The purification of edible oils and fats

Originally, oils were not refined but with the introduction of solvent extraction, refining became necessary. Crude cottonseed oil was refined by treating the oil with caustic soda and the same process was used for all other oils that needed refining. The subsequent introduction of centrifugal separators converted the original batch process into a continuous process. Degumming was introduced to obtain lecithin but limited to soya bean oil. Physical refining was introduced for high acidity oils like palm oil after the oil had been degummed to low residual phosphorus levels in the dry degumming process, in which the oil is first of all treated with an acid and then with bleaching earth. In Europe, further degumming processes were developed that allowed seed oil to be physically refined and later phospholipase enzymes were introduced to reduce oil retention by the gums and improve oil yield. Given these various oil purification processes, the refiner must decide which process to use for which oil in which circumstances. The paper provides a survey of what to do and when. It also discusses several topics that require further investigation and development.     

Effects of Degumming and Bleaching on 3-MCPD Esters Formation During Physical Refining

Crude palm oil (CPO) was physically refined in a 200-kg batch pilot refining plant. A study of the possible role of degumming and bleaching steps in the refining process for a possible critical role in the formation of 3-chloropropane-1,2-diol (3-MCPD) esters was evaluated. For the degumming step, different percentages of phosphoric acid (0.02–0.1%) as well as water degumming (2.0%) were carried out. Six different types of bleaching clays, mainly natural and acid activated clays were used for bleaching process at a fixed dose of 1.0%. Deodorization of the bleached oils was performed at 260 °C for 90 min. Analyses showed that 3-MCPD esters were not detected in the CPO. Phosphoric acid degumming (0.1%) in combination with acid activated clays produced the highest levels (3.89 ppm) of 3-MCPD esters in the refined (RBD) oil. The esters were at the lowest levels (0.25 ppm) when the oil was water degummed and bleached with natural bleaching clays. However, the refined oil qualities were slightly compromised. Good correlation of 0.9759 and 0.9351 was obtained when concentration of the esters was plotted against acidity of the bleaching earths for the respective acid and water degumming processes. The findings revealed the contribution of acidic conditions on the higher formation of 3-MCPD esters. In order to lower the esters formation, it is important to reduce acid dosage based on the crude oil qualities or to find alternatives to acid degumming process. Neutralization of the acidity prior to deodorization was effective in reducing the formation of 3-MCPD esters.     

Effect of processing on the composition and oxidative stability of coconut oil

The effect of various processing procedures on the composition and oxidative stability of coconut oil has been studied. The crude oil is relatively stable but major reductions in oxidative stability occur during the bleaching of oil degummed with phosphoric acid; during alkali refining; during the deodorization of oil degummed with citric acid and bleached; and during the deodorization of oil processed with a combined phosphoric acid degumming and bleaching operation. The reasons for the loss of oxidative stability during processing are discussed with reference to changes in the composition of the oil. Residual traces of citric acid or phosphoric acid play an important role in stabilizing processed oils. The tocopherol content is also important, although no additional stabilization of the oil occurs on adding levels of tocopherol above those present naturally in the crude oil. A combined phosphoric acid degumming and bleaching process leads to smaller losses of tocopherols than sequential treatments.     

Pretreatment of corn oil for physical refining

Crude corn oil that contained 380 ppm of phosphorus and 5% of free fatty acids was degummed, bleached, and winterized for physical refining. The pretreatment and the steam-refining conditions were studied in pilot plant scale (2 kg/batch). The efficiency of wet degumming and of the total degumming processes, at different temperatures, was evaluated. TriSyl silica was tested as an auxiliary agent in the reduction of the phosphorus content before bleaching. The experimental conditions of the physical refining were: temperature at 240 or 250°C; 8 to 18 mbar vacuum, and distillation time varying from 1 to 3 h. Degumming at 10 or 30°C resulted in the removal of more phosphorus than at 70°C. Water degumming was more efficient than the processes of total degumming or acid degumming. Corn oil, degummed at 10 or 30°C, after bleaching passed the cold test, irrespective of the degumming agent used. Degumming and winterization took place simultaneously at these temperatures. The pretreatment was able to reduce the phosphorus content to less than 5 ppm. The amount of bleaching earth was reduced by carrying out dry degumming or by using silica before bleaching. Corn oil acidity, after physical refining, varied from 0.49 to 1.87%, depending on the residence time. Contrary to alkali refining, physical refining did not promote color removal due to the fixation of pigments present in the crude corn oil.     

