Changes in free amino acids and sugars in potatoes due to sulfate fertilization and the effect on acrylamide formation.

To examine how sulfur deprivation may affect acrylamide formation in cooked potatoes, three varieties of potato were grown under conditions of either severe sulfur deprivation or an adequate supply of sulfur. In all three varieties sulfur deprivation led to a decrease in acrylamide formation, even though the levels of sugars, which are acrylamide precursors, were higher in tubers of the sulfur-deprived plants. In one variety the concentration of free asparagine, the other precursor for acrylamide, was also higher. There was a very close correlation between the concentration of asparagine in the tubers expressed as a proportion of the total free amino acid pool and the formation of acrylamide upon cooking, whereas sugars were poorly correlated with acrylamide. In potatoes, where concentrations of sugars are usually limiting, competition between asparagine and other amino acids participating in the Maillard reaction may be a key determinant of the amount of acrylamide that is formed during processing.

Effect of citric acid and glycine addition on acrylamide and flavor in a potato model system.

Acrylamide levels in cooked/processed food can be reduced by treatment with citric acid or glycine. In a potato model system cooked at 180 degrees C for 10-60 min, these treatments affected the volatile profiles. Strecker aldehydes and alkylpyrazines, key flavor compounds of cooked potato, were monitored. Citric acid limited the generation of volatiles, particularly the alkylpyrazines. Glycine increased the total volatile yield by promoting the formation of certain alkylpyrazines, namely, 2,3-dimethylpyrazine, trimethylpyrazine, 2-ethyl-3,5-dimethylpyrazine, tetramethylpyrazine, and 2,5-diethyl-3-methylpyrazine. However, the formation of other pyrazines and Strecker aldehydes was suppressed. It was proposed that the opposing effects of these treatments on total volatile yield may be used to best advantage by employing a combined treatment at lower concentrations, especially as both treatments were found to have an additive effect in reducing acrylamide. This would minimize the impact on flavor but still achieve the desired reduction in acrylamide levels.

Formation of high levels of acrylamide during the processing of flour derived from sulfate-deprived wheat.

When wheat was grown under conditions of severe sulfate depletion, dramatic increases in the concentration of free asparagine were found in the grain of up to 30 times as compared to samples receiving the normal levels of sulfate fertilizer. The effect was observed both in plants grown in pots, where the levels of nutrients were carefully controlled, and in plants grown in field trials on soil with poor levels of natural nutrients where sulfate fertilizer was applied at levels from 0 to 40 kg sulfur/Ha. Many of the other free amino acids were present at higher levels in the sulfate-deprived wheat, but the levels of free glutamine showed increases similar to those observed for asparagine. In baked cereal products, asparagine is the precursor of the suspect carcinogen acrylamide, and when flours from the sulfate-deprived wheat were heated at 160 degrees C for 20 min, levels of acrylamide between 2600 and 5200 microg/kg were found as compared to 600-900 microg/kg in wheat grown with normal levels of sulfate fertilization.

Measurement of acrylamide and its precursors in potato, wheat, and rye model systems.

The relationship between acrylamide and its precursors, namely, free asparagine and reducing sugars, was studied in cakes made from potato flake, wholemeal wheat, and wholemeal rye, cooked at 180 degrees C, from 5 to 60 min. Between 5 and 20 min, major losses of asparagine, water, and total reducing sugars were accompanied by large increases in acrylamide, which maximized in all three products between 25 and 30 min, followed by a slow linear reduction. Acrylamide formation did not occur to a large degree until the moisture contents of the cakes fell below 5%. Linear relationships were observed for acrylamide formation with the residual levels of asparagine and reducing sugars for all three food materials.

The effect of cooking on acrylamide and its precursors in potato, wheat and rye.

