bk-2016-1237.ch001

Chapter 1

Downloaded by 80.82.78.170 on December 11, 2016 | http://pubs.acs.org Publication Date (Web): November 18, 2016 | doi: 10.1021/bk-2016-1237.ch001

On the Role of Amadori Rearrangement Products as Precursors of Aroma-Active Strecker Aldehydes in Cocoa Sandra Hartmann and Peter Schieberle* Deutsche Forschungsanstalt für Lebensmittelchemie, Lise-Meitner Strasse 34, Freising, Germany *E-mail: [email protected]; phone: +498161 712932; fax: +498161 712970

The unique aroma of cocoa develops during fermentation and roasting. In particular, amino acids are well known to thermally generate the so-called Strecker aldehydes. In addition, 1-amino-1-desoxyketoses, known as Amadori rearrangement products (ARPs), are also suggested as key intermediates in the formation of these odorants. To study their role as precursors of cocoa aroma compounds, selected ARPs as well as the respective amino acids were monitored during different processing steps in order to correlate their amounts with the formation of the corresponding Strecker aldehydes. The amounts of ARPs formed during cocoa fermentation and a comparison with the amounts of Strecker aldehydes generated after roasting showed a good correlation between ARP formation/degradation and Strecker aldehydes formation. Interestingly, the ARPs were already available in unfermented cocoa beans. To unequivocally elucidate the role of ARPs as precursors of the Strecker aldehydes in “competition” with the free amino acids, an isotope enrichment experiment was performed. The obtained data clearly suggested the ARPs as effective precursors of Strecker aldehydes.

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


Introduction Next to taste, the aroma is one of the most important quality attributes of cocoa. However, the fine flavor does not only depend on the genetics and the growing conditions, but is significantly influenced by fermentation, drying, and roasting processes (1–3). During the thermal treatment of cocoa, a non-enzymatic browning reaction, the so-called “Maillard reaction”, becomes predominant and leads to sensory changes due to the formation of organic volatiles (4–7). During this reaction, also Strecker aldehydes are generated by oxidative transamination and decarboxylation of the parent amino acids (8, 9). Next to the amino acids, also Amadori rearrangement products (ARPs) have been verified as potential precursors of the Strecker aldehydes (10, 11). For example, Weigl performed a fractionation of aroma precursors in properly fermented, unroasted cocoa (12). Each fraction was added to coconut fat, then roasted and the concentrations of the formed Strecker aldehydes were determined. In the low molecular fraction of the extract, he found high amounts of the odorants after heating and identified ARPs as their possible precursors. The aim of this work was to study the formation of selected Strecker aldehydes in correlation with the generation and degradation of their possible precursors in order to get a better insight into the aroma formation during cocoa processing. For this purpose, in particular the question when and from which intermediate Strecker aldehydes are formed during different cocoa processing steps was clarified.

Materials and Methods Cocoa Beans Cocoa beans from the Dominican Republic which were fermented in plastic boxes, sun-dried before and after roasting were kindly provided by a food manufacturer. The unfermented, 1-, 3-, and 5-day fermented as well as the respective roasted cocoa samples were available for analyses. Chemicals The following compounds were obtained from commercial sources: acetone capillary GC grade ≥ 99.9%, benzolylchloride, L-isoleucine, L-leucine, L-methionine, L-phenylalanine, 2-methylbutanal, 3-methylbutanal, 3-(methylthio)propanal, phenylacetaldehyde, trichloroacetic acid, [2H8]-Lvaline (Sigma-Aldrich Chemie, Steinheim, Germany). Methanol 99.8%, dichloromethane p.A., diethyl ether p.A., and sodium sulfate p. A. (Merck, Darmstadt, Germany). [13C6,15N]-L-isoleucine, [2H3]-L-leucine, [2H3]-L2 methionine, and [ H5]-L-phenylalanine, (Cambridge Isotope Laboratories, Saarbruecken, Germany). [2H2]-3-Methylbutanal (Dr. Ehrenstorfer, Augsburg, Germany). The following isotopically labeled standards were synthesized according to the given references; [2H2]-2-methylbutanal (13), [2H3]-3(methylthio)propanal (14), and [13C2]-phenylacetaldehyde (15). The unlabeled Amadori rearrangement compounds N-(1-deoxy-d-fructosyl)-L-isoleucine, 2 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

