The Refining and Fractionation of Palm Oil
Crude palm oil (CPO), like other crude edible oils, contains impurities such as gums, pigments, trace metals and a host of other fat soluble compounds consisting primarily of free fatty acids (FFA) and odoriferous matters. When required for edible purposes, it is important that all constituents which give an unpleasant taste, odour and colour are removed. Trace metals which can give rise to poor storage stability in the refined oil, must also be reduced to acceptable levels.
In most instances, crude palm oil in Malaysia is refined through a physical/steam refining process. However, chemical/alkali refining of crude palm oil is also carried out by small number of refineries. The methods differ basically in the way free fatty acids (FFA) are removed from the crude oil.
CHEMICAL / ALKALI-REFINING
The process of alkali-refining treats the crude oil in three (3) stages of processing, namely, degumming – cum – neutralization, bleaching and deodorization.
• Degumming cum Neutralization
The crude oil is first gum-conditioned with phosphoric acid followed by caustic neutralization. In this process, the free fatty acids are first reduced to below 0.25% as soap-stocks with an alkali (caustic soda). The soap-stock, a product of alkali treatment, is normally split with mineral acid (sulphuric acid) and sold as a by-product called Palm Acid Oil (PAO).
The neutralized oil, after separating from the soap-stock, is washed with water and dried under vacuum.
After the phosphoric acid treatment for gum removal and neutralization for free fatty acids reduction, the oil still contains undesirable impurities, odour and colour pigments which need to be treated before the deodorization process. Most of the impurities and residual soaps are removed in the bleaching process. The neutralized oil and 1%-2% of bleaching earth are mixed
under vacuum and at temperatures of 90°C – 130°C with a reaction time of 20-40 minutes.
Thereafter, the earth along with the impurities and some colouring substances are removed from the oil by filtration to obtain a clear and light coloured oil.
Deodorization removes odours and flavours of the neutralized, bleached oil resulting in a bland finished product. It is carried out under vacuum (2 – 6 mbar) and the FFA in the oil steam is stripped at the temperature of 200°C – 250°C to below 0.1%. Deodorization also eliminates peroxides, and the colour of the oil to Red 3 max. (5 1/4” Lovibond cell).
Physical or steam refining of palm oil is widely used in Malaysia since 1976. It gives the benefit of improved yields, efficiency in process, reduced processing time and the immediate recovery of high purity palm fatty acid distillates (PFAD). It also reduces effluent problems arising from the treatment of soap-stock, as in the case of alkali refining.
• Pre-treatment and Bleaching
The crude oil is treated with phosphoric acid at a temperature of 90°C-130°C to remove impurities such as gums, trace metals, etc. The acid-treated oil is then bleached under vacuum with 1%-2% of bleaching earth, followed by filtration.
The bleached oil is then steam-stripped under vacuum (2-6 mbar) at a temperature of 250°C – 260°C to remove the free fatty acids to below 0.1%. The RBD palm oil which leaves the deodorizer is bland in flavour and the colour is below Red 3 (5 1/4” Lovibond cell). Simultaneously, the process also produces the Palm Fatty Acid Distillate (PFAD), a by-product which has an FFA of 80-95%.
Palm oil, with an iodine value (IV) of around 52 and a saturated to unsaturated fatty acids ratio of about 50:50, is made of triglycerides of a wide range of melting points. This unique composition enables it to be crystallized under a controlled temperature programme. The mixture of oil and crystalized solid fat is then filtered to obtain a low melting fraction (olein) and a high melting fraction (stearin).
In Malaysia, there are three (3) processes for fractionation, namely dry, detergent and solvent fractionation. The main process is the dry fractionation. The detergent process is used in a smaller scale whilst the solvent fractionation is used for the production of specific fractions for specialty fat products.
• Dry Fractionation
This is a physical process normally carried out on neutralized palm oil and refined, bleached and deodorized palm oil. The feed oil is stirred slowly under a controlled cooling programme in a crystallizer to allow the higher melting triglycerides to precipitate. The crystallized mass is then filtered in an automated membrane filter to separate olein, the liquid fraction from stearin, the solid fraction. The stearin fraction is then melted using hot water heat exchanger before storage. In the whole process, no additive or any processing chemical is required.
