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648 Schwarz Eur. J. Lipid Sci. Technol. 102 (2000) 648649

Formation of trans polyalkenoicfatty acids during vegetable oilrefining

Walter Schwarz

SETUZA AS, Ust nad Labem, Czech Republic

Vegetable oils constitute an indispensable part of our dietbecause they provide not only an important source of es-sential fatty acids, but also oil soluble vitamins. In the lastdecades their consumption has been steadily increasingwhile animal fats decreased. In developed countries, veg-etable oils intended for usual consumption are refined.During the refining process, undesirable substances,such as those of mucilaginous character, free fatty acids,pigments, and volatiles imparting unpleasant odours andflavours, are removed. The process results in productsthat consumers meet on the market vegetable oils ofsparkling light yellow colour, neutral flavour, and odour.

As shown by recent research, small amounts of transun-saturated fatty acids are formed during refining, particu-larly in the last refining stage, i.e. during deodorisation.Initial work in this field was published by Wolff[13] in theearly 90s. During the final refining stage, i.e. in the de-odorisation, stripping steam is blown in oil under very lowpressure and at temperatures above 200 C, to removesensory, undesirable volatiles. Under these conditions, al-so other substances present in oil, such as free fattyacids, tocopherols, phytosterols, and other volatile sub-

stances, are partially entrained and distill with the strip-ping steam. In the chemical (alkali) refining process, freefatty acids present in oil are neutralised by sodium hy-droxide, and the resulting soaps are to be washed out, theoil is then bleached and deodorised. In recent years,physical refining is an increasingly used refining method,not only for soybean oils, but due to economic reasons,also for other vegetable oils and fats [4, 5]. The physicalrefining is a process with increasing requirements for theefficiency of deodorising columns, because free fattyacids are not removed before the deodorisation itself,which is why higher deodorisation temperatures (by1030C) are used.

Raw vegetable oils include just a negligible amount oftransunsaturated fatty acids, ranging between 0.10.3%of the total fatty acid content. As it follows from our owntests of refining processes on industrial scale, no transisomerisation occurs during preceding refining stages,

For the above-mentioned reasons, the value of partialcatalytic hydrogenation is lower now, however, it will veryprobably be always a process of great interest and need-ful for both edible and nonedible purposes. For the pro-ducers of shortenings and emulsified fats, there still existsthe demand to diminish the trans fatty acids content by

the tune of the hydrogenation mode. Moreover, it seemsthat a decisive solution is the application of such process-es as fractionation, total hydrogenation, and interesterifi-cation. Big companies have introduced transzero mar-garines a couple of years ago, so the good and clear wayhas been done also for minor producers. However, to pro-duce fat products without transacids is not a real prob-lem, the problem is to conserve some, mainly texturalquality parameters, which are in the specific products in-duced with transfatty acids.

Finally, I would like to compare todays importance of hy-drogenation to a traffic situation in front of the crossroad.

For a mode of partial catalytic hydrogenation, it is likely tostop the lighting of green colour and will switch to orangecolour, or somewhere even red one. However, this is nottrue for the hydrogenation itself, because in all the regimevariances and applications of hardened fats, it is and willbe the powerful technological step in any time. Here, I seethe crossroad lights shining a bright green colour.

References

[1] J.W.E. Coenen: Hydrogenation of edible oils. J. Am. OilChem. Soc. 53 (1976) 382389.

[2] R. D. OBrien:Fats and Oils. Formulating and Processing forApplications. Technomic Publishing Co., Lancaster-Basel1998.

[3] V. Koman, J. Prokaj, S. Schmidt:Study on hydrogenation ratein dependence on catalytic inhibitors (in Slovak). Bull. FoodRes. 22 (1983) 239250.

[4] S. Schmidt, P. Bohov, J. Prokaj:The influence of partial cat-alytic hydrogenation on the structure of triacylglycerols inrapeseed oil (in Slovak). Bull. Food Res. 28 (1989) 395404.

[5] A. Celkov, B. Kavuliak, S. Schmidt:Effect of hydrogen distri-bution on hydrogenation of monoenoic oil (in Slovak). Bull.Food Res. 27 (1988) 431438.

[6] A. Celkov, S. Schmidt, J. Zajc: Effect of ultrasound onhydrogenation of sunflower oil. Fat Sci. Technol. 91 (1989)6265.

[7] J. Klvanov: Transfatty acids in fat products and metabolism(in Slovak). Lab. Diagnost. 4 (1997) 251255.