Zu den Veränderungen von Rapsöl und Leinöl während der Verarbeitung

Changes of rapeseed and linseed oil during processing During processing of crude oil in a large oil mill, three samples each of rapeseed and linseed were investigated at each processing stage, i.e. press oil, solvent-extracted oil, mixed oil, and degummed/caustic refined oil. In the case of rapeseed also bleached and desodorized oils (230°C; 3.0 mbar for 2 h) were investigated. Rapeseed and linseed oil showing the typical major fatty acids contained less than 1% trans-isomeric fatty acids (trans fatty acids = TFA). Linseed oil had a similar TFA-concentration as rapeseed oil, and the concentrations did not change during the processing stages up to degummed/caustic refined oil, and were also unchanged in the bleached rapeseed oil. Desodorization of rapeseed oil, however, trebled the TFA concentration to 0.58%. The detected tocopherol patterns were typical of rapeseed and linseed oils. There was no difference between mixed oil and degummed/caustic refined oil in the total concentration of tocopherols. Neither had bleaching any effect. Rapeseed oil desodorization diminished total tocopherol concentration by 12% from 740 mg/kg to 650 mg/kg. Due to degumming/caustic refining the phosphorus concentration of both oils decreased to less than a tenth compared to mixed oil. Other elements determined in degummed/caustic refined rapeseed oil were not detectable (manganese < 0.02 mg/kg, iron < 0.4 mg/kg, copper < 0.02 mg/kg, lead < 10 μg/kg) or only as traces zink 0.1 mg/kg, cadmium 2 μg/kg). In linseed oil, which initially showed a higher trace compounds concentration, a significant decrease was found by degumming/caustic refining. Iron could not be detected. There were traces of zinc, manganese, copper, lead, and cadmium. There was no difference between the acid values of rapeseed and linseed crude oil. Acid value decreased drastically already during the degumming/caustic refining stage. The crude linseed oils had a higher peroxide value, anisidine value and diene value than the corresponding crude rapeseed oils. With peroxide values of ≤ 0.1 mEq O₂/kg found in almost all investigated rapeseed oils, no effect of refining could be detected. The anisidine value showed an increase after bleaching. Desodorization trebled the diene value.     

Integrated pollution control in seed oil refining

In edible oil refining, the various processes in current use lead to different by-products and/or waste products and they may also cause some form of pollution. The processes are reviewed in this paper and current or possible means of disposal of these by-products/waste products are discussed to highlight the areas requiring most attention. These areas turn out to be gum disposal when the degumming operation is carried out at a stand-alone refinery, and soapstock effluent resulting from the alkali refining process. Other waste products and pollution sources are found to be unimportant or manageable. Accordingly, a major step forward in pollution abatement in seed oil refining can be achieved by making two changes. The first one entails carrying out the degumming operation at the oil mill rather than at the refinery. This should be done in such a way that the degummed oil is amenable to physical refining. The acid refining process is recommended for this degumming step and consequently, acid refined oil with appropriate quality guarantees will then become the article of trade. The second one involves a switch from alkali refining crude or waterdegummed oil to the physical refining of acid refined oil. For this latter step, a counter-current process is recommended because of its low stripping steam requirement. Dry condensation of the distillate will further alleviate pollution problems associated with deodorization and physical refining. Finally, some processes, that may contribute to pollution control but that still require development, are mentioned.     

The Total Degumming Process – Theory and Industrial Application in Refining and Hydrogenation

The degumming of vegetable oils prior to physical refining is a crucial preliminary step. The degumming process is not only largely responsible for the quality of the final product, but it also determines the amount of bleaching earth to be used, which has a substantial effect on the yield improvement which can be attained by this route. Investigations show clearly that iron, as a pro-oxidant, strongly influences the stability of refined oils, and that oil, degummed before bleaching and physical refining, may contain a maximum of 0.2 ppm Fe, if it is to yield a stable product. The Total Degumming Process has been developed on the basis of these findings, to make it possible to degum oil to a residual Fe-level below 0.2 ppm and a residual phosphorus content below 10 ppm. The principles and industrial application of the process have been considered. The results of industrial production using different raw materials of various qualities have been used to make a comparison between the conventional refining process (neutralization – bleaching – deodorization) and the Total Degumming Process in combination with physical refining. The combination of the Total Degumming Process and a simplified caustic refining process, and the use of Totally Degummed Oil for hydrogenation have also been considered.     

