Future prospects for palm oil refining and modifications

ARTICLE

Auteur(s) : Véronique Gibon, José Vila Ayala, Peggy Dijckmans, Jeroen Maes, Wim de Greyt

Desmet Ballestra Group, Minervastraat 1, B-1930 Zaventem, Belgium

Introduction

Most of natural oils have only a limited application in their original form, as a consequence of their specific chemical composition. They therefore often undergo a chemical or physical modification. Quite a number of questions arise with respect to the effect of the chemical modification processes on the nutritional quality of oils. The issue whether trans fatty acids have a negative impact on health also forces producers to find alternatives like dry fractionation and enzymatic interesterification. Due to the continuous technological developments, a whole variety of products normally processed by solvent or detergent fractionation can now be obtained with a high degree of selectivity by dry fractionation. The original booming of the dry fractionation process has helped palm oil to conquer a strong position on the commodity market. Today, palm oil is without doubt the most widely fractionated oil. New demands for special cuts drifted the industry toward a more sophisticated approach with the production of higher cold stability oils (super- and topoleins), of vitamin enriched fractions (high carotene, tocopherol and tocotrienol contents) and of specialty fats (cocoa butter equivalents and substitutes).

Enzymatic interesterification (non specific) has lately received increasing interest as an alternative to partial hydrogenation for the production of low-trans hard fat suitable for shortenings and stick or tub-type margarines. In this context, palm stearin is a suitable alternative to improve tolerance to high temperature and for crystal morphology and stability, although blending with softer oils (including palm liquid fractions) remains necessary to impart plasticity to the final product. On the other side, specialty fats are widely used in food industry for applications where specific physical properties are important. Most confectionery products have a high fat content and meltdown in the mouth is extremely critical. The standard of excellence is cocoa butter and for this reason, specialty fats are often designed to have positive traits of cocoa butter or properties that make them more suitable for specific applications. Low-trans specialty fats can be developed from palm oil (or palm kernel oil) fractions. On the other hand, fat modification technologies like dry fractionation as such or eventually combined with specific enzymatic interesterification constitute a new interesting approach to design special new products.

Quality parameters of crude palm oil

An acidity below 3% guarantees fresh fruits, good storage and controlled transportation; a freshly expelled crude palm oil, promptly dried and cooled, shows slow hydrolysis and oxidation; a high acidity is usually combined with high iron and copper levels, both having important pro-oxidant potential; low grade crude palm oil suffers auto-oxidation of carotene.

A good test to asses the quality of a crude palm oil is the deterioration of bleachability index, shortly said DOBI. The DOBI is basically the ratio between the content of carotenes and secondary oxidation products. The most easily refined palm oil has a DOBI above 2.5; a DOBI below 2 testifies an oil with very poor quality and more difficult to refine. A crude oil with a DOBI above 3 can be considered as an oil of very satisfactory quality. It’s a matter of fact that good-quality refined oils can not be produced from low quality oils. A bleachability test was developed (table 2) to predict the final color of crude palm oil. Six crude palm oils with an initial acidity from 7.2 to 1.9% were prepared. The low DOBI values combined with elevated levels of fatty acids and peroxides gave the indication that some samples were difficult to bleach. Oils were bleached and steam refined and the final color of the heat bleached oils was observed. Conclusion was clear: a heat-bleached palm oil with color below 2R can only be produced when crude palm oil has a DOBI above 2.5.
Table 1 Specifications and quality requirements for crude palm oil.

Palm oil refining

The free fatty acid content of a palm deodorizer distillate is usually 80-85% when the oil is refined according to the physical process. If two condensing units are introduced in the scrubber section, each of them set at an optimized temperature, two different distillates can be obtained with improved purity and added commercial value. With this double condensing system, the content of acylglycerols and other minor components in the condensed fatty acids is reduced and the free fatty acid purity is significantly improved (figure 2).

