Evaluation of Key Aroma Compounds in Processed Prawns (Whiteleg

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Evaluation of Key Aroma Compounds in Processed Prawns (Whiteleg Shrimp) by Quantitation and Aroma Recombination Experiments Veronika Mall, and Peter Schieberle J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b00636 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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Journal of Agricultural and Food Chemistry

Evaluation of Key Aroma Compounds in Processed Prawns (Whiteleg Shrimp) by Quantitation and Aroma Recombination Experiments

Veronika Mall and Peter Schieberle#

Deutsche Forschungsanstalt für Lebensmittelchemie, Lise-Meitner-Straße 34, D85354 Freising, Germany

#

Corresponding Author Prof. Dr. Peter Schieberle Phone: +49 8161 71 2932 Fax: +49 8161 71 2970 E-mail: [email protected]

ACS Paragon Plus Environment


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ABSTRACT. In our previous study on the aroma compounds of heated prawn meat,

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the main odorants in blanched (BPM) and fried prawn meat (FPM), respectively,

3

were characterized by means of gaschromatography/olfactometry and aroma extract

4

dilution analysis. In this follow-up study, these aroma compounds were quantified by

5

means of stable isotope dilution assays and odor activity values (OAV; ratio of

6

concentration to odor detection threshold) were calculated. Results revealed 2-

7

acetyl-1-pyrroline and (Z)-1,5-octadien-3-one as the most potent odor-active

8

compounds in both prawn samples. In FPM, as compared to BPM, higher OAVs

9

were determined for 2-acetyl-1-pyrroline, 2-acetyl-2-thiazoline, 3-methylbutanal, 3-

10

(methylthio)propanal, phenylacetaldehyde, 3-hydroxy-4,5-dimethyl-2(5H)-furanone,

11

4-hydroxy-2,5-dimethyl-3(2H)-furanone,

12

trimethylpyrazine. Aroma recombination experiments corroborated that the overall

13

aroma of the blanched as well as the fried prawn meat, respectively, could well be

14

mimicked by the set of key odorants quantitated in this study.

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KEYWORDS. stable isotope dilution assay; odor activity value; aroma recombinate;

16

crustaceans,

prawns;

2,3-diethyl-5-methylpyrazine

litopenaeus


and

vannamei.

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17

INTRODUCTION

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Crustaceans and among them, in particular prawns have become quite popular

19

in the last decades as healthy and delicious food. In order to characterize the aroma

20

compounds eliciting the desirable aroma of heated prawn meat, the main odorants of

21

blanched (BPM) and fried prawn meat (FPM) were recently identified by us based on

22

results from aroma extract dilution analysis (AEDA).1 Following this approach, forty-

23

three or forty-five odorant compounds respectively were found in BPM and FPM with

24

2-acetyl-1-pyrroline,

25

2-aminoacetophenone featuring the highest flavor dilution (FD) factors in both

26

samples. Among the set of fifteen compounds only occurring after heat treatment in

27

FPM were 2,3-diethyl-5-methylpyrazine and trimethylpyrazine, γ-octalactone and

28

γ-nonalactone as well as 2,6-dimethoxyphenol.1

(Z)-1,5-octadien-3-one,

3-(methylthio)propanal

and

29

AEDA is a useful screening method to locate aroma-active compounds in a bulk

30

of odorless volatiles. Ranking the odorants by means of FD factors gives an idea on

31

the impact of each aroma compound on the overall aroma of the investigated food

32

sample. However, as the odorants are completely vaporized during the procedure

33

and, thus, matrix effects are not taken into account, quantitation of the aroma

34

compounds and calculation of odor activity values should always follow this

35

screening approach.2,3

36

Until now, only few investigations exist aiming at quantitating the aroma

37

compounds of crustacean meat. In spiny lobster, American lobster and mangrove

38

crab, respectively, quantitation of odorants was carried out using internal standards,

39

and the OAVs of several odorants were calculated using odor thresholds available

40

from earlier publications.4-6 In these studies the highest OAVs were found for 2-