Processing of prepressed solvent extracted oil from indian soybean-refining loss studies

Degumming of soybean oil considerably reduces the free fatty acid (FFA) content of the oil. Lowering of refining loss when oil is degummed prior to caustic treatment is attributed to this reduction in FFA and the excess carry over of neutral oil, because gums are excellent emulsifiers. Pretreatment of solvent extracted soybean oil with phosphoric acid followed by water degumming and caustic refining results in a lower refining loss and considerable reduction in caustic requirement in comparison to the conventional process based on water degumming followed by gum conditioning with phosphoric acid and caustic refining. The process gives sharp and fast separation of gums from the oil.     

Physical refining of edible oils

Crude oils obtained by oilseed processing have to be refined before the consumption in order to remove undesirable accompanying substances. The traditional alkali refining is often replaced by physical refining in which the use of chemicals is reduced. The most widely used method is steam refining. The crude oil quality is very important in order to obtain high quality refined oil. Furthermore, the oil should be efficiently degummed to remove phospholipids as well as heavy metals and bleached to remove pigments. The most important step consists of the application of superheated steam under low pressure and at temperatures higher than 220 °C. Both free fatty acids and objectionable volatiles, formed by cleavage of lipid oxidation products, are removed. A disadvantage is the partial loss of tocopherols. Side reactions, particularly isomerization of polyunsaturated fatty acids, should be minimized. The quality of physically refined oil is close to that of alkali refined oils, but losses of neutral oil are lower and the environment is less polluted. Among other methods of physical refining the application of selective membranes is promising.     

Continuous ultrasonic degumming of crude soybean oil

Ultrasonic energy has been applied to continuous degumming for the efficient removal of phospholipids from crude soybean oil. The crude oil and water (2.0% by weight) were pumped through an ultrasonic processing cell, oil and hydrated gums were separated by centrifugation, and the recovered oil was vacuum bleached. The degummed and bleached oil had a residual phosphorus content of less than 10 ppm and was subsequently deacidified-deodorized in all-glass laboratory deodorization equipment. Odor and flavor evaluation indicated that the salad oil produced by the process of ultrasonic degumming/deodorization-deacidification was equivalent in quality and stability to a conventionally processed salad oil.     

Steam-Refined soybean oil: I. effect of refining and degumming methods on oil quality

A lot of commercially extracted crude soybean oil was water degummed with and without a phosphoric acid pretreatment. The degummed oils were bleached and then deacidified-deodorized in a single step to yield physically (steam) refined soybean salad oils. Their flavor and oxidative stability were compared to caustic-refined oils given otherwise identical processing treatments. Physically refined oils without a phosphoric acid pretreatment were of poor initial quality compared to those given the phosphoric acid pretreatment. However, caustic- and steam-refined oils processed with the phosphoric pretreatment were of comparable quality. Presented in part at the AOCS-AACC Symposium, Current Concepts of Food Ingredients, Chicago, March 1977.     

Experience of Prerefining of Vegetable Oils with Acids

Prerefining of vegetable oils with acids serves general purposes. One is to remove impurities like phosphatides etc. from the crude oils to such a degree that the oil can then be physically refined. Another purpose is to facilitate subsequent alkali refining and to reduce pollution in effluent from alkali refining. After a review of some earlier work results from tests in laboratory scale on soya oil, rapeseed oil and linseed oil will be presented and discussed. Finally, an industrial procedure for acid refining, called “special degumming” will be described.     

Effects of processing steps on the contents of minor compounds and oxidation of soybean oil

The processes of degumming, alkali refining, bleaching and deodorization removed 99.8% phospholipids, 90.7% iron, 100% chlorophyll, 97.3% free fatty acids and 31.8% tocopherols from crude soybean oil. The correlation coefficient between the removals of phosphorus and iron in soybean oil during processing was r = 0.99. The relative ratios of α-, β -, γ- and δ-tocopherols in crude oil, degummed oil, refined oil, bleached oil and deodorized soybean oil were almost constant, γ- and δ -tocopherols represented more than 94% of tocopherols in soybean oil. The order of oxidation stability of oil is crude > deodorized > degummed > refined > bleached oil.     