Normal deodorizers operate at 3-4 mbar. Today, special vacuum production units, ice condensing, have been developed to reach lower pressures (below 2 mbar) and at the same time to reduce emission and effluents by a more efficient condensation of the volatiles. The ice condensing system is becoming more and more the standard in new refining plants. In this system, the vapor is condensed on surface condensers working alternatively at very low temperature. The remaining non condensable matters are removed by mechanical pumps in series with liquid ring pump or by vacuum steam ejector system. The ice condensing system significantly reduces the motive steam consumption but also requires extra electrical energy (figure 3).

Specially refined palm oil

Applied to very good quality crude palm oil (low acidity and high DOBI), a chemical refining plant is able to purify crude palm oil so efficiently that is can be deodorized directly (without bleaching) or in the worst case with very small amounts of non-activated bleaching earths or silica. The thermal bleaching would only operate during deodorizing, resulting in reduced losses in tocopherols and tocotrienols and excellent oxidative stability for the final product. Thermal bleaching conducted at lower temperature is resulting in limited destruction of carotene together while keeping high amounts of tocopherols and tocotrienols in the oil (figure 4).

Production of white soaps from palm oil is not evident: the main problem is too dark color which is usually achieved due to the saponification step. Good results can only be expected when a crude palm oil of high quality is used and refining procedure performed under optimized conditions. Very good saponification colors (around 2.5R) can be obtained from a very light RBD palm oil that can be produced from crude oil having a DOBI above 3 (table 4).
Table 3 Characteristics of Golden Palm Oil at different process conditions (physical refining).

Dry fractionation of palm oil

Commodity oils (oleins, superoleins, stearins and soft palm mid fractions) are produced in one or two steps, main applications being ingredients for margarines or shortenings and frying or salad oils. The hard palm mid fraction (third step) is a specialty fat widely used in the confectionery industry as an ingredient for cocoa butter equivalent (CBE) formulation. Palm oil is indeed a valuable source of POP (2-oleo-diplamitin) and, at high POP levels, it becomes (after fractionation) suitable for cocoa butter replacement fats. The content of POP in the hard palm mid fraction is significantly increased during the multi-step dry fractionation process; final enrichment in the fraction can easily exceed 65% of POP (table 5). This specific composition offers to the hard palm mid fraction a very steep melting profile similar to the one of cocoa butter for specialty fats applications (figure 7). Specialty fats are indeed widely used in food industry for applications where specific physical properties are important. They are used in combination with other ingredients such as cocoa powder, sugar flavors, milk products, nuts… Most confectionery products have a high fat content and meltdown in the mouth is extremely critical. The standard of excellence is cocoa butter and for this reason, specialty fats are often designed to have positive traits of cocoa butter or properties that make them more suitable for specific applications. These include melting and solidification properties, fine crystal structure, snap, gloss, color and flavor release. Miscibility with cocoa butter is another important aspect that needs to be considered. A true cocoa butter equivalent (CBE) is an all-vegetable non hydrogenated product (generally based on four raw materials: illipe, shea butter, sal fat and palm oil) containing almost the same fatty acids and acylglycerol present in a typical cocoa butter; as a result of the closely similar chemical composition, the physical properties of a CBE also closely match to cocoa butter. CBE are fully compatible with cocoa butter and can be used in any proportion up to 100% for complete replacement; they also require the same tempering procedures as cocoa butter to prevent fat bloom.

Cocoa butter substitutes (CBS) are principally made of palm kernel oil after fractionation. Palm kernel stearin is particularly enriched in lauric and myristic fatty acids and, for this reason, compatibility with cocoa butter is poor. Cocoa butter would not exceed 5% of CBS to avoid softening. The stearin by itself, or after full hydrogenation, is an excellent CBS, with a very high solid fat content at 20 °C making it a material of choice for molded products with good fat bloom resistance and good snap; there is no need for tempering as the formed crystals are stable. Cocoa butter replacers (CBR) are produced by hydrogenation of liquid oil, frequently followed by dry fractionation. Raw materials can include rapeseed, sunflower, soybean and also palm oil or palm olein. The fractionated CBR have better melting profile and better eating quality than the non fractionated ones but they are also more expensive. CBR can tolerate up to 25% of cocoa butter on fat base when used in confectionery coatings. It must be mentioned that today, the high trans content of CBR makes this alternative less desirable.