N-(1-deoxy-d-fructosyl)-L-leucine, N-(1-deoxy-d-fructosyl)-L-methionine, N-(1-deoxy-d-fructosyl)-L-phenylalanine, their [13C6] labeled isotoplogues, as well as the ARPs containing the respective labeled amino acid [13C6,15N]-Lisoleucine, [2H3]-L-leucine, [13C,2H3]-L-methionine, and [2H5]-L-phenylalanine were synthesized according to (16, 17). As an example, the abbreviation Fru-Leu is applied to assign the Amadori reaction product derived from D-glucose and L-leucine and [13C6]-Fru-Leu is used for the labeled isotopologue. Quantitation of Strecker Aldehydes


Sample Preparation Cocoa was homogenized using a freezer mill (SPEX freezer mill 6870, SPEX SamplePrep, Metuchen, NJ). Afterwards, the samples were suspended in 250 mL diethyl ether and the isotopically labeled aldehydes were added. After stirring for 2 h, the extract was filtered off and dried over anhydrous sodium sulfate. A solvent assisted flavor evaporation (SAFE) (18) was applied in order to eliminate non-volatile compounds. Finally, the distillate was concentrated using a Vigreux column (60 x 1 cm). This procedure was carried out at least in a quadruplicate.

Mass Spectral Analyses For mass spectral analyses, a Leco Pegasus 4D GCxGC-TOF/MS instrument (St. Joseph, MI) was applied, consisting of an Agilent Technologies GC model 7890A (Böblingen, Germany), a dual-stage quad-jet thermal modulator and a secondary oven coupled to a time-of-flight mass spectrometer providing unit mass resolution. The GC oven was operated by the Leco Chroma TOF software (version 4.50.8.0) and the autosampler by Maestro (Maestro 1 version 1.4.16.9, Gerstel GmbH & Co.KG, Mühlheim, Germany). In the first dimension, a DB-FFAP column (30 m x 0.25 mm i.d., film thickness 0.25 µm, Agilent J&W GC columns, Waldbronn, Germany) equipped with a deactivated pre-column (2 m x 0.32 mm i.d.) was used, while the second dimension was equipped with a OV-1701 column (2.5 m x 0.18 mm i.d., film thickness 0.18 µm, Agilent J&W GC columns). Mass spectra were acquired within m/z 35-250 in the EI modus, at a rate of 100 spectra/s. The temperature of the ion source was 230 °C, the detector voltage showed 1650 V, the modulation time was 4 s, and the energy was -70 eV. Data were evaluated by means of GC Image and GC Project (version 2.2b0, Lincoln, NE).

Analysis of 2- and 3-Methylbutanal For the SPME-measurement, a Supelco StableFlexTM SPME Fiber 65 µm PDMS-DVB Coating was used. The SPME agitator front inlet was 1 cm fiber and was extracted for 15 min (40 °C agitator temperature). The primary oven temperature was programmed: 35 °C (8 min) at 12 °C/min to 230 °C (2 min). The 3 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

secondary oven started at 45 °C (8 min) and was raised by 12 °C/min to 240 °C (2 min).

Analysis of 3-(Methylthio)propanal and Phenylacetaldehyde


A PTV injection port (Thermo Fisher Scientific, Dreieich, Germany) was used in the splitless mode and was operated by a multipurpose autosampler (Gerstel GmbH & Co.KG). The primary oven temperature was programmed: 40 °C (2 min) at 6 °C/min to 230 °C (7 min). The secondary oven started at 60 °C (2 min) and was raised by 6 °C/min to 240 °C (7 min). Quantitation of Amino Acids Sample Preparation The amino acids were quantitated according to (19). Cocoa was homogenized with a freezer mill (SPEX freezer mill 6870). Trichloroacetic acid (10%; 20 mL) as well as a defined amount of the labeled amino acid was added, stirred for 1 h and afterwards, the extract was filtered off. The pH value of the aqueous solution was adjusted to 10 by means of an aqueous sodium hydroxide solution (2 M). The derivatization reagent benzoylchloride solution (0.03 M; 5 mL) was added and the mixture was again stirred for 2 h at room temperature. Afterwards the pH was adjusted to 3 with hydrochloric acid. The benzamides formed were extracted 3-times with dichloromethane and the combined organic phase was dried over anhydrous sodium sulfate. The solvent was removed by means of a rotary evaporator. Afterwards, the residue was dissolved in a mixture of water and acetonitrile (8:2, v/v) and was filtered off by a PTFE-filter (Spartan 13/0.45 µm RC, Whatman, Vienna, Austria). This procedure was carried out at least in a quadruplicate.

