bk-2016-1237.ch010

Chapter 10

Downloaded by PURDUE UNIV on November 28, 2016 | http://pubs.acs.org Publication Date (Web): November 18, 2016 | doi: 10.1021/bk-2016-1237.ch010

Relationship between Alkylpyrazine and Acrylamide Formation in Potato Chips J. Stephen Elmore,1,* Samuel Snowden,1 Adrian Briddon,2 Nigel G. Halford,3 and Donald S. Mottram1 1Department

of Food and Nutritional Sciences, University of Reading, Whiteknights, Reading RG6 6AP, United Kingdom 2AHDB Potato Council, Sutton Bridge Crop Storage Research, East Bank, Sutton Bridge, Spalding, Lincolnshire PE12 9YD, United Kingdom 3Plant Biology and Crop Science Department, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom *E-mail: [email protected]

Levels of acrylamide and fourteen alkylpyrazines were measured in chips made from twenty cultivars of potato, which had been stored for two and six months. While a significant correlation existed between acrylamide and total pyrazines (r2 = 0.609; p = 0.001) for the studied samples, this correlation was not as great as those that existed between the 14 pyrazines themselves (r2 = 0.607–0.965). Levels of pyrazines varied significantly with cultivar (p = 0.0001), while for the cultivars Innovator and Lady Rosetta most pyrazines increased on storage. The ratio between acrylamide and total pyrazines was examined. Low-acrylamide chips that are relatively high in alkylpyrazines (Verdi, Lady Claire, Lady Rosetta and Fontane) may be of interest to manufacturers.

There is a broad consensus that acrylamide at the levels found in food may be harmful to humans and potentially increases the risk of developing cancer. For this reason, a concerted effort has been made by the European snack foods industry, which has led to a significant reduction in acrylamide levels in potato chips over the past 10 years (1). A number of strategies have been suggested for acrylamide reduction in cooked foods (2), although many have an adverse effect on product © 2016 American Chemical Society Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


quality because they affect the Maillard reaction, which is the primary mechanism for acrylamide formation but is also crucial for the development of flavor and color in cooked foods (3). Few studies have examined the relationship between acrylamide formation and alkylpyrazine formation in model systems and cooked foods. Alkylpyrazines are a very important class of Maillard-derived compounds, possessing nutty, earthy, roasted aromas (4). A strong relationship existed between acrylamide and unsubstituted pyrazine formation in model systems containing asparagine, glucose and one other amino acid heated at 160 °C for 20 min (5), although correlations between acrylamide and other pyrazines were weaker. When 1:1 asparagine:glucose model systems were heated at different temperatures (from 140 °C to 170 °C) for 15 min, a linear relationship existed (r2 = 0.9996) between levels of acrylamide and total pyrazines (6). Conversely, little or no correlation was shown in the relationship between alkylpyrazines and acrylamide in six types of cocoa beans roasted at 116 °C for 23 minutes. In addition, neither pyrazines nor acrylamide were associated with acceptability when cocoa liquor was consumed by a trained panel (7). As part of a project looking at the effect of different agronomic and post-harvest strategies on acrylamide formation in cooked potatoes, twenty potato cultivars were grown in the UK in 2011 (8). After harvest, tubers were stored for 2 or 6 months at 8 °C. Some of the tubers from each cultivar and storage treatment were made into chips, while others were freeze dried in order to measure their sugar and free amino acid content. In many of the varieties not generally used for making chips, sugar levels are too high to prepare product with acceptable color attributes. However, this set of samples provides a unique opportunity to link precursor levels with products of the Maillard reaction, in particular alkylpyrazines and acrylamide. If a good correlation exists between these compounds, then the use of a simple GC-MS method may provide a means of predicting acrylamide levels in cooked food, without the need to purchase a significantly more expensive LC-MS/MS system to enable direct measurement of acrylamide itself.