41

acetyl-1-pyrroline, 2,3-butanedione, trimethylamine, 3-(methylthio)propanal, (Z)-4-



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heptenal and 3-methylbutanal. However, exact quantitation methods using e.g. stable

43

isotope labeled isotopologues as internal standards have not yet been applied on the

44

aroma compounds of crustacean meat. Therefore, the aim of this study was to

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continue our recent investigation1 on the key aroma compounds of blanched and

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fried prawn meat with their quantitation by means of stable isotope dilution assays

47

and subsequent calculation of OAVs. Following, aroma recombination experiments3

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should allow the unequivocal characterization of the main odorants provoking the

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desired roasted and fishy odor of heated prawn meat.

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MATERIALS AND METHODS

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Material. Frozen whiteleg shrimp (Litopenaeus vannamei), raw with head and

52

carapace, were purchased at a local supermarket. After thawing, the prawns were

53

cooked in unsalted water for one min. The carapace was removed, the meat was

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frozen with liquid nitrogen and minced to a fine powder. In a second set of

55

experiments, thawed prawns were pan-fried at 160 °C without addition of fat. After six

56

min of evenly frying both sides, the prawns were cooled and their carapace was

57

removed. Then, the meat was frozen with liquid nitrogen and minced to a fine

58

powder.

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Chemicals. The following chemicals were obtained from the sources given in

60

parentheses. Dichloromethane, diethyl ether, ethanol, (99 %), sodium hydroxide,

61

anhydrous sodium sulfate (Merck, Darmstadt, Germany); thiourea (Fluka, Neu-Ulm,

62

Germany); [2H3]-methyl iodide, rhodium on alumina, methanol-d, deuterium oxide,

63

palladium on barium sulfate (5%) (Sigma-Aldrich Chemie, Taufkirchen, Germany).

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Dichloromethane and diethyl ether were freshly distilled before use.

65 66

Reference Odorants. These were purchased from commercial sources or were synthesized as reported recently.1 ACS Paragon Plus Environment

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Isotopically Labeled Internal Standards. The isotopically labeled internal

68

standards, either labeled with deuterium or carbon-13, were synthesized as

69

previously described: [2H2–5]-2-acetyl-1-pyrroline,7 [2H4]-2-acetyl-2-thiazoline,8 [2H2-4]-

70

2-aminoaceto-phenone,9 [2H2]-butanoic acid,10 [2H2–7]-(E,E)-2,4-decadienal,11 [2H2]-γ-

71

decalactone,12 [2H2]-decanoic acid,13 [2H3–4]-2,3-diethyl-5-methylpyrazine,8 [2H3]-5-

72

ethyl-3-hydroxy-4-methyl-2(5H)-furanone,14

73

furanone,14

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methoxybenzaldehyde,16

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[2H2]-3-methylbutanoic

76

2,4-nonanedione,19

77

[2H2]-(E,Z)-2,6-nonadienal,11 [13C2]-(E,E,Z)-2,4,6-nonatrienal,22 [2H2]-(Z)-1,5-octadien-

78

3-one,11

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acetaldehyde,24 [2H3]-trimethylpyrazine.25 [2H2]-(Z)-4-Heptenal was obtained from

80

Aromalab

81

[2H5-8]-2,6-dimethoxyphenol were made available by introducing deuterium via H/D

82

exchange to the unlabeled reference compound.26

[13C2]-3-hydroxy-4,5-dimethyl-2(5H)-

[13C2]-4-hydroxy-2,5-dimethyl-3(2H)-furanone,15 [2H3]-2-methoxyphenol,8 acid,17

[2H2]-1-octen-3-one,11

AG

(Planegg,

[2H2]-3-methyl-butanal,17

[2H6–8]-3-methylindole,18

[2H3]-3-(methylthio)propanal,20

[2H3]-3-methyl-

[2H3]-(E,E)-2,4-nonadienal,21

[13C2]-2,3-pentanedione,23

Germany).