The relationship between acrylamide and its precursors, namely free asparagine and reducing sugars, was studied in simple cakes made from potato flake, wholemeal wheat and wholemeal rye, cooked at 180 degrees C, from 5 to 60 min. Between 5 and 20 min, large losses of asparagine, water and total reducing sugars were accompanied by large increases in acrylamide, which maximized in all three products between 25 and 30 min, followed by a slow linear reduction. Acrylamide formation did not occur to any extent until the moisture contents of the cakes fell below 5%. A comparison of each type of cake with a commercial product, made from the same food material, showed that acrylamide levels in all three commercial products were well below the maximum levels in the cooked cakes.

Kinetic models as a route to control acrylamide formation in food.

A kinetic model for the formation of acrylamide in potato, rye and wheat products has been derived, and kinetic parameters calculated for potato by multi-response modeling of reducing sugar (glucose and fructose), amino acid, asparagine and acrylamide concentrations with time. The kinetic mechanism shares, with Maillard browning, a rate limiting (probably dicarbonylic) intermediate, and includes reaction steps of this intermediate which are competitive with respect to acrylamide formation. A pathway representing physical and/or chemical losses of acrylamide accounts for the measured reduction of acrylamide yield at long reaction times. A mechanistic hypothesis regarding the competing reactions of Strecker aldehyde formation and tautomerization followed by beta-elimination to give acrylamide, features in the kinetic model and can be used to determine the factors which steer the reaction towards acrylamide. A predictive application of this model is for 'what-if' experiments to explore the conditions which lead to reduced acrylamide yields.

Acrylamide in potato crisps prepared from 20 UK-grown varieties: effects of variety and tuber storage time.

Twenty varieties of field-grown potato were stored for 2 months and 6 months at 8 °C. Mean acrylamide contents in crisps prepared from all varieties at both storage times ranged from 131 µg/kg in Verdi to 5360 µg/kg in Pentland Dell. In contrast to previous studies, the longer storage period did not affect acrylamide formation significantly for most varieties, the exceptions being Innovator, where acrylamide formation increased, and Saturna, where it decreased. Four of the five varieties designated as suitable for crisping produced crisps with acrylamide levels below the European Commission indicative value of 1000 µg/kg (Saturna, Lady Rosetta, Lady Claire, and Verdi); the exception was Hermes. Two varieties more often used for French fries, Markies and Fontane, also produced crisps with less than 1000 µg/kg acrylamide. Correlations between acrylamide, its precursors and crisp colour are described, and the implications of the results for production of potato crisps are discussed.

Effect of cultivar and storage time on the volatile flavor components of baked potato.

Tubers of five cultivars of potato were stored at 4 degrees C for 2, 3, and 8 months and baked in a conventional oven. The flavor compounds from the baked potato flesh were isolated by headspace adsorption onto Tenax and analyzed by gas chromatography-mass spectrometry. On a quantitative basis, compounds derived from lipid and Maillard reaction/sugar degradation dominated the flavor isolates, with sulfur compounds, methoxypyrazines, and terpenes making smaller contributions. Levels of 37 of the >150 detected compounds were monitored in each cultivar with time of storage. Many significant differences were found in levels of individual compounds, compound classes, and total monitored compounds for the individual effects of cultivar and storage time and for their two-way interaction. Differences may be explained by variations in levels of flavor precursors and activities of enzymes mediating flavor compound formation among cultivars and storage times. In addition, differences in agronomic conditions may partly account for variations among cultivars. Overall, of the compounds monitored, those most likely having the greatest flavor impact were 2-isopropyl-3-methyoxypyrazine, 2-isobutyl-3-methoxypyrazine, dimethyl trisulfide, decanal, and 3-methylbutanal, with methylpropanal, 2-methylbutanal, methional, and nonanal also being probable important contributors to flavor.

Acrylamide is formed in the Maillard reaction.

Reports of the presence of acrylamide in a range of fried and oven-cooked foods have caused worldwide concern because this compound has been classified as probably carcinogenic in humans. Here we show how acrylamide can be generated from food components during heat treatment as a result of the Maillard reaction between amino acids and reducing sugars. We find that asparagine, a major amino acid in potatoes and cereals, is a crucial participant in the production of acrylamide by this pathway.