Experimental Potato Samples Twenty different potato (Solanum tuberosum) cultivars were studied (8): Lady Claire (LC), Lady Blanca (LB), Lady Olympia (LO), Lady Rosetta (LR), Daisy (Dy), King Edward (KE), Maris Piper (MP), Fontane (F), Hermes (Her), Markies (Mk), Harmony (Har), Pentland Dell (PD), Desiree (Des), Challenger (Ch), Ramos (R), Innovator (I), Umatilla Russet (UR), Russet Burbank (RB), Saturna (S), and Verdi (V). The varieties chosen covered a range of uses, in particular chips, French fries and domestic use (i.e. mashed, jacket and roast potatoes). Potatoes were all grown at the same site, so that the effect of location on tuber composition was minimized. 134 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

After lifting, tubers were stored at 8 °C for either two or six months at the Potato Council Sutton Bridge Crop Storage Research (SBCSR) facility (Spalding, UK). Tubers were treated with the anti-sprout agent chlorpropham (CIPC), with applications just after storage commenced and two further applications for the sixmonth samples (all applications at 28 mL per tonne of ProLong (50% w/v CIPC in methanol)).


Chips Potatoes from each variety at each storage point (40 treatments) were made into chips at SBCSR. Three batches were prepared and analyzed for each treatment. Peeled potato slices (0.12–0.15 cm thick, 300 g fresh weight) were washed in cold water for 45 s and then cooked in 15 L of high oleic sunflower oil for 3 min at a starting temperature of 177 °C, using a Yeoman D-11E30 single tank 9 kW electric fryer (Bartlett, Exeter, UK). Chips were allowed to cool, and then heat-sealed in laminated foil before being stored at –18 °C until analysis.

Analysis of Volatile Alkylpyrazines Volatile analysis was carried out by automated headspace solid-phase microextraction (SPME) followed by gas chromatography-mass spectrometry (GC-MS), using an Agilent 110 PAL injection system and Agilent 7890 gas chromatograph with 5975C mass spectrometer (Agilent, Santa Clara, CA). The SPME fiber stationary phase was composed of 75 µm divinylbenzene/Carboxen™ on polydimethylsiloxane; Supelco, Bellefonte, PA). Crushed chips (0.50 g) were placed in a 20-mL headspace vial and 5 mL of water containing 50 µg/L 1,2-dichlorobenzene (DCB) were added. The vial was sealed with a magnetic screw cap with PTFE/silicone septum (Supelco). The samples were then equilibrated for 10 minutes at 37 °C before being extracted for 30 min. Sample was agitated at 500 rpm (5 s on, 2 s off) during equilibration and extraction. After extraction, the contents of the fiber were desorbed onto the front of a Supelcowax-10 fused silica capillary column (60 m × 0.25 mm i.d., 0.25 μm film thickness; Supelco). The GC temperature program and the fiber desorption step commenced at the same time. During the desorption period of 45 s, the oven was held at 30 °C. After desorption, the oven was held at 30 °C for a further 255 s before heating at 5 °C/min to 230 °C. Helium was used as the carrier gas at a constant flow rate of 0.9 mL/min. A series of n-alkanes (C5–C22) was analyzed, under the same conditions (10 µL of a 100 ppm solution in diethyl ether added to an empty vial), to obtain linear retention index (LRI) values for the alkylpyrazines. The mass spectrometer operated in electron impact mode with an electron energy of 70 eV, scanning from m/z 29 to m/z 280 at 1.9 scans/s. Compounds were identified by first comparing their mass spectra with those contained in the NIST/ EPA/NIH Mass Spectral Database or in previously published literature. Wherever possible, identities were confirmed by comparison of LRI values, with either those of authentic standards, or published values. 135 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Acrylamide Analysis Ground samples (0.50 g) were weighed into 50 mL Falcon tubes and extracted with 40 mL of water containing 50 µg/L 13C3-acrylamide at room temperature (8). After shaking for 20 min, tube and contents were centrifuged at 9000 rpm for 15 min at 15 °C. Two milliliters from the aqueous layer were passed through a 0.2-µm syringe filter into a 2-mL vial and analyzed by LC-MS/MS, using an Agilent 1200 HPLC system connected to an Agilent 6410 triple quadrupole mass spectrometer with electrospray ion source in positive ion mode. An isocratic separation was carried out at room temperature using a 100 × 3.0 mm i.d. Hypercarb column with a 10 × 3.0 mm i.d. Hypercarb pre-column (both 5 µm particle size; Thermo Fisher, Waltham, MA). The mobile phase was 0.1% aqueous formic acid at a flow rate of 0.3 mL/min. Injection volume was 25 µL. The transitions m/z 72→55 and m/z 72→27 were measured for acrylamide and the transition m/z 75→58 was measured for 13C3-acrylamide. Concentrations of acrylamide in chips were expressed as μg/kg fresh weight.

