134 June 2010, Vol. 22, No. 6 Lipid Technology

Analysis

Determination of total polar material in frying oilusing accelerated solvent extraction

Zainal and Heinz-Dieter Isengard

Zainal is a research scientist at the University of Hohenheim, e-mail: zainal@unihohenheim.deand Heinz-Dieter Isengard is head of the Department of Food Analysis at the Institute of Food Science and Biotechnology of theUniversity of Hohenheim, Garbenstr. 25, 70593 Stuttgart, Germany, tel: +497113974670, fax +497113974674e-mail: heinz-dieter.isengard@unihohenheim.de

Summary

During use, frying fats undergo chemical deterioration. This leads to the formation of compounds that are more polar than thetriacylglycerols of the fat. Collectively these are called total polar material (TPM), and the mass concentration of TPM is used as anindicator of the quality of frying fats. According to official methods, the fat or oil is separated into polar and non-polar fractions bypreparative column chromatography (PCC). The chromatographic step is time-consuming and needs large amounts of solvents. Here weshow that the chromatographic step can be replaced by accelerated solvent extraction (ASE). This was demonstrated for high-oleicsunflower oil in which potatoes, carrots and parsnips were fried. The materials separated by these two procedures are shown to be identicalas judged by HPLC of the TPM. The two procedures show good agreement.

Introduction

During the frying process the oil deteriorates as a result ofhydrolytic, oxidative and thermal reactions. Polar compoundsare formed from the non-polar triacylglycerols. These arereferred to as the total polar material (TPM), whereas the remain-ing triacylglycerols and other compounds of low polarity makeup the non-polar fraction. The ratio and the amount of these twofractions depend on the kind of frying oil, storage condition,and processing conditions like frying temperature, frying timeand the food being fried. The TPM in the frying oil and the friedgoods may be harmful for human health. Some countries havetherefore set a limit of TPM in frying oil between 24 and 30%. InGermany, the amount of TPM in frying oil has to be lower than24% [1], otherwise it has to be discarded.

The determination of TPM in frying oil is therefore an impor-tant analysis. Two standard procedures exist. They are based onpreparative column chromatography, one using a long column[2], the other using pipette “micro columns” tips [3] instead.Both divide the oil into polar and a non-polar fractions. The non-polar fraction is eluted from the column and determined gravi-metrically. TPM is calculated as the difference of sample weightand mass of non-polar components. A recent alternative methodmeasures the dielectric constant of the oil [4] which increaseswith the mass concentration of polar compounds. The chroma-tographic methods are time consuming and require trained per-sonnel to carry out the measurements.

Here we show how accelerated solvent extraction (ASE),usually used for fat extraction from samples [5, 6], compareswith the procedure including the chromatographic step in TPMmeasurement.

In this example, refined high-oleic sunflower oil was used asfrying medium. Three different vegetable products were fried:potatoes, carrots and parsnips. Each sample was cut into stripsand weighed in portions of approximately 350 g each before fry-ing. A commercial deep fryer with a capacity of 4 litres was used.The frying temperature was 1808C and the frying time was

5 min. Each type of product was fried up to 25 times. The fryingoils (stored in small bottles in a refrigerator) were analysed after5, 10, 15, 20 and 25 times of frying for each product. They wereanalyzed using the preparative column chromatographymethod and with the ASE method.

Preparative column chromatography (PCC)method

In the PCC method according to the DGF method (DGF CIII 3b)the glass column is 45 cm long and has an inner diameter of2.1 cm. It is packed with 25 g of prepared silica gel with a watercontent of 5%. 2.50 g € 0.10 g sample are weighed into a 50mlvolumetric flask and 20 ml of this solution is transferred to thecolumn. The non-polar fraction is eluted with 150 ml of mixedsolvent (petroleum ether/diethyl ether 87:13 by volume) and col-lected in a weighed flask. The polar fraction is retained in thecolumn. The solvent is evaporated and the mass of non-polarmaterial determined by back-weighing the flask. The total polarmaterial is calculated from the sample mass and the mass ofnon-polar material. It is expressed as a percentage of the samplemass.

Accelerated solvent extraction (ASE)

The main principle of the ASE method corresponds to the pre-parative column chromatography method. 1.25 g € 0.10 g sam-ple are weighed into a 25 ml volumetric flask, eluent is filled upto the mark. The extraction cell is filled with a cellulose filter,25 g of prepared silica gel (water content of 5%) and another cel-lulose filter. Then 40 ml of solvent mixture (petroleum ether/diethyl ether 87:13 (v / v)) are filled in. The cell is then sealed andplaced in the ASE oven. The extraction is carried out with the fol-lowing parameters: temperature 608C, static time 3 min, cyclenumber 6, flush volume 50%, pressure 100 bar. The solvent with

DOI 10.1002/lite.201000019

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Lipid Technology June 2010, Vol. 22, No. 6 135

the non-polar fraction passes to the bottle. Afterwards, it is trans-ferred into an empty and pre-weighed flask. The solvent is thenevaporated by using a rotary evaporator at about 558C and at600 € 100 mbar. The flask is then dried for one hour in an ovenat 103 8C. After cooling, the flask is weighed again, the mass dif-ference being the non-polar fraction. The percentage of TPM iscalculated as in the PCC method.