HPLC-MS/MS Mass spectra were recorded by means of a triple-quadrupole tandem mass spectrometer (TSQ Quantum Discovery; Thermo Electron, Dreieich), coupled to a Surveyor high-performance liquid chromatography (HPLC) system (Thermo Fisher Scientific), using the selected reaction monitoring mode. This HPLC was equipped with a thermostated autosampler (20 °C) and an Aqua C18 5U 125 Å column (150 x 4.6 mm i.d., Phenomenex, Aschaffenburg, Germany), kept at 30 °C and connected to a 4 x 2.0 mm i.d. polar RP precolumn (Phenomenex). The sample (10 µL) was separated at a flow rate of 0.2 mL/min. The solvent system was composed of acetonitrile/0.1% aqueous formic acid (8+2) for 10 min and afterwards a linear gradient was applied by increasing acetonitrile from 20% to 100% within 17 min. The mass spectrometer was operated in the positive electrospray ionization mode (ESI+) with a spray needle voltage of 3.3 kV and a spray current of 5 µA. The temperature of the capillary was 290 °C, the capillary 4 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

voltage was 35 V, the sheath and auxiliary gas were adjusted to 35 and 10 arbitrary units, respectively. The collision cell was operated at a collision gas pressure of 1 mTorr. Quantitation of Amadori Rearrangement Products


Sample Preparation The Amadori rearrangement products were quantitated according to (17). Cocoa was homogenized with a freezer mill (SPEX freezer mill 6870). The samples were suspended in anhydrous methanol (75 mL), the isotopically labeled standards were added and the mixture was stirred for 60 min. The extract was filtered off and the solution was concentrated to 5 mL in order to purify it by means of solid-phase extraction. For this purpose, a C18-T cartridge (55 µm, 140 Å, Phenomenex) was used, which had been conditioned by anhydrous methanol. After removing of methanol, the residue was dissolved in a mixture of water and acetonitrile (8:2, v/v), and then filtered off by a PTFE-filter (Spartan 13/0.45 µm RC, Whatman). This procedure was carried out at least in a quadruplicate.

HPLC-MS/MS Mass spectra were recorded as described in (17). A TSK Gel Amide-80 150 x 2 mm i.d. column (Tosoh Bioscience, Tokyo, Japan), kept at 30 °C and connected to a 4 x 2.0 mm i.d. polar RP precolumn (Phenomenex) was used. The solvent system was composed of acetonitrile/0.1% aqueous formic acid (9+1) for 3 min with a gradient to 0.1% aqueous formic acid/acetonitrile (9+1) within 12 min, then held for 5 min. Model Experiments Fru-Leu, Fru-Ile, Fru-Met, and Fru-Phe (in acetontrile) or the amino acids L-leucine, L-isoleucine, L-methionine, and L-phenylalanine (in water) were singly added to 2.5 g of coconut oil. The precursors were roasted singly in a roasting block for 30 min at 120 °C. For the analyses of the Strecker aldehydes, the model system was cooled down, spiked with the respective isotopically labeled standard, extracted with diethyl ether (50 mL) for 1 h, worked-up as described in “Quantitation of Strecker aldehydes” and analyzed by means of GCxGC-TOF/MS. This procedure was carried out at least in a quadruplicate. Isotope Enrichment Analyses Methanol (2 mL) was added to the homogenized cocoa sample. For a better homogenization, silica gel (10 % with water) was added. Then, four isotopically labeled amino acids [13C6,15N]-L-isoleucine, [2H3]-L-leucine, [13C,2H3]-L-methionine, and [2H5]-L-phenylalanine (in methanol) were spiked to the cocoa beans in the same concentration as the measured unlabeled amino acids 5 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

present in the beans. In a parallel series of experiments, four ARPs (in methanol) with labeled amino acid moiety were spiked to the cocoa sample, also in the same amount as measured for the unlabeled ARPs in the beans. The samples were put into a desiccator and the solvent was gently removed with a rotary evaporator. Afterwards, the spiked samples were roasted for 30 min at 120 °C in a roasting block. This procedure was carried out at least in a quadruplicate. The analyses of the Strecker aldehydes were done as reported above.


