flowchart showing both processing routes is as below.
The difference in both routes lies in the method in which the FFAs are removed. In chemical refining, the free fatty acids and most of the phosphatides are removed during alkali neutralization; during physical refining, the free fatty acids are distilled during deodorization.
The traditional edible oil processing system consists of caustic neutralization, bleaching, and deodorization. Caustic neutralization of vegetable oils with high phosphatide contents delivers a soapstock that is a mixture of sodium salts of fatty acids, neutral oil, water, unused caustic, and other compounds resulting from the reactions of the caustic with various impurities in the oil. Disposal of this soapstock or the waste streams from soapstock processing systems has become increasingly more expensive. A second problem associated with chemical neutralization is the loss of neutral oil, which reduces the overall yield from the crude oil. Elimination of the caustic refining step is economically attractive, but it means that degumming or some other pretreatment process or system must assume all the functions of the alkali refining process, except for FFA removal.
Physical refining can remove the FFA, as well as the unsaponifiable and other impurities, by steam stripping, thus eliminating the production of soapstock and keeping neutral oil loss to a minimum. However, degumming and pretreatment of the oil are still required to remove those impurities that darken or otherwise cause a poor-quality product when heated to the temperatures required for steam distillation. Crude oil pretreatment is normally a two-step operation — the addition of a chemical is required to remove any trace quantities of gums remaining after water degumming and bleaching. Following pretreatment, all the FFA and any remaining trace impurities are removed by steam distillation in a single unit. Soapstock acidulation is eliminated with physical refining, and a higher-grade distilled fatty acid is recovered directly from the oil without major pollution problems
The refining method of choice is determined by the characteristics of the individual crude fats and oils.
- Phosphatides and Free Fatty Acid (FFA) content.
Low-phosphatide and relatively high FFA crude oils, such as palm, palm kernel, and coconut, are almost always physically refined.
- Amount of undesirable products in the oil.
A wider range of undesirable products are better removed by an alkali treatment. Some oils, such as cottonseed, cannot be physically refined; an alkali treatment is required to remove the gossypol pigment. This pigment is sensitive to heat and oxidation and forms colour compounds that are difficult to remove from the oil except by reaction with caustic soda.
- Local environmental concerns
Chemical refining generates soap stock, a by-product of the reaction of FFA with alkali. Soap stock is normally split using a strong acid to yield acid oil. In this process, waste water is generated which requires appropriate treatment.
A comparison of advantages and disadvantages as shown below.
Advantages of physical refining lie in higher overall yield, use of less chemicals, and production of less effluent.
Chemical refining is still the most widely applied process for soft oils with low free fatty acid (FFA) content and high phosphorus levels (soybean oil, rapeseed oil, sunflower oil etc.) due to its tolerance towards crude oil quality and it usually gives a good refined oil quality.
Physical refining was developed for high(er) FFA oils and low phosphorus level (such as palm oil) for which chemical refining is not economically attractive. Physical refining results in more easily valorised side products (e.g. deodoriser distillate), but generally requires better quality crude oil. It is therefore more suitable for integrated crushing–refining plants with better control over the incoming crude oil quality.
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