Physical refining was utilized as early as 1930 as a process for the preneutralization of products with a high initial FFA content. In this case, preneutralization was followed by caustic refining. Later, it was found possible to physically refine lauric oils and tallow if the proper pretreatment was applied before steam distillation. Physical refining became a reality in the 1950s for processing palm oil, which typically contains high FFA and low gum contents. The palm oil process subjected the crude feedstock first to pretreatment and then to acidification.
The pretreatment consisted of a degumming step and an earth bleaching step, which together remove certain non-volatile impurities by filtration. Volatile and thermally unstable components are removed during the conditions of steam distillation under vacuum, which originally gave the process its name of steam refining. However, for vegetable oils, such as soybeans, that contain relatively low levels of FFA and higher amounts of phosphatides, physical refining only recently became a possibility.
The physical refining process consists of several steps and each step removes certain type of impurities as shown in the figure below.
The first step of physical refining is degumming, which consists of the removal of the phosphatides which will interfere with the stability of the oil products in later stage. It is vital to remove the phosphatides content in the crude oil because the presence of this component will impart undesirable flavour and colour, and shortened the shelf life of oil.
Research shows that to obtain good quality and stable oil after refining, the phosphorous content of the crude oil should not exceed 20 ppm, and the phosphorous content of the pre-treated oil immediately prior to physical refining should not exceed 5 ppm. The reduction to these levels brings very low levels of trace metals such as copper and iron.
Complex esters which contain phosphorus, nitrogen bases, sugars, and long-chain fatty acids are classed as phospholipids. The phosphatides in oils are fatty acid esters of glycerol—which, at the same time, are also esters of phosphoric acid. The phosphoric acid may also be linked with a nitrogen base or a sugar, and a cation such as magnesium, calcium, or sodium.
As an industry standard, plants usually measure the elemental Phosphorus (P) in the oil, as an indication of the presence of phospholipids. Elemental phosphorus measurements do not indicate the quantity of each of the common phospholipid species found in the oil, namely phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI) and phosphatidic acid (PA). Nor do they identify other compounds containing phosphorus, including degraded forms of phospholipids (such as lyso-phospholipids)
The affinity for water of the ester group determines the overall affinity for water of the phospholipid: the higher the water affinity, the higher the emulsification power. The relative affinity of phospholipids for water is usually called “hydratability.” Those phospholipids responsible for higher oil losses due to oil emulsion, i.e. PC and PI, are usually designated as hydratable phospholipids (HP), while PA and PE salts of calcium (Ca), magnesium (Mg) and iron (Fe) are usually called non-hydratable phospholipids (NHP).
The presence of phosphatides in many fats and oils is one of the most important reasons why the term “bleaching” is recognized as inadequate when describing the improvement in the quality of a crude oil which is being sought. Mere removal of pigment is not the only requirement; it may not even be the most important.
Phosphatides, as the principal constituent of gums in the crude oil, severely interfere with the efficiency of subsequent process steps if allowed to remain. Thus, in alkali neutralization, phosphatides’ presence causes increased amounts of neutral oil to be emulsified and hence, lost in the soapstock; diminishes the adsorptive action of clay and carbon by adhering to their surfaces; similarly poisons nickel catalyst; darkens the colour of an oil if they become broken down by heat; and can lead to impaired flavour stability. The main responsibility for this last ill-effect, however, is linked with traces of pro-oxidant iron liable to persist in refined oil along with the gummy material rather than phosphatides themselves.
A wide variation exists in the typical phosphatide content of different vegetable oils. For a long time, crude oils—such as olive, palm, palm kernel, babassu, and coconut—were known to contain a very small amount, perhaps a small fraction of 1%, of phosphatides; soybean, maize, sunflower, linseed, rapeseed, and cottonseed hold substantial amounts, best dealt with early in their processing.
Briefly, phosphatidic acid in its undissociated form is soluble in oils but not in water; if it can be caused to dissociate, it hydrates, flocculates to micelles or liquid crystals, and can be encouraged to pass to the aqueous phase by settling or centrifuging. Similarly, when the magnesium or calcium salts, which are insoluble in water, are treated with an acid or cation precipitant to remove the metal, they too will hydrate and can be separated. This process is known as degumming.