Oxidative stability of high-fatty acid rice bran oil at different stages of refining

The contents of natural antioxidants and the oxidative stability of rice bran oils at different refining steps were determined. Tocopherols and oryzanols were constant in crude and degummed oils but decreased in alkali-refined, bleached and deodorized oils. The process of degumming, alkali-refining, bleaching and deodorization removed 34% of the tocopherols and 51% of the oryzanols. During storage of deodorized oil for 7 wk, 34% of the tocopherols and 19% of the oryzanols were lost. The maximum weight gain, peroxide value and anisidine value were obtained from alkali-refined oil during storage. The order of oxidation stability was crude ≥ degummed > bleached = deodorized > alkali-refined oil.     

Chemical degumming of canola oils

Crude oil produced from 2 varieties of canola (low thioglucoside rapeseed), i.e., Candle and Tower, were chemically degummed using 54 reagents. Phosphorus, iron, calcum and free fatty acid levels, and Lovibond colors were measured. Inorganic and organic acids or their anhydrides gave the best results in terms of phosphorus removal. Phosphoric, nitric and citric acids, and maleic anhydride were used in laboratory refining tests to determine the effects of chemical degumming on the refining process and on refined oil quality. Citric acid and maleic anhydride gave residual phosphorus levels of less than 50 mg/kg, and after refining, resulted in the best quality oil in terms of color, flavor and stability. The chemical degumming agents behaved similarly for expressed, solvent-extracted and blended oils of both varieties tested. One anomalous result was observed: Candle oil was not efficiently degummed by hydrocholoric acid, whereas the Tower oil gave excellent results. The experiments suggest that chemical degumming can significantly improve the quality of crude canola oil, and will lead to improved final products at lower cost to the refiner.     

Effect of refining of crude rice bran oil on the retention of oryzanol in the refined oil

The effect of different processing steps of refining on retention or the availability of oryzanol in refined oil and the oryzanol composition of Indian paddy cultivars and commercial products of the rice bran oil (RBO) industry were investigated. Degumming and dewaxing of crude RBO removed only 1.1 and 5.9% of oryzanol while the alkali treatment removed 93.0 to 94.6% of oryzanol from the original crude oil. Irrespective of the strength of alkali (12 to 20° Be studied), retention of oryzanol in the refined RBO was only 5.4–17.2% for crude oil, 5.9–15.0% for degummed oil, and 7.0 to 9.7% for degummed and dewaxed oil. The oryzanol content of oil extracted from the bran of 18 Indian paddy cultivars ranged from 1.63 to 2.72%, which is the first report of its kind in the literature on oryzanol content. The oryzanol content ranged from 1.1 to 1.74% for physically refined RBO while for alkali-refined oil it was 0.19–0.20%. The oil subjected to physical refining (commercial sample) retained the original amount of oryzanol after refining (1.60 and 1.74%), whereas the chemically refined oil showed a considerably lower amount (0.19%). Thus, the oryzanol, which is lost during the chemical refining process, has been carried into the soapstock. The content of oryzanol of the commercial RBO, soapstock, acid oil, and deodorizer distillate were in the range: 1.7–2.1, 6.3–6.9, 3.3–7.4, and 0.79%, respectively. These results showed that the processing steps—viz., degumming (1.1%), dewaxing (5.9%), physical refining (0%), bleaching and deodorization of the oil—did not affect the content of oryzanol appreciably, while 83–95% of it was lost during alkali refining. The oryzanol composition of crude oil and soapstock as determined by high-performance liquid chromatography indicated 24-methylene cycloartanyl ferulate (30–38%) and campesteryl ferulate (24.4–26.9%) as the major ferulates. The results presented here are probably the first systematic report on oryzanol availability in differently processed RBO, soapstocks, acid oils, and for oils of Indian paddy cultivars.     