Dry fractionation is classically operated on refined palm oil but the high vitamin content of the crude oil makes the dry fractionation process (combined with special refining procedures) an attractive route for designing special (red) products: carotenes, tocopherols and tocotrienols concentrate markedly in the liquid fractions (oleins and superoleins) and quantities are still present in the solid fractions (stearins and palm mid fractions). A series of dedicated products (red palm superolein) are available on the market (figure 4).

Minor components like diacylglycerols, which can not be removed during the refining process of palm oil, have several disadvantages during dry fractionation; di- and triacylglycerols show eutectic interactions making separation difficult and fractionation incomplete. Also, high melting diacylglycerols (mainly 1,3-dipalmitoyl glycerol) develop cloudiness upon storage at room temperature affecting the cold resistance and the cloud point of the olein fractions (table 6). Finally, diacylglycerols have detrimental effect on crystallization performances of the speciality fats (CBE) and for this reason they need to be kept as low as possible during the whole dry fractionation process. Good quality crude palm oil (low acidity, high DOBI, good oxidative sate) is therefore mandatory as highly affecting the quality parameters of the palm fractions.
Table 5 Approximate concentration changes of the main triacylglycerol components during multi-step dry fractionation of palm oil (P: palmitic acid; S: stearic acid; O: oleic acid; L: linoleic acid; St: saturated acid; U: unsaturated acid).

Enzymatic interesterification

On the contrary, a regio-selective enzyme is required for specific enzymatic interesterification; in this case, acyl exchange is exclusively operating at specific positions of the glycerol backbone (most of the time at sn 1,3 positions). Specific enzymatic interesterification is mainly used for the production of specialty fats like low calorie or easily absorbable fats, infant formulation or CBE. When used in CBE formulation, the hard palm mid fraction is blended with other source to readjust the composition (table 8). To avoid this blending, cocoa butter equivalents can alternatively be produced by sn 1,3 selective enzymatic interesterification, idea being to preserve the unsaturated content at sn-2 position while increasing the stearic content at sn 1-3 positions. Applied to palm oil, the strategy would be to modify a “POP fat” into a “POP/POS/SOS fat”, as close as possible to the composition of cocoa butter (figure 9). Regio-selective enzymatic interesterification eventually followed by stripping of the acids or methyl esters in excess and dry fractionation for further enrichment makes possible compositional adjustment the main objective being a maximal achievable content in StUSt (SOS, POS and SOS) for use in confectionery.
Table 7 Quality parameters and thermal characteristics of a palm stearin and soybean oil blend (70%/30% w/w) submitted to chemical and non specific enzymatic interesterification (Lipozyme TL IM – Novozymes) (Random: theoretical prediction based on the law of probability. P: palmitic acid; S: stearic acid; O: oleic acid; L: linoleic acid; Ln: linolenic).

Further reading

2 Gibon V, De Greyt W, Kellens M. Palm Oil Fractionation. EJLST 2007 ; 109 : 336-49.

3 De Greyt W, Kellens M. Deodorization. In : Shahidi F, ed. Bailey’s Industrial Oil and Fat Products, Vol. 5. Hoboken, NJ (USA) : John Wiley and Sons, InC, 2005.

4 Calliauw G, Gibon V, De Greyt W, Plees L, Foubert I, Dewettinck K. Phase composition during palm olein fractionation and its effect on soft PMF and superolein quality. JAOCS 2007 ; 84 : 885-91.

5 Costales-Rodrigez R, Gibon V, Verhe R, De Greyt W. Chemical and enzymatic interesterification of a blend of palm stearin/soybean oil for low-trans margarine formulation. JAOCS 2009 ; 86 : 681-97.

6 Wong SOO. Speciality Fats versus Cocoa Butter. Malaysia, 1991.