[2H3]-4-hydroxy-3-

[13C2]-phenyl-

[2H1-3]-Benzothiazole

and

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Synthesis of [2H3]-2-Acetylpyridine. [2H3]-2-Acetylpyridine was synthesized

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according to the route outlined previously.27 2-Acetylpyridine (0.1 g; 0,83 mmol) was

85

added to a solution of 1,5,7-triazybicyclo[4.4.0]dec-5-ene (11.5 mg; 0083 mmol) in

86

chloroform-d (3 mL) which was let stir for 12 h at room temperature. The H-D-

87

exchange was quenched by the addition of hydrochloric acid (1 mL; 1 mol/L). The

88

organic layer was then washed with water (2 x 2 mL) and saturated brine (1 mL)

89

before drying over anhydrous sodium sulfate and purifying by means of SAFE-

90

distillation. Stabilization tests showed that this standard should only be used in

91

neutral conditions to avoid H-D-exchange.



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Synthesis of [2H1-3]-Benzothiazole. A mixture of benzothiazole (0,135 g; 1 mmol),

93

palladium on barium sulfate (5%; 0,25 g, equates 12,5 mg Pd) and deuterium oxide

94

(1,5 mL) were placed in a small autoclave, sealed and heated at 160 °C for seven d

95

in an aluminum heating block. Afterwards, the content of the autoclave tube was

96

filtered through a glass frit (G4), washed with deionized water (3 x 2 mL) followed by

97

diethyl ether (3 x 2 mL). After separation, the aqueous phase was extracted with

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diethyl ether (3 x 25 mL) and the organic layers, containing the deuterated reaction

99

product were combined and dried over anhydrous sodium sulfate.27

100

Synthesis of [2H5–8]-2,6-Dimethoxyphenol. A mixture of 2,6-dimethoxyphenol

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(0,154 g; 1 mmol), palladium on barium sulfate (5%; 0,25 g, equates 12,5 mg Pd)

102

and deuterium oxide (1,5 mL) were placed in a autoclave, sealed and heated at 160

103

°C for seven d in an aluminum heating block. Purification of the raw product was

104

carried out as described above.27

105

Synthesis of [2H3]-Methanethiol. A mixture of [2H3]-methyl iodide (2.68 g;

106

19 mmol) and thiourea (1.52 g; 20 mmol) was dissolved in ethanol/ water (1:1, by

107

vol.; 20 mL) and refluxed for 12 h. Afterwards, 100 µL of the reaction mixture was

108

added to an aqueous solution of sodium hydroxide (2 mol/L; 200 µL) in a gas tight

109

vessel sealed with a septum. After 20 min, defined headspace volumes were

110

withdrawn with a gas tight syringe and subjected to headspace-HRGC/MS (Figure 1).

111

Because of its high volatility, the isotopically labeled internal standard [2H3]-

112

methanethiol was synthesized freshly on the day of analysis.

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Quantitation of Odorants. Freshly prepared, powdered fried or blanched prawn

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meat was mixed with diethyl ether and spiked with defined amounts of the labeled

115

internal standards depending on the respective amount of the analyte present in the

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sample which had been estimated in a preliminary experiment. The mixture was

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extracted for 2 h while stirring, before the extract was subjected to SAFE distillation28 ACS Paragon Plus Environment

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and subsequent concentration via vigreux column and microdistillation to 200 µL.1

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For the quantitation of 3-(methylthio)propanal and 3-hydroxy-4,5-dimethyl-2(5H)-

120

furanone, the extract was divided into acidic and neutral/basic fraction prior to

121

analysis to avoid coelution of compounds.1

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The samples were subjected to either HRGC/MS or two-dimensional HRGC-

123

HRGC/MS and the intensities of the respective ions given in Table 1 were monitored.

124

Concentrations were calculated from the relative abundances of ions selected for

125

analyte and internal standard. The data obtained were corrected by calibration

126

factors which were determined by analyzing mixtures of defined amounts of the

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unlabeled aroma compound and the corresponding labeled standard in ratios ranging

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from 5:1 to 1:5.