Statistical Analysis Two-way analysis of variance (ANOVA) and Fisher’s Least Significant Difference (LSD) test were used (p < 0.05) to determine differences between concentrations of alkylpyrazines and acrylamide due to potato variety and storage time (XLStat 2015; Addinsoft, Paris, France). Student’s t-test was used to see if there was a significant effect of storage on pyrazine levels for each variety (p < 0.05). XLStat 2015 was also used to perform linear regression analysis.

Results and Discussion Alkylpyrazines Fourteen alkylpyrazines were found in all chip samples in quantifiable amounts. The fourteen pyrazines are listed in Table 1, along with their LRI on a Supelcowax-10 column, their mass spectral base peak and their mean concentrations in all cultivars (n = 6, i.e. stored 2 and 6 months). Measurement of the peak areas of the pyrazines (and also the dichlorobenzene internal standard) was achieved by multiplying the peak area of a characteristic spectral peak in the pyrazine of interest (always the base peak) by a factor, obtained by dividing the total ion current peak area of the pure compound by the peak area of the base peak. Approximate quantities of the pyrazines were obtained by comparison of their calculated peak areas with that of the 1,2-dichlorobenzene internal standard, assuming a response factor of one for all pyrazines and the internal standard.

136 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Table 1. Mean Alkylpyrazine Concentrations (mg/kg; n = 6) in Chips Prepared from 20 Potato Cultivars Grown in the United Kingdom Pyrazineb

