Download AROMA COMPOUNDS IN FRENCH FRIES FROM THREE POTATO

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.
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1.
2.
3.
4.
5.
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J. Agric. Food Chem. 55: 1427-1436.
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N.G. (2007) J. Agric. Food Chem. 55: 5363-5366.
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