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High Resolution Gas Chromatography/Mass Spectrometry (HRGC/MS). The

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system described in the following was used for the quantitation of 2- and

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3-methylbutanoic acid and decanoic acid. A Varian 3800 gas chromatograph

132

(Agilent, Waldbronn, Germany) was combined with an ion trap detector type Saturn

133

2000 (Agilent). The samples were injected cold-on-column on a DB-FFAP capillary

134

(30 m × 0.25 mm i.d.; 0.25 µm film thickness) (J&W Scientific, Folsom, CA) with a

135

starting temperature of 40 °C (held isothermally for 2 min), followed by a temperature

136

gradient of 6 °C/min up to 230 °C (held isothermally for 5 min). The characteristic

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ions were monitored in the chemical ionization mode (MS-CI) with methanol as

138

reactant gas.

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Two-Dimensional

High

Resolution

Gas

Chromatography/Mass

140

Spectrometry (HRGC-HRGC/MS). For the quantitation of all other odorants (except

141

methanethiol, trimethylamine and ammonia), two-dimensional HRGC-HRGC/MS was

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applied using a Trace 2000 series gas chromatograph (Thermo Fisher Scientific,

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Braunschweig, Germany) equipped with a moving capillary stream switching system ACS Paragon Plus Environment


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(MCSS) (Fisons Instruments, Mainz-Kastel, Germany) and linked to a second gas

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chromatograph CP 3800 (Agilent) coupled with an ion trap mass spectrometer Saturn

146

2000 (Agilent). Samples were injected cold-on-column and after chromatography on

147

the first capillary, the analyte of interest and the internal standard were transferred

148

into a cold trap (-100 °C) by means of the MCSS system. By heating the trap, the

149

sample was transferred to the second capillary. Analytes were determined by means

150

of MS-CI with methanol as the reactant gas. In the first oven, a fused silica capillary

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DB-FFAP (30 m × 0.32 mm i.d.; 0.25 µm film thickness) (J&W Scientific) was used in

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combination with a DB-1701 (30 m × 0.25 mm i.d.; 0.25 µm film thickness) (J&W

153

Scientific) in the second oven. The oven temperature was held at 40 °C for 2 min,

154

then raised at 6°C/min to 230 °C and finally held for another 5 min. The cut time

155

intervals in the first dimension were determined by injection of the reference

156

compounds in a previous experiment.

157

Quantitation of Methanethiol by Means of Static Headspace Analysis.

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Methanethiol was determined by means of SIDA of static headspace samples using

159

a headspace-HRGC/MS as described previously.29 The freshly prepared fried or

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blanched prawn meat (20 g), was ground and put in a headspace vial (100 mL)

161

sealed with an airtight septum. The sample was spiked with a defined volume of the

162

labeled [2H3]-methanethiol (200 – 1000 µL= 0.04 – 0.21 µg) by injection through the

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septum using a gastight syringe. After equilibration while stirring for 30 min at 40 °C

164

the sample was subjected to HS-HRGC/MS using a Trace GC Ultra (Thermo Fisher

165

Scientific) connected to an ion trap detector Varian 2100 T (Agilent). The headspace

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sample was injected with a gas tight syringe. The gas chromatograph featured a Cold

167

Trap 915 (Thermo Fisher Scientific), cooled with liquid nitrogen, where the volatile

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aroma compounds were trapped and co-injected air could be purged. A DB-5 thick

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film capillary column (30 m × 0.25 mm i.d.; 1.0 µm film thickness) (J&W Scientific) ACS Paragon Plus Environment

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was held at a starting temperature of 0 °C for 2 min, before rising the temperature at

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6 °C/min up to 100 °C, followed by a temperature gradient of 40 °C/min up to 240 °C

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(held for 5 min).