Good technical/commercial reasons exist for reducing the phospholipid content early in the processing, in those crude oils where it is relatively high. A summary of the advantages of doing so follows:
- The deposition of sludges, especially when the crude oil is wetted, is a nuisance to storage and transport.
- In alkali refining, emulsification of neutral oil in soapstock and, hence, oil loss are significantly encouraged by phosphatides.
- Phospholipids clog the surface of adsorbent clay or carbon and, therefore, compete with pigments and any other unwanted minor components which are desirable to remove.
- Phospholipids obstruct and poison the surface of a nickel catalyst irreversibly.
- At the temperatures of deodorization and, more surely, at those of physical refining, phospholipid breakdown leads to the darkening of colour and impaired flavour stability.
- Hydrated gums carry with them during their removal a proportion of unwanted minor components like trace metals.
- Subsequent dewaxing, if needed, is all the easier in the absence of gums.
- Degumming of soybean oil provides the principal supply of lecithin, for which an established market exists.
There are generally 3 types of degumming processes
a) Dry Degumming
b) Water Degumming
c) Acid Degumming
d) Enzymatic Degumming
When phosphatide content is low, as is the case for palm oil, palm kernel, coconut, and tallow, dry degumming is the common way. Typical composition of the main components of Malaysian crude palm oil (CPO) is shown below. Note the phosphorus content in the oil. The phosphorus content of CPO is quite variable both in amount and in properties. Whereas normally in the range of 10 to 20 ppm, phosphatide values exceeding 30 ppm have been observed for CPO of high acidity.
In dry degumming, the oil is treated with an acid (phosphoric or citric) to decompose the metal ion/ phosphatide complexes and is then mixed with bleaching earth. The acid disrupts non-hydratable phosphatides by decomposing magnesium and calcium complexes; it coagulates the phosphatides and sequestrates iron and copper, which are all further adsorbed on bleaching earth. In short the acid dissociates the non-hydratable phosphatides into phosphatidic acid and calcium or magnesium bi-phosphate salt.
The earth containing the degumming acid, phosphatides, pigments, and other impurities is then removed by filtration. Its main advantage is that it does not generate an aqueous effluent, apart from the water involved in the vacuum system. Seed oils that have been water or acid degummed may also be dry-degummed to ensure a low phosphorus oil to steam distillation.
Phosphoric acid is used only in small quantity (0.1-0.3%) of a 85% solution because excess is not adsorbed onto the earth causing problem of hydrolysis or causing the phosphorylation of mono or diglycerides during physical refining. Excess cannot be eliminated with calcium carbonate as it reverses the reaction from phosphatidic acid to non-hydratable phosphatides.
The dry degumming process uses standard bleaching equipment. Acid, usually 85% phosphoric, is dispersed in 80 to 100°C oil at 0.05 to 1.2% of the oil. Excessive amount of phosphoric acid must be avoided as it may promote the splitting of some triglycerides and a reaction with the oil, known as phosphorylation which results in darkening of the oil during steam refining/deodorization. This hazard is avoided by using instead a 0.05% w/w dose of citric acid as a 50% solution in water. This, although usually rather more expensive than phosphoric acid, works very satisfactorily.
To get rid of pro-oxidant trace metals in refining fats and oils is important; the most common are iron and copper. Iron at 0.1 ppm and copper at 0.01 ppm can lower the stability of a deodorized oil. Various acids readily combine with these trace metals forming chelate complexes, and this renders the metals inactive as catalysts. A citric-acid solution is most popular for this purpose, which is described as sequestering. Other acids which behave in a similar way are ascorbic (vitamin C), phosphoric, tartaric, and EDTA. The complexes formed when trace metals are sequestered are very readily adsorbed by activated clays; hence, this provides a double security. A typical precaution is to add ca. 0.05% w/w of citric acid/oil as a 50% solution in water to an oil about to be bleached. Some sequesterants do no more than capture trace metals, but others, notably citric acid, will scavenge free oxygen, split soap, and convert nonhydratable phosphatides to a hydratable form.