Correspondence: Walter Schwarz, SETUZA AS, CZ-401 29Ust nad Labem, Czech Republic. Phone: +420-47529-1111,Fax: +420-47529-3999; e-mail: walter.schwarz@setuza.cz

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i.e. during hydration (degumming), deacidification, andbleaching. The content of all original transfatty acids re-mains at their original level. During the deodorisationcarried out at a temperature of 230 C and a pressure of670Pa in a Bernardiniequipment the content of transun-saturated fatty acids increased three to ten times. The

highest increase was observed in polyenoic fatty acidswith a double bond predominantly in position 9. Trienoicfatty acids isomerise significantly faster than dienoicacids. Monoenoic fatty acids are the most resistant totrans isomerisation, and it is interesting that only thecontent of trans-11 octadecenoic acid increased, whilethe content of trans-9 octadecenoic isomer remainedunchanged.

Crude rapeseed oil contains also a very small amount ofconjugated fatty acids which are formed already in theseeds by enzymatic oxidation of essential fatty acids tothe corresponding hydroperoxides with conjugated dou-ble bonds. In the course of deacidification and bleachingin the refining process their content remains unchanged(0.100.25%). The conjugated diene content roughlydoubles in laboratory tests, but only at high temperatures,as tested in the range of 210230C under nitrogen [6].Conjugated trienoic systems probably decompose anddisappear completely [7].

Deodorisation temperature is the main factor influencingthe speed of isomerisation reactions. In deodorisationcolumns, used for physical refining of vegetable oils on in-dustrial scale, the content of transunsaturated fatty acids

took place within the range of 230270C, e.g. in case oferucic acid-free rapeseed oil, the transfatty acid contentrose up to 5.2%. At normal operating conditions of250C, the content of transacids increased to 2.02.4%.Also, under conditions of physical refining, a more signifi-cant increase is observed in trans isomers of linolenicacid than linoleic acid. Monoenoic fatty acids were practi-cally not isomerised. At the same time, conjugated dienesincreased to 0.5%, and conjugated triene systems againprobably decomposed to a level under the detection limit.

Suitable modifications of deodorisation columns, particu-larly the increase of live steam/oil contact area, resulted in

the decrease of necessary operation temperature downto 230240 C, while other quality parameters were re-tained, which also brought about a drop of the formationof corresponding transfatty acids [8]. Under such opera-tion conditions, the level of unsaturated transfatty acidsoscillated between 0.61.0%, which is within the limits re-quired in trade relations [9]. Similar and maybe better re-sults are now being achieved in the deodorisation of soy-bean and sunflower oils by physical refining. In thissphere of vegetable oil processing, there is no other stepon the horizon bringing a further reduction of the content

Eur. J. Lipid Sci. Technol. 102 (2000) 648649 Formation of transpolyalkenoic fatty acids 649

of transunsaturated fatty acids. The only degree of free-dom is the operating pressure, whose further reduction isimprobable due to technical reasons.

Conclusions

Edible oils as rich sources of essential fatty acids are alsoa negligible source of trans unsaturated fatty acidsformed during deodorisation, which is the last stage of therefining process. The isomerisation rate is controlled bytemperature, while the quantity of originating trans fattyacids is influenced also by the kind of oil used and the du-ration of heating period. A suitable design of deodorisa-tion equipment used for physical refining resulted in thedecrease of the content of transunsaturated fatty acids inedible oils under 1%.

References

[1] R. L. Wolff:Further sturies on artificial geometrical isomers ofalpha-linolenic and in edible linolenic acid-containing oils. J.Am. Oil Chem. Soc. 70 (1993) 219.

[2] R. L. Wolff:Heat-induced geometrical isomerisation of alpha-linolenic acid: Effect of temperature and heating time on theappearance of individual isomers. J. Am. Oil Chem. Soc. 70(1993) 425.

[3] J. Cmolk, J. Pokorny:Physical refining of edible oils. Eur. J.Lipid Sci. Technol. 102 (2000) 472486.

[4] H. Staage:The physical refining process. J. Am. Oil Chem.Soc. 62 (1985) 299308.

[5] J. Cmolk, W. Schwarz, Z. Svoboda, J. Pokorny, Z. Rblov,M. Dolezal, H. Valentov:Effects of plant scale alkali refiningand physical refining on the quality of rapeseed oil. Eur. J.Lipid Sci. Technol. 102 (2000) 1522.

[6] K. Kovari, J. Denise, F. Zwobada, Z. Kemeny, K. Recseg, G.Henon:Kinetics of trans isomer fatty acid formation duringheating. Olaj, Szappan, Kozmet. 47 (1998) 712.

[7] W. Schwarz, J. Cmolk, J. Pokorny :Isomerisierung der Poly-enfettsuren whrend der Desodorisierung von Speiselen.Fat Sci. Technol. 97 (1995) 491495.

[8] Neue innere Einrichtung zweier Desodorierungsstufen: 3013340 CDE Lurgi l Gas Chemie GmbH, Frankfurt a/M 1997.

[9] J. Cmolk, W. Schwarz, Z. Svoboda, J. Pokorny, Z. Rblov,M. Dole al:Physical refining of rapeseed oil. Presentation onthe 22nd World Congress and Exhibition of the ISF, KualaLumpur (Malaysia), 1997.