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Differentiation of 2- and 3-Methylbutanoic Acid. The differentiation of

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2-methylbutanoic acid and 3-methylbutanoic acid was carried out by means of mass

175

spectrometry in the EI mode as described previously.30 Using [2H2]-3-methylbutanoic

176

acid as internal standard, the total of both isomers was quantitated by monitoring

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mass trace m/z 85 and the standard ion m/z 87. Since 3-methylbutanoic acid

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features a much lower odor threshold of 490 µg/kg compared to 2-methylbutanoic

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acid (2200 µg/kg),31 a differentiation is crucial for their contribution to the overall

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aroma. Thus, the isomer ratio was determined by means of HRGC/MS using the

181

characteristic fragments obtained by electron impact ionization (MS-EI; m/z 60 for

182

3-methylbutanoic acid and m/z 74 for 2-methylbutanoic acid, respectively) to be

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45% 3-methylbutanoic acid and 55% 2-methylbutanoic acid.

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Ion Chromatography. Determination of the amount of trimethylamine was

185

carried out by use of an ion chromatography system ICS-2000 with an AS

186

autosampler (Dionex, Sunnyvale, USA). Freshly prepared blanched or fried prawn

187

meat (50 g), respectively, was minced, mixed with double distilled water (80 mL),

188

stirred for 2 h and then subjected to a careful SAFE distillation.28 The aqueous

189

distillate obtained was then filled up to 100 mL in a measuring flask with doubly

190

distilled water and directly subjected to ion chromatography. The stationary phase

191

was a cation exchange column IonPac CS18 250 × 2 mm (Dionex) with a precolumn

192

IonPac CG18 50 × 2 mm (Dionex), heated to 40 °C. An aqueous solution of methane

193

sulfonic acid was generated by the eluent generator and used as mobile phase with a

194

flow rate of 0.3 mL/min. The following gradient was used for optimum detection of

195

trimethylamine and minimum matrix interferences: starting with a concentration of 3 ACS Paragon Plus Environment


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mM methanesulfonic acid for 10 min, raising the concentration to 10 mM in the

197

following 5 min, and again to 15 mM in the following 10 min. After the run, the

198

concentration was lowered to 3 mM to reach the starting conditions again. Analytes

199

were detected by means of suppressed conductivity detection (data collection rate

200

5.0 Hz) using the integrated conductivity detector and an electrolytic suppressor cell

201

(Dionex CSRS 300 2mm), running in auto-recycling mode with a suppression current

202

of 14 mA. Since ion chromatography featured good linearity, quantitation was carried

203

out by means of external calibration. Aqueous solutions of trimethylamine with

204

increasing concentrations (0.4 – 10 mg/L) were subjected to ion chromatography and

205

correlated to the peak areas.

206

Enzymatic Assay. For the determination of free ammonia in processed prawn

207

meat, an enzymatic bio analysis kit (Boehringer Mannheim/R-Biopharm, Darmstadt,

208

Germany) was used. The reaction of 2-oxogutarate and ammonia in the presence of

209

glutamate dehydrogenase (GDH) and reduced nicotine-amide-adenine dinucleotide

210

(NADH) was used for the assay. The amount of the oxidized NAD+, stoichiometric to

211

the amount of free ammonia was determined photometrically.

212

Sensory Evaluation. Sensory analyses were carried out in a sensory room

213

designed for this purpose with individual sections for each panelist. The room

214

temperature was adjusted to 20 – 25 °C and analyses were carried out in tinted light.

215

Sensory analyses were carried out by a trained panel consisting of 15 to 21 panelists

216

(male and female, age 24 to 41), who had participated in weekly sensory sessions to

217

train their ability to recognize and describe different aroma qualities.

218

Aroma Profile Analyses. The following reference substances were used as

219

aroma descriptors: metallic ((Z)-1,5-octadien-3-one), seasoning-like (3-hydroxy-4,5-

220

dimethyl-2(5H)-furanone), roasted, popcorn-like (2-acetyl-1-pyrroline), cucumber-like

221

((E,Z)-2,6-nonadienal), fishy ((Z)-4-heptenal), fatty ((E,E)-2,4-decadienal), caramelACS Paragon Plus Environment

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like

(4-hydroxy-2,5-dimethyl-3(2H)-furanone),

cooked

potato-like

223

(3-(methylthio)propanal), and fishy, rotten (trimethylamine). The odor attribute sea

224

breeze-like was rated without a reference substance. For the aroma evaluation of

225

fried and blanched prawn meat, respectively, the samples were freshly prepared (20

226

g) and kept in glass vessels at 60 °C in a water bath before evaluation. For aroma

227

profile analyses, the intensities of the respective aroma qualities were ranked on a

228

scale from 0 to 3 (0 = not perceivable, 3 = very high intensity) in steps of 0.1.