• Detergent Fractionation
In the process, the crude palm oil is first cooled to allow the solid fat to crystallize. Then an aqueous surfactant solution comprising a mixture of sodium lauryl sulphate and magnesium sulphate is introduced to ‘trap’ the solid crystals which are then separated by a centrifuge from the liquid fraction. The ‘trapped’ solid fat in the detergent solution is then heated to release the melted stearin which, in turn is separated from the aqueous phase by another centrifuge. Both the liquid and the melted stearin are washed thoroughly with hot water to remove traces of detergent and dried under partial vacuum to obtain the final olein and stearin.
• Solvent Fractionation
Due to the higher capital investment and operation cost, this process is used only for obtaining high value fractions such as palm mid-fraction for the production of cocoa butter equivalent (CBE). In this process, the crude palm oil is first dissolved in a suitable solvent (usually hexane or acetone) and the miscella thus obtained is chilled under a controlled cooling progamme to allow the solid triglycerides to crystallized. The solid phase is then separated from the liquid phase in an air-tight filter. The olein miscella and stearin miscella are then separately distilled and stripped to recover the solvent which is recycled to the process.
DOUBLE FRACTIONATION PROCESS
In double fractionation process, the olein from the first fractionation is refractionated at a lower temperature (than the first stage) to obtain another liquid fraction with higher unsaturation, called superolein. The iodine value of superolein is generally above 59. The characteristics can be referred to Malaysian Standard for Palm Superolein Specification (MS1762:2004) or Codex Standard for Named Vegetable Oils (CODEX STAN 210-1999). The oil has better clarity and lower tendency to turn cloudy compared to normal olein. Superolein is also suitable as cooking oil and frying oil. Blending normal olein or superolein with unsaturated oils allows for oils with different composition and clarity to cater for different market requirements.
The Refining and Fractionation of Palm Kernel Oil
As in crude palm oil (CPO), crude palm kernel oil (CPKO) can also be refined by either caustic soda or through a physical process. However, as crude palm kernel oil is almost free of carotenes, and contains fatty acids of short carbon chain length, the amount of bleaching clay used is less compared to palm oil, and the temperature required for deodorization is lower (usually <240°C).
Palm kernel oil is fractionated industrially to yield palm kernel stearin and palm kernel olein. In contrast to the palm stearin from palm oil, palm kernel stearin is of a higher value fraction as the latter can be used for the production of cocoa butter substitute (CBS). Three fractionation processes are commercially available, namely by hydraulic pressing, detergent fractionation followed by centrifugal separation and solvent crystallization followed by air-tight filtration. Usually either the crude oil or the RBD oil is used as a feedstock.
• Dry Fractionation
The palm kernel olein and palm kernel stearin are the products separated by pressing of the crystallized palm kernel oil which has been tempered in a specially designed cold room. With improvement in the design of crystallization chamber and the hydraulic membrane presses, this process can now be semi or fully automated.
• Detergent Process
In this process, the palm kernel oil is first diluted with palm kernel olein and then cooled under a preset programme in a crystallizer . A detergent solution containing surfactant and electrolyte is added to ‘wet’ the stearin crystals which can then be separated from the olein using a centrifuge. The stearin is again separated from the detergent solution by heating followed by another centrifugation. After washing the separated phases, palm kernel stearin and palm kernel olein are obtained.
• Solvent Process
This is an expensive process where a solution of palm kernel oil in a solvent (either acetone or hexane) is cooled to a very low temperature for crystallization. The solid and the liquid phases are then separated in an airtight filter operated under vacuum. Simultaneous distillation and stripping of the solvent from the solid and liquid phases produce the palm kernel stearin and palm kernel olein. The high production costs due to the high energy requirement make this process less competitive to the others.
HYDROGENATION AND HYDROGENATED PRODUCTS
Hydrogenation is carried out either via a batch-wise or continuous process by reacting the palm kernel oil product with hydrogen gas in the presence of a nickel catalyst. The extent of hydrogenation depends on the desired iodine value, slip melting point and solid fat content in the final product. This is shown in Table 6 below. These are only guideline data and may vary depending on the processing conditions.