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Expression of Multidisciplinary Flavour Science AROMA COMPOUNDS IN FRENCH FRIES FROM THREE POTATO VARIETIES J.S. ELMORE, J.A. Woolsgrove, D.K. Wang, A.T. Dodson, and D.S. Mottram Department of Food Biosciences, University of Reading, Whiteknights, Reading RG6 6AP, UK Abstract Sugars and free amino acids were measured in three potato varieties widely available in the United Kingdom. French fries were cooked for 6, 9 and 12 min at 180 °C, and the effects of cooking time and variety on volatile composition were examined. Maillard reaction-derived aroma compounds increased as cooking time increased. Varieties Desiree and Maris Piper were relatively high in sugars and aroma compounds derived from sugars, e.g. 5-methylfurfural and dihydro-2-methyl3[2H]-furanone, whereas variety King Edward was relatively high in free amino acids and their associated aroma compounds, such as pyrazines and Strecker aldehydes. Introduction Popular British potato varieties Desiree, King Edward and Maris Piper are available all year. They are picked in early autumn and then stored. To prevent sprouting, potatoes are either sprayed with a sprout suppressant or are stored at around 4 °C (1). However, at low temperatures, breakdown of starch to sugar causes the phenomenon of low temperature sweetening, resulting in increased browning on cooking. This is also associated with increased formation of acrylamide, a suspected carcinogen (2). As part of a study to examine the effect of potato storage time on acrylamide formation in French fries, and the relationships between acrylamide formation, colour and aroma composition, we examined how the aroma compounds in French fries formed from the Maillard reaction are related to levels of free amino acids and sugars in the raw potato. Experimental Potatoes of the above three varieties were purchased pre-packed (2.5 kg bags) from three different supermarkets in February 2008, i.e. after approximately 5-6 months storage. French fries (300 g; 5-8 cm × 1 cm × 1 cm) were cooked in sunflower oil at 180°C for 6, 9 and 12 min. The cooked fries were blended with water (50g fries: 150 g water). Aroma volatiles were collected from 50 g of blended sample at 30 °C onto a Tenax TA trap, which was then heat-desorbed using a Perkin-Elmer ATD and analysed by GC-MS (DB-5MS column: 60 m × 0.32 mm, 1 μm film thickness; J & W) (3). Compounds were identified by comparing their mass spectra and linear retention indices with those of authentic compounds. Chromatographic peak areas of identified compounds were measured relative to that of 100 ng of 1,2-dichloro-benzene (DCB), which was added to the Tenax trap immediately after collection. 305 Table 1. Major Maillard reaction-derived compounds in French fries from three varieties of potato cooked at 180 °C for 6, 9 and 12 min (n = 3). Peak areas in a 50 g sample (French fries:water = 1:3) are measured relative to that of 100 ng of 1,2-dichlorobenzene internal standard (= 100) Pea k No. LRI 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 501 557 600 604 653 664 694 745 739 807 824 826 831 836 909 913 917 962 999 20 21 22 23 1003 1004 1050 1078 a,b Compound Desiree 9 12 6 min min min acetone 6.3 2-methylpropanal 508 2,3-butanedione 6.3 2-butanone 9.4 3-methylbutanal 264 2-methylbutanal 1300 2,3-pentanedione 6.46 dimethyldisulphide 13.9 2-methylpropanoic acid 5.13 dihydro-2-methyl-3[2H]-furanone 1.08 methylpyrazine 16.1 3-methylbutanoic acid 2.46 furfural 8.20 2-methylbutanoic acid 8.74 2-acetylfuran 2.73 2,6(and 2,5)-dimethylpyrazine 21.7 ethylpyrazine 3.65 5-methyl-2-furfural 1.72 2-ethyl-6-methylpyrazine 3.03 2-ethyl-3-methylpyrazine + trimethylpyrazine 4.11 2-ethyl-5-methylpyrazine 3.99 phenylacetaldehyde 7.10 3-ethyl-2,5-dimethylpyrazine 3.92 Maris Piper King Edward std 6 9 6 9 min min 12 min min min 12 min error CTa Vb 7.0 729 11.2 17.2 397 2000 14.9 15.4 13.1 4.65 31.2 6.75 21.0 20.1 5.44 40.7 7.24 5.97 7.76 10.9 944 12.1 21.4 390 2010 20.3 16.7 14.2 10.6 41.7 4.42 34.5 18.9 10.0 50.6 13.4 14.2 12.6 4.9 447 6.2 6.3 232 1310 5.54 6.26 1.07 0.04 5.62 0.69 1.65 2.09 0.09 5.73 1.29 0.03 0.51 4.7 530 10.1 13.8 356 1370 14.0 14.4 4.66 2.50 27.4 3.85 10.0 9.30 1.70 27.5 8.72 1.65 7.65 10.3 1100 18.5 23.5 686 3150 32.0 16.0 24.7 8.81 49.1 8.50 32.8 29.0 4.20 45.1 16.3 5.82 11.9 8.0 737 6.8 15.0 560 1820 8.34 22.2 8.71 1.35 37.6 6.25 3.87 14.9 1.78 53.3 9.75 0.85 16.2 6.5 894 8.9 16.6 749 2530 12.4 27.3 10.2 3.01 52.3 6.90 9.19 17.7 2.95 73.4 17.8 1.88 23.5 5.8 1170 12.4 23.5 784 2990 19.9 34.8 8.26 6.44 86.4 4.31 12.2 11.8 5.02 118 31.4 5.35 42.3 0.6 58.6 1.0 1.4 47.0 195 1.84 2.21 1.78 0.70 5.32 0.68 2.92 2.24 0.68 7.48 2.11 1.04 2.81 ns *** ** *** ** * *** ns * *** *** ns ** ns ** ** ** ** ** ns * ns ns *** ns ns ** ns * ** ns ns ns ** *** ** * *** 9.22 8.40 11.6 8.57 