LRI

Cultivara

base peak

Dy

Har

Her

KE

LC

LR

MP

Mk

PD

RB

S

UR

V

Des

R

Ch

I

F

LO

LB

137

Methyl

1294

94

7.5c

10.3

8.3

13.7 1.7

4.9

8.9

4.3

18.4 13.5

5.0

11.3

1.5

16.5

6.9

8.7

17.8 7.0

10.9 14.6

2,5-Dimethyl

1348

108

8.4

5.8

10.6

16.5 1.6

5.6

10.4

4.8

18.9 21.4

4.8

13.2 1.2

20.5

9.3

9.8

17.9 6.6

14.8 20.6

2,6-Dimethyl

1353

108

3.1

3.3

3.0

6.2

0.4

1.5

3.1

1.5

8.4

5.7

1.2

4.3

0.4

6.6

2.7

3.3

6.6

2.1

4.1

6.7

Ethyl

1359

107

3.9

4.3

4.0

7.1

1.2

2.5

4.5

2.6

7.9

7.8

2.8

5.6

1.1

7.5

3.8

4.7

8.9

3.5

5.4

7.2

2,3-Dimethyl

1371

108

1.0

1.1

1.1

1.6

0.3

0.6

1.0

0.6

2.8

2.8

0.5

1.5

0.2

2.1

0.8

1.3

2.0

0.7

1.4

2.1

2-Ethyl-6-methyl

1406

121

3.9

2.4

3.5

7.2

0.7

1.7

3.4

1.8

7.2

7.4

1.7

5.2

0.5

6.1

3.6

4.2

6.8

2.6

4.6

6.2

2-Ethyl-5-methyl

1413

121

3.6

2.4

4.2

6.7

0.9

2.5

4.2

2.2

5.9

10.0

2.3

5.5

0.7

7.9

4.0

4.0

7.5

2.6

5.9

8.4

Trimethyl

1425

42

1.7

1.1

2.0

2.9

0.3

1.0

1.6

1.0

4.2

4.2

0.8

2.3

0.3

3.6

1.8

1.7

3.2

1.1

2.7

4.3

2-Ethyl-3-methyl

1426

121

3.0

2.3

3.1

5.5

0.6

1.5

2.9

1.5

6.9

6.5

1.5

4.2

0.4

5.8

2.7

3.7

5.9

2.4

3.8

5.0

Vinyl

1461

106

0.6

1.2

0.5

1.0

0.1

0.3

0.8

0.3

1.8

1.3

0.4

1.0

0.2

1.4

0.5

0.6

1.7

0.5

0.9

1.2

3-Ethyl-2,5-dimethyl

1463

135

6.4

2.9

8.8

11.9 1.4

4.0

7.1

3.6

14.2 19.1

3.3

10.7 0.8

14.1

7.6

5.8

12.2 4.9

9.8

13.4

2-Methyl-5/6-vinyl

1512

120

1.4

1.4

1.3

3.2

0.2

0.6

1.7

0.5

3.7

3.4

0.6

2.4

0.2

2.9

1.4

1.1

3.4

0.9

2.1

3.3

2-Methyl-5/6-vinyl

1519

120

0.7

0.8

0.6

1.4

0.1

0.4

0.9

0.3

2.0

2.0

0.4

1.2

0.1

1.7

0.7

0.7

2.0

0.6

1.2

1.6

3-Vinyl-2,5-dimethyl

1560

133

1.0

0.6

1.4

2.4

0.1

0.6

1.5

0.6

2.7

3.9

0.6

1.9

0.2

2.6

1.5

1.2

2.9

0.9

2.0

2.9

46.1 39.8

52.3

87.2 9.7

25.7 105

109

25.7

70.3 7.9

99.3

47.3 50.9

TOTAL

27.7 51.9

a

98.9 36.5

69.6 97.6

Cultivar names and corresponding codes are listed in Experimental. b Identities of compounds in bold were confirmed using reference compounds run under the same conditions. c Values are relative to the GC-MS peak area of 1,2-dichlorobenzene added to the chips at 0.5 mg/kg, assuming a response factor of 1.

Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Table 2. Effect of Tuber Storage on Mean Total Alkylpyrazine Concentrations (mg/kg; n = 3) in Chips Prepared from 20 Potato Cultivars Grown in the United Kingdom 2-months storage

6-months storage

pa

Daisy

71.8

113

NS

Harmony

89.1

70.1

NS

Hermes

95.1

114

NS

King Edward

191

158

NS

Lady Claire

18.3

20.4

NS

Lady Rosetta

34.9

75.9

*

Maris Piper

115

93.0

NS

Markies

44.0

58.7

NS

Pentland Dell

222

197

NS

Russet Burbank

199

237

NS

Saturna

56.2

46.7

NS

Umatilla

150

131

NS

Verdi

15.3

16.1

NS

Desiree

214

184

NS

Ramos

89.7

99.5

NS

Challenger

86.0

118

NS

Innovator

150

245

*

Fontane

56.9

89.2

NS

Lady Olympia

192

86.8

NS

Lady Blanca

242

148.4

NS


cultivar

a

Significant effect of storage time on reducing sugar concentration (*p < 0.05; NS, not significant).