Degumming and bleaching ofLesquerella fendleri seed oil

A lesquerella species (Lesquerella fendleri) being investigated as a domestic source of seed oil containing hydroxy fatty acids shows good agronomic properties and is being tested in semi-commercial production.Lesquerella fendleri seeds contain 25% oil, of which 55% is lesquerolic acid (14-hydroxy-cis-11-eicosenoic). Oils produced in pilot-plant quantities by screw press, prepress-solvent extraction and extrusion-solvent extraction processes have been refined in the laboratory by filtering, degumming and bleaching. Two American Oil Chemists’ Society (AOCS) standard bleaching earths and two commercial earths were compared for effectiveness in bleaching these dark, yellow-red, crude lesquerella oils. Free fatty acids (1.3%), iodine value (111), peroxide value (<4 meq/kg), unsaponifiables (1.7%) and hydroxyl value (100) were not significantly affected by degumming and bleaching, but phosphorus levels of 8–85 ppm in the crude oils were reduced to 0.5–1.1 ppm in the degummed and bleached oils. Crude oils had Gardner colors of 14, which were reduced to Gardner 9–11 in the degummed and bleached oil, depending on bleach type and quantity used. AOCS colors in the range of 21–25R 68–71Y were obtained. By including charcoal in the bleaching step, a considerably lighter oil could be obtained (Gardner 7).     

Natural refining of extruded-expelled soybean oils having various fatty acid compositions

Simple, low-capital-investment oil refining techniques, which may also meet the needs of natural or organic food industries, were explored to process extruded-expelled (E-E) soybean oils with various fatty acid compositions. Most settled E-E oils are naturally low in phosphatides (<100 ppm phosphorus) and were easily water degummed to low phosphorus levels (<55 ppm). Free fatty acids were reduced to 0.04% by adsorption with 3% Magnesol^®. Magnesol reduced residual phosphorus contents to negligible levels. This material also adsorbed primary and secondary oil oxidation products. Our adsorption refining procedure was much milder than conventional refining, as indicated by little formation of primary and secondary lipid oxidation products and less loss of tocopherol. The remaining challenge to effective natural refining is the removal of off-flavor components. Our adsorption treatment reduced the natural flavor of soybean oil but flavor was still present, probably too strong for many consumers. Polyunsaturated oils oxidized more easily than did the other types of oils; therefore, precautions should be taken when refining such oils. High-oleic soybean oil, on the other hand, had excellent oxidative stability and better flavor characteristics after degumming and adsorption with Magnesol compared with other oils.     

Membrane degumming and dewaxing of rice bran oil and its refining

A combined degumming-dewaxing batch by filtration through a ceramic membrane followed by earth bleaching and physical or alkali refining was studied for crude rice bran oil. The results were compared with the conventional centrifugal process for gum and wax removal. The characteristics of the refined oils obtained by the two processes were comparable. However, the former process was promising with respect to higher recovery of oil and better recovery of the byproducts gum and wax. Oil content of the mixed gum-wax phase was 7.6–8.1%. The recovery of oil using the membrane technique was always 2–3% higher than the centrifugal process. The membrane process was also found to be more effective and the quality of the final product was acceptable.     

Enzymatic degumming

The oldest enzymatic degumming process (the Lurgi EnzyMax® process) was launched in 1992. It used porcine phospholipase A₂, which has the disadvantages of limited availability and not being kosher/halal. To overcome these disadvantages, various microbial enzymes have been developed; they have different specificities and therefore offer different advantages. Phospholipase C for instance has the advantage that it leads to the formation of diacylglycerols that remain in the oil being degummed. This constitutes a significant yield improvement which also results from the formation of lysophospholipids that retain less oil than their precursors. In the laboratory, a fine dispersion of the aqueous enzyme solution in the oil can be maintained so that the phospholipase enzymes can be made to interact with non‐hydratable phosphatides (NHP) in the oil phase and catalyse their hydrolysis. On an industrial scale, dispersions coalesce before the enzymatic NHP hydrolysis is complete. Accordingly, enzymatic degumming processes that claim NHP‐removal and a low residual phosphorus content in the enzymatically degummed oil are invariably preceded by an acid treatment in which a degumming acid (citric acid) is finely dispersed into the oil to be degummed and made to react with the NHP present in the oil before the enzyme is added. This enzyme then only interacts with the phospholipids present in the water phase. This raises the question whether the yield increase resulting from the use of enzymes should be realised by treating the oil to be degummed or the gums that have already been isolated from the oil during a degumming treatment. Lack of experimental evidence prevents a firm answer to this question but the arguments in favour of treating the gums look more impressive than what can be said in favour of treating the oil. In short: Enzymes do not degum the oil but can be used to de‐oil the gums.