12.2 11.0 11.3 11.5 0.70 0.61 2.12 0.29 7.87 6.68 7.70 6.47 12.6 8.50 11.1 8.57 15.3 9.08 10.2 18.4 21.9 13.3 14.6 25.4 40.8 27.5 21.8 49.2 2.61 1.71 1.31 3.35 ** ** ** ** *** ** ** *** Probability that there is a difference between means for (a) cooking time and (b) variety; ns, no significant difference between means (p > 0.05); * significant at the 5 % level; ** significant at the 1 % level; *** significant at the 0.1 % level. Expression of Multidisciplinary Flavour Science Portions of the flesh of each raw potato sample were freeze-dried for precursor analysis. Free amino acids (excluding arginine) were measured using EZ-Faast for GC-MS (Phenomenex); sugars were measured by high-performance anion-exchange chromatography (Dionex) (4). Results Table 1 contains those compounds derived from the Maillard reaction, which were present in at least one sample, with a peak area of more than 5% of the peak area of the DCB standard. Of these compounds, 20 increased with cooking time, while none decreased, and 16 varied with variety. Three compounds, which could be formed by sugar breakdown alone, namely 2-acetylfuran, dihydro-2-methyl-3[2H]-furanone and 5-methylfurfural, were highest in Desiree, while many of the remaining Maillardderived compounds were highest in King Edward, e.g. Strecker aldehydes and dimethyl disulfide, formed relatively early in the Maillard reaction, and alkylpyrazines, which are stable end-products (5). Table 2 shows the levels of the key aroma and acrylamide precursors in the three varieties. King Edwards were relatively low in sugars and high in amino acids, Desiree were relatively high in sugars and amino acids, while Maris Piper were relatively high in sugars and low in amino acids. Desiree was much higher in asparagine than the other two varieties and has been shown to produce significantly more acrylamide in French fries than Maris Piper (2). As the potatoes had been stored, sugar levels were relatively high, compared with freshly-harvested potatoes, and the French fries obtained were golden brown in colour. It was noted that King Edwards from one supermarket were threefold higher in fructose and glucose, compared to those from the other two supermarkets (data not shown). Table 2. Levels of key aroma and acrylamide precursors (mmol per kg dry weight) in 3 varieties of potato (n= 9). Values in the same column with different letters are significantly different. Variety Desiree King Edward Maris Piper a Glucose 32.4 28.5 32.9 Fructose 29.7 (b) 23.2 (a) 31.3 (b) Total sugars 68.4 (ab) 56.4 (a) 71.4 (b) Asparagine 74.0 (b) 33.8 (a) 29.6 (a) Total amino acidsa 266.5 (b) 257.9 (b) 179.5 (a) excluding arginine A principal component analysis was performed on the statistically significant data, to visualise the relationships between precursors and aroma compounds (Figure 1). Sugar-derived aroma compounds were separated from the rest along principal component (PC) 1, and were associated with the sugars and varieties Desiree (DES) and Maris Piper (MP). Most of the remaining aroma compounds were clustered with King Edwards (KE) and the amino acids alanine, serine and glycine, which were separated from the other amino acids along PC 2. These amino acids have been shown to be important in the formation of low-molecular weight pyrazines (5), such as those in Table 1, which were highest in the French fries made from King Edwards. As well as showing how cooking time can affect flavour formation in French fries, these results also show the effects of sugars and amino acids on aroma formation. As reducing sugars increase during potato storage, levels of sugar-derived 307 Expression of Multidisciplinary Flavour Science compounds, such as furfural, should become relatively more important in the aroma profile of the cooked French fries and may act as a predictor of acrylamide formation. 1 PC 2 (21.8%) 0.5 Asn Met IleTyr DES Lys Val 9 Thr total AA Phe Leu sucrose 13 18 15 DES DES 10 MP MP total sugars 3 0 fructose glucose 7 -0.5 -1 -1 Figure 1. MP KE -0.5 0 PC 1 (43.4%) 4 KE Gln 22 Ser Gly 21 2 Ala 6 KE 17 16 23 20 5 11 19 8 0.5 1 PCA plot showing relationships between aroma compounds and their precursors in French fries cooked for 9 min: z aroma compound, ¡ potato variety, { amino acid, sugar. References 1. 2. 3. 4. 5. Kumar D., Singh B.P., Kumar P. (2004) Ann. Appl. Biol. 145: 247-256. Burch R.S., Trzesicka A., Clarke M., Elmore J.S., Briddon A., Matthews W., Webber N. (2008) J. Sci. Food Agric. 88: 989-995. Methven L., Tsoukka M., Oruna-Concha M.J., Parker J.K., Mottram D.S. (2007) J. Agric. Food Chem. 55: 1427-1436. Elmore J.S., Mottram D.S., Muttucumaru N., Dodson A.T., Parry M.A.J., Halford N.G. (2007) J. Agric. Food Chem. 55: 5363-5366. Nursten H.E. (2005) The Maillard Reaction: Chemistry, Biochemistry and Implications; Royal Society of Chemistry. 308