Lojzova et al. also tried to correlate pyrazines with acrylamide in potato chips but failed to do so as they used a non-polar HP-VOC fused silica capillary column (Agilent) to separate the pyrazines. They found that resolution of three isomeric pairs of pyrazines was not possible (9). The polar Supelcowax-10 column used in our work was able to separate all of the pyrazines from each other, even those of the same molecular mass, and provided good quality data. There was a highly significant effect of cultivar on the pyrazine content of the chips. Those potatoes that are used to make chips for commercial purposes were relatively low in pyrazines, with cultivars Verdi and Lady Claire always possessing the lowest two concentrations of each pyrazine. Lady Rosetta, Markies and Saturna were also consistently low in each pyrazine. Relatively 138 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


high producers of all pyrazines included Pentland Dell, Russet Burbank, Desiree, Innovator and Lady Bianca; none of these varieties are generally used for chip production. Cultivars that were low in pyrazines were generally low in reducing sugars and free amino acids and vice versa. However, there was one exception: Harmony behaved very differently from the other cultivars, as it was very high in reducing sugars, yet possessed average levels of free amino acids. It was the only cultivar to possess high levels of one pyrazine (vinylpyrazine) and low levels of another (3-ethyl-2,5-dimethylpyrazine), with intermediate values for the rest. Cultivars Lady Rosetta and Innovator showed a significant increase in virtually all 14 pyrazines as a result of storage (p < 0.05), while few other cultivars were affected by storage. It might be expected that this increase was due to the significant increase in reducing sugars that had occurred in these cultivars during storage (10.8 to 29.7 mmol/kg in Lady Rosetta and 45.3 to 84.2 mmol/kg in Innovator). However, sugar levels had also significantly increased in Daisy (31.5 to 58.4 mmol/kg), Markies (10.3 to 18.3 mmol/kg) and Ramos (18.6 to 37.61 mmol/kg) at 6 months without this being reflected in changes in pyrazine levels. Table 2 shows the effect of storage on total pyrazine levels in the 20 cultivars.

Correlations between Alkylpyrazines and Acrylamide Table 3 is a matrix showing the correlations between all the pyrazines and also the correlations between the pyrazines and acrylamide in the 120 samples analyzed. All correlations (r2) were highly significant (p < 0.0001), although it is clear that correlations between acrylamide and the pyrazines (r2 = 0.41–0.71) are weaker than those between the 14 pyrazines (r2 = 0.61–0.97). Acrylamide formation in heated potato products is generally thought to be related to levels of reducing sugars in the raw material. However, when levels of reducing sugars exceed those of total free amino acids, as is the case with cultivar Harmony in this sample set, this relationship breaks down. In fact, for Harmony, acrylamide values were substantially less than would be expected. As the precursor composition of Harmony is at odds with those of the other cultivars, it was decided to perform the correlation analysis again but without Harmony present. When it was removed from the data set, all correlations between the acrylamide and the pyrazines increased but the greatest correlation value (r2) was still only 0.751 for 2,6-dimethylpyrazine. Some increases also occurred within the pyrazines: the lowest value of r2 within the pyrazines for methyl vs 3-vinyl-2,5-dimethyl increased from 0.610 to 0.638, while the highest value of r2 for 2,6-dimethyl vs 2,3-dimethyl showed no increase in r2. A plot of acrylamide against total pyrazines is shown in Figure 1 and it can be seen that the 6 chip samples made from cultivar Harmony are outliers. When the Harmony samples were removed from this plot, r2 increased from 0.609 to 0.679. The relationship between acrylamide and reducing sugars, excluding cultivar Harmony (r2 = 0.728, p < 0.0001), was substantially stronger than the relationship between total pyrazines and total sugars (r2 = 0.469, p < 0.0001; data not shown). 139 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

140


Table 3. Correlations (r2) between 14 Alkylpyrazines and Acrylamide in Chips Prepared from 20 Potato Cultivars Grown in the United Kingdom Methyl