High performance liquid chromatography(HPLC)

A C18 column (4.66150 mm) with modified silicagel (3.5 lm)was used as stationary phase and a mixture of acetone and aceto-nitrile (80% / 20% by volume) as mobile phase. The flow rate was1 ml/min. The injection volume was 10 ll, the stop time 15 min,the column temperature 308C and detection was made by usinga refractive index detector (RID) at a temperature of 408C. The

samples, oils and obtained non-polar fractions, were dilutedwith 2.5 ml acetone, put into a vial and run through the HPLC.

Correlation between preparative columnchromatography and ASE

The percentage of TPM was determined gravimetrically by thePCC and the ASE methods. In Figure 1 the results obtained byPCC and by ASE are plotted against each other. The points in thegraph are the values obtained after 5, 10, 15, 20 and 25 fryingprocesses of potatoes in high-oleic sunflower oil (HOSO). The twomethods show a good correlation. This indicates that the ASEmethod can also be applied as separation technique for deter-mining the total polar material (TPM) in frying oil. The corre-sponding data were y = 1.0058x with R2 = 0.9755 for carrots andy = 1.013x with R2 = 0.9609 for parsnips.

Compared to the PCC method, the ASE method has the follow-ing advantages: Solvent consumption is only (40 + 150) mlinstead of (40 + 80 + 150) ml. Sample mass is only 1.25 mginstead of 2.5 mg. Solvent dilution is 25 ml instead of 50 ml. Seasand (4 g) is replaced by two filters. Preparation time is reducedfrom about 30 min to about 10 min. The running time is only40 min instead of 70 min. Handling is easy and less complicated.

Response of high performance liquidchromatography (HPLC)

In addition to the good correlation between the PCC and ASEmethods, HPLC analysis shows that the two methods result inthe same fractionation.

Fig. 2 shows for the example of potatoes that both methodsperform the same separation. The non-polar fraction of oil waseluted from the column after a retention time of more than4 min. The oils before separation show the same peaks for reten-tion times of less than about 4 min. These correspond to thepolar fractions. After separation by PPC and ASE respectively,these peaks do no longer appear. The separation was, thus, suc-cessful with both methods. The peaks resulting from the non-polar fraction (retention times higher than 4 min) have practi-

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Figure 1. Correlation of results obtained for TPM percentage byPCC and by ASE after 5, 10, 15, 20 and 25 times of frying ofpotatoes in high-oleic sunflower oil

Figure 2. Chromatograms of high-oleic sunflower oil after 5 frying processes with potato strips and of the non-polar fractions sepa-rated by PCC and by ASE

136 June 2010, Vol. 22, No. 6 Lipid Technology

cally the same retention times for both of the separation meth-ods. PCC yields higher peaks. This is due to the fact that for ASEonly half of the amount of sample is used. The chromatogramsfor carrots and parsnips show the same phenomena.

Conclusion

There is a good correlation between the preparative columnchromatography and the accelerated solvent extraction resultsfor measuring total polar material in frying oils. Both tech-niques yield the same composition of the non-polar fraction asshown by HPLC. The ASE procedure is quicker and requires lessexperimental effort.

Accelerated solvent extraction is therefore a valuable alterna-tive to the column chromatographic step in determining TPM infrying oils.

References

[1] Deutsche Gesellschaft f�r Fettforschung (DGF). Recom-mendations of the 3rd International Symposium on deep-fat frying optimal operations, 20–21 March 2000, Hagen/Westphalia (Germany). Eur. J. Lipid Sci. Technol. 2000, 102,594.

[2] DGF. (2006) Deutsche Einheitsmethoden zur Untersuchung vonFetten, Fettprodukten, Tensiden, und verwandten Stoffen. Sec-tion Fat, CIII 3b (84), DGF, Wissenschaftliche Verlagsge-sellschaft, Stuttgart (Germany).

[3] DGF. (2006) Deutsche Einheitsmethoden zur Untersuchung vonFetten, Fettprodukten, Tensiden, und verwandten Stoffen. Sec-tion Fat, CIII 3e (06), DGF, Wissenschaftliche Verlagsge-sellschaft, Stuttgart (Germany).

[4] Isengard, H. D. and Landend�rfer, T., Schnelle Bestim-mung des Verdorbenheitsgrades von Frittierfetten. Lebens-mittelchemie 2009, 63, 23.

[5] Richter, B. E., et al., Accelerated solvent extraction: Atechnique for sample preparation. Anal. Chem. 1996, 68,1033 –1039.

[6] Br�hl, L. and Matth�us, B., Extraction of oilseeds by SFE –a comparison with other methods for determination ofthe oil content. Fresenius J. Anal. Chem. 1999, 364, 631–634.

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