For oils rich in phosphatides especially in non-hydratable phosphatides, the dry degumming could not be applied as it would require a huge quantity of bleaching earth. Conventionally these oils were refined by water degumming process. Soybean oil when water-degummed yield lecithin which has various applications. In this process, warm water is added to the crude oil at 80-85ºC and the mixture is agitated slowly for approximately 20 minutes, a process usually referred to as “hydration”. The water dosage used is usually based on the expected amount of phospholipids in the crude oil. The “hydratable” phospholipids agglomerate at the interface of the oil and water, capturing some non-hydratable phospholipids with them. Oil is also trapped by the phospholipids, forming an emulsion, referred to as “gums” or “wet gums”. The residual phosphorus levels found in the water degummed oil average between 80 and 130 ppm, and correlate very well with the remaining metal (Ca, Mg, Fe) content of the oil because the metals are bound to the phospholipids, particularly PA, forming non-hydratable salts.
Water-degummed oil still contains phosphatides, only the hydratable phosphatides are removed with water degumming. Typically, oils will have an 80- to 200-ppm phosphatide content after water degumming, depending on the type and quality of the crude oil. The nonhydratable phosphatides, which are the calcium and magnesium salts of phosphatidic acid and phosphatidyl ethanolamine, remain in the oil after water degumming. The amount of the nonhydratable phosphatides in the oil is related to the general quality of the oil and, in particular, to the degree of action of the enzyme phospholipase. This enzyme is responsible for the production of phosphatidic acid from hydratable phosphatides
Acid degumming leads to lower residual phosphorus content than water degumming and, therefore, is a good alternative if dry degumming and physical refining are to be the next refining steps. The acid degumming process might be considered as a variant of the water degumming process in that it uses a combination of water and acid. Crude oil, either water degummed or not, is treated by an acid, usually phosphoric acid, citric acid or malic acid in the presence of water. The nonhydratable gums, consisting mainly of the calcium and magnesium salts of phosphatidic acid and phosphatidyl ethanolamine, can be conditioned into hydratable forms with a degumming acid. For oils containing relatively low amounts of Non Hydratable Phospholipids (e.g. sunflower oil) this process can lead to degummed oil with a lower residual phosphorus content (5 to 30ppm) than water degumming. The acid degummed oil is then dry degummed and physically refined. The process is usually carried out at elevated temperature around 90°C,
The practical limit for oil losses in the water/acid degumming processes is composed of the amount of phospholipids to be removed, and the oil they emulsify. The only way to reduce fatty matter loss even further would be through changes in the emulsifying nature of the phospholipids themselves. This is what enzymes can do.
The use of enzymes for degumming is usually referred to as “enzymatic degumming,” and process conditions are designed to optimize the activity of the enzyme used. This technique is useful for both crude and crude degummed oils (oils that were previously water degummed). One way to reduce the emulsification properties of the phospholipids is to cleave or cut their polar and non-polar parts from one another. Enzymes are an effective way to achieve this reaction. They are natural, selective catalysts, reacting at moderate temperature and pH. Compared with chemical processes, enzyme-catalyzed reactions are very selective, greatly reducing or eliminating the formation of undesired by-products. The enzymes of interest which are active on phospholipids are called phospholipases.
The use of chemicals (acid and caustic) has a dual role – changing hydratability of some phospholipids, but also optimizing the enzyme performance regarding the pH of the oil-water mixture. If only hydratable phospholipids are targeted for the enzymatic reaction, the use of additional chemicals (acid and caustic) may be eliminated.
The process advantages include:
1. Enzymatic reactions are usually carried out under mild conditions.
2. The enzymes are highly specific.
3. The process has acceptable reaction rates.
4. Only small quantities of the enzyme are required to carry out the chemical reactions.
5. Degummed oils with low phosphorus and iron contents are produced even with poor-quality starting oils.