Methyl

1.00

2,5-Dimethyl

0.87

2,5-Dimethyl

2,6-Dimethyl

Ethyl

2,3-Dimethyl

2-Ethyl-6methyl

2-Ethyl-5methyl

Trimethyl

2-Ethyl-3methyl

Vinyl

3-Ethyl2,5dimethyl

2-Methyl-5/6vinyl

2-Methyl-5/6vinyl

3-Vinyl2,5dimethyl

Total pyrazines

Acrylamide

1.00

2,6-Dimethyl

0.92

0.88

1.00

Ethyl

0.95

0.92

0.87

1.00

2,3-Dimethyl

0.74

0.79

0.75

0.77

1.00

2-Ethyl-6-methyl

0.83

0.92

0.90

0.91

0.76

1.00

2-Ethyl-5-methyl

0.69

0.88

0.70

0.82

0.67

0.84

1.00

Trimethyl

0.80

0.95

0.85

0.87

0.81

0.90

0.86

1.00

2-Ethyl-3-methyl

0.90

0.93

0.91

0.95

0.81

0.97

0.81

0.91

1.00

Vinyl

0.84

0.73

0.78

0.85

0.70

0.74

0.64

0.73

0.80

1.00

3-Ethyl-2,5-dimethyl

0.69

0.89

0.72

0.79

0.75

0.86

0.87

0.91

0.86

0.65

1.00

2-Methyl-5/6-vinyl

0.71

0.80

0.84

0.75

0.68

0.86

0.73

0.82

0.81

0.77

0.79

1.00

2-Methyl-5/6-vinyl

0.79

0.86

0.83

0.87

0.76

0.88

0.81

0.88

0.89

0.90

0.83

0.92

1.00

3-Vinyl-2,5-dimethyl

0.61

0.82

0.69

0.74

0.68

0.84

0.86

0.87

0.80

0.68

0.90

0.86

0.91

1.00

Total pyrazines

0.90

0.98

0.91

0.95

0.82

0.96

0.88

0.96

0.97

0.81

0.91

0.86

0.92

0.86

1.00

Acrylamide

0.69

0.58

0.71

0.55

0.56

0.50

0.43

0.59

0.57

0.64

0.47

0.58

0.57

0.41

0.61

Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

1.00


Figure 1. Relationship between total pyrazine content and acrylamide content in 120 potato chip samples (cultivar Harmony is plotted as circles) These results suggest that pyrazine formation in potato chips is not directly proportional to acrylamide formation. Cultivars that give chips low in acrylamide are obviously of interest to manufacturers. However, low-acrylamide cultivars where pyrazines are formed at relatively high levels compared to acrylamide may be even more desirable, as they may be perceived as being more flavorsome than varieties where pyrazine formation is relatively low. For example, 3-ethyl-2,5-dimethylpyrazine has long been regarded as a key contributor to potato chip aroma (10). Table 4 shows the acrylamide/total pyrazine ratio (A/TP; acrylamide measured in μg/kg, total pyrazines measured in mg/kg) for chips made from the 20 cultivars after both storage periods. The effect of cultivar on A/TP was highly significant ( 0.05)

Conclusions The correlation between acrylamide and total alkylpyrazine formation in potato chips from 20 UK-grown cultivars was highly significant, although it may not be strong enough to be used as a predictor of acrylamide formation. In addition, there were no individual pyrazines that could be used as predictors. 142 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

It may be a useful exercise to relate acrylamide levels to those of other Maillard-derived compounds that are considered important in chip flavor, such as methional and 3-methylbutanal (11), although because these compounds are Strecker aldehydes formed relatively early in the Maillard reaction, it is likely that they will react further, to give compounds such as pyrazines (12).


Acknowledgments The study was financially supported by the Biotechnology and Biological Sciences Research Council (BBSRC) of the United Kingdom and industry partners through the Sustainable Arable LINK programme ‘Producing Low Acrylamide Risk Potatoes’ (http://www.acrylamide-potato.org.uk/).

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