Comment on “Soxhlet extraction of acrylamide from potato chips” by J. R. Pedersen and J. O. Olsson, Analyst, 2003, 128, 332

In their paper J. R. Pedersen and J. O. Olsson questioned the procedures commonly applied for the extraction of acrylamide from, e.g., potato chips (US terminology). After a Soxhlet extraction with methanol lasting for 10 days, they found 13,850 µg kg⁻¹ acrylamide, rather than the about 2,000 µg kg⁻¹ determined by conventional extraction. They concluded that the current methods should be verified.

The current procedures might have been verified better than the suggested Soxhlet method. We conclude that the extra amount of acrylamide found by these authors was formed during the extraction.

Experiment 1^b: Formation in the extract from potato slices free of acrylamide

Raw, non-heated potato slices were used for the experiment in order to demonstrate the formation of acrylamide during Soxhlet extraction. Using both the static extraction technique with water (60 °C, ultrasonic bath) and Accelerated Solvent Extraction (ASE) less than 5 µg kg⁻¹ were analysed whereas after Soxhlet extraction for 2 days, 5,500 µg kg⁻¹ acrylamide were found in the methanol extract using GC-MS/MS. After 7 days, concentration in the methanol extract was increased to 18,500 µg kg⁻¹.

However, the potential of acrylamide formation of the potato slices was high: after heat treatment in an oven at 160 °C during 30 min, from 800 µg kg⁻¹ (blend) up to 10,600 µg kg⁻¹ (black pieces) acrylamide were found by the static extraction method (water, 60 °C, ultrasonic bath). The amounts of acrylamide determined in raw potato slices after Soxhlet extraction with methanol were in reasonable accordance with the amounts formed by heating to 160 °C, i.e. with the potential of the sample to form acrylamide.

Experiment 2^a: Formation in the extract from precursors

The experiment was designed to verify the hypothesis that the acrylamide was formed in the methanol extract. In Soxhlet extraction, the sample is warmed only modestly, whereas the extract is constantly kept at the boiling point. Methanol does not only extract the acrylamide, but also its precursors, asparagine, the reducing sugars and possibly other catalysts, i.e. the extract contains the starting materials for acrylamide formation and exposes them to moderate heating. The boiling point of methanol is substantially below the temperatures reached during frying or roasting, but acrylamide is formed even at ambient temperature when the reaction is supported by ammonium carbonate.¹

It was assumed that the 6.2 g of potato chips weighed in by Pedersen and Olsson contained 60 mg asparagine and 30 mg fructose. These two components were directly added to 100 ml methanol (free of acrylamide; asparagine was not fully dissolved) and kept at the boiling point. After 5 and 15 days, the methanol contained 215 and 750 µg l⁻¹ acrylamide, respectively (analyzed by GC-MS). Calculated to the 6.2 g potato chips (absent in our experiment), this corresponds to a concentration of 3,500 and 12,100 µg kg⁻¹, respectively. This is in reasonable agreement with the acrylamide Pedersen and Olsson determined in the Soxhlet extraction of the potato chips, taking into account that the concentrations of asparagine and the reducing sugars in the chips was unknown to us, asparagine not fully dissolved, and ammonium extracted from the potato might have catalyzed the acrylamide formation.

We conclude that the high concentration of acrylamide found by Pedersen and Olsson is an artefact of their extraction procedure and there are no good reasons to question the methods used so far.

References

1. M. Biedermann and K. Grob, Mitt. Lebensm. Hyg., 2003, 94, 406–422.

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