229

Determination of Odor Thresholds. Odor thresholds in water were determined

230

following a previously published protocol.31 For evaluation of the impact of the basic

231

compounds trimethylamine and ammonia, their thresholds were determined in a

232

phosphate buffer solution adjusted to the natural pH 6.9 of processed prawn meat

233

following the same protocol.

234

Aroma Recombination Experiments. Aroma models of blanched and fried prawn

235

meat, respectively, were prepared containing all odorants with odor activity values ≥

236

1 in their actual concentrations in an aqueous matrix adjusted to the natural pH 6.9 of

237

prawn meat by means of phosphate buffer. Exceptions were trimethylamine and

238

2-acetyl-1-pyrroline, which were added in their 3 fold and 5 fold concentration to the

239

aroma models, respectively, on the basis of preliminary sensory experiments (Tables

240

2 and 3).

241

The aroma model and the freshly prepared food sample were each placed in

242

closed glass vessels (20 g each) and presented to the panelists who were asked to

243

evaluate the recombinate in the same way as described above for the aroma profile

244

analysis. Additionally, the panelists were asked to rate the overall similarity of the

245

model to the original sample on a scale from 0 to 3 (0 = no similarity, 1 = slightly

246

similar, 2 = the food sample can be recognized, 3 = high similarity) in steps of 0.1.



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Omission Experiments. In further mixtures of blanched and fried prawn meat

248

odorants, several compounds were omitted and the models were again presented to

249

the sensory trained panel (Table 3).

250

251

Triangle Test. Triangle tests were carried out according to ISO 4120:2004.

RESULTS AND DISCUSSION

252

Quantitation of Key Aroma Compounds in Blanched (BPM) and Fried (FPM)

253

Prawn Meat. In our previous study,1 by application of aroma extract dilution analysis

254

(AEDA) and subsequent identification experiments 43 or 45 odor-active compounds,

255

respectively, were found in BPM and FPM. These odorants featuring high FD factors

256

in our previous study were now selected for stable isotope dilution assays (Table 1).

257

Additionally, an enzymatic assay to determine the content of free ammonia and an

258

ion chromatographic assay for the quantitation of trimethylamine were carried out.

259

Thus, 35 or 36 odorants, respectively, were quantitated in BPM and FPM.

260

Application of stabile isotope dilution assays to the processed prawn meat

261

samples revealed decanoic acid (BPM: 240 µg/kg; FPM: 810 µg/kg) and butanoic

262

acid (BPM: 450 µg/kg; FPM: 610 µg/kg) with the highest amounts in both samples

263

(Table 2). Compounds determined in concentrations ranging from 100 to 10 µg/kg

264

were 2- methylbutanoic acid (BPM: 34 µg/kg; FPM: 30 µg/kg) and 3-methylbutanoic

265

acid (BPM: 28 µg/kg; FPM: 50 µg/kg), 4-hydroxy-3-methoxybenzaldehyde (BPM: 28

266

µg/kg; FPM: 32 µg/kg), 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone (BMP: 17 µg/kg;

267

FPM: 9.8 µg/kg) and the isomers of methylbutanal (2-methylbutanal: BMP: 13 µg/kg;

268

FPM: 25 µg/kg; 3-methylbutanal: BMP: 12 µg/kg; FPM: 23 µg/kg).

269

On the other hand, some aroma-active compounds were found in very small

270

amounts (

Evaluation of Key Aroma Compounds in Processed Prawns (Whiteleg

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