Results and Discussion Formation of Strecker Aldehydes and Their Precursors For the quantitation of Strecker aldehydes during cocoa processing, unfermented, 1-, 3-, and 5-day fermented/unroasted as well as the respective roasted cocoa samples were analyzed to clarify whether the odorants are formed enzymatically during maturation/fermentation or thermally during roasting (Table 1). During fermentation, all Strecker aldehydes were already available in unfermented cocoa beans, but were additionally generated. Phenylacetaldehyde was available with 18 µg/kg in the raw cocoa bean and finally reached a maximum of 76 µg/kg in the 5-day fermented beans (factor 4), followed by 3-methylbutanal which increased in its concentration by a factor of 3, 3-(methylthio)propanal by a factor of 3, and 2-methylbutanal by a factor of 2. After 5 days of fermentation and roasting, most of the Strecker aldehydes were additionally formed. Only 2-methylbutanal remained at the same concentration. These results agreed with the data reported previously (20, 21). Afterwards, amino acids as well as ARPs were quantitated in differently fermented cocoa beans. In Figure 1, the concentrations of L-isoleucine, L-leucine, L-methionine, and L-phenylalanine present at different fermentation days are shown. In agreement with the literature (21), amino acids were already present in unfermented beans and increased during fermentation. Interestingly, also the ARPs Fru-Ile, Fru-Leu, as well as Fru-Phe could already be determined in unfermented cocoa beans (Figure 2) and during fermentation the concentration increased significantly for all ARPs. A total of 3 days of fermentation were sufficient to reach a maximum. Fru-Met increased from < limit of detection (LoD = 30 µg/kg (17)) to 8 mg/kg, Fru-Leu from 6 to 116 mg/kg (factor 19), Fru Phe from 5 to 78 mg/kg (factor 16), and Fru-Ile from 6 to 48 mg/kg (factor 8). The formation of ARPs during fermentation has already been described by Pammer (22) who determined the concentration of Fru-Leu, Fru-Ile, and Fru Phe in unfermented and 7-day fermented cocoa beans. Following, the degradation of these precursors after thermal treatment was evaluated. In Figure 3 the concentrations of precursors in 5-day fermented unroasted and the respective roasted sample are presented. While the amino acids decreased slightly, the ARPs were substantially degraded.

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

Table 1. Changes in the concentrations (µg/kg) of Strecker aldehydes in unroasted (U) and roasted (R) cocoa samples from the Dominican Republic, which were not-fermented (0d), 1-, 3-, and 5-day (d) fermented. conc.a

odorant 0dU

1dU

3dU

5dU

3-methylbutanal

267 ±9%

553 ±11%

1161 ±17%

868 ±8%

2-methylbutanal

333 ±14%

630 ±7%

674 ±4%

610 ±1%

4 ±2%

5.86 ±2%

13 ±1%

11 ±16%

18 ±16%

67 ±12%

58 ±20%

76 ±9%

3-(methylthio)propanal


phenylacetaldehyde

conc.a

odorant

a

0dR

1dR

3dR

5dR

3-methylbutanal

195 ±12%

361 ±19%

1070 ±7%

1403 ±20%

2-methylbutanal

268 ±11%

378 ±2%

395 ±2%

602 ±13%


11 ±7%

14 ±3%

18 ±6%

24 ±8%

phenylacetaldehyde

60 ±7%

78 ±9%

121 ±10%

162 ±17%

Mean value out of at least four work-ups of one batch and root mean square deviation.

Figure 1. Formation of amino acids during fermentation of cocoa beans.



Figure 2. Formation of Amadori rearrangement products during fermentation of cocoa beans.

Figure 3. Degradation of amino acids and Amadori rearrangement products after roasting of 5-day fermented cocoa samples. AA: Amino acid; ARP: Amadori rearrangement products; U: unroasted; R: roasted. 8 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Efficacy of Amino Acids and Amadori Products To Form Strecker Aldehydes


In the next part of this study, the question should be clarified which of the precursors are more effective in Strecker aldehyde formation: the amino acid or the ARP. For this purpose, the yields of Strecker aldehydes were determined after roasting of each precursor singly in coconut fat. Fat was used, since cocoa contains just a small amount of water compared to the concentration of its fat content. In addition, coconut fat is nearly odorless and offers a similar composition as cocoa butter. In most cases, the ARPs were the more effective precursor compared to the respective amino acids (Table 2). Only 2-methylbutanal was similarly generated from the ARP (6.16 mol%) compared to the amino acid (7.28 mol%).

Table 2. Yields of Strecker aldehydes formed after single roasting of unlabeled ARPs or amino acids, respectively, in coconut fat. yielda [mol%]

Strecker aldehyde

a

ARP

amino acid

3-methylbutanal

4.00 ±8%

0.55 ±13%

2-methylbutanal

6.16 ±7%

7.28 ±7%


9.29 ±9%

1.87 ±2%

phenylacetaldehyde

6.25 ±6%

0.48 ±3%

Mean value out of at least four work-ups of one batch and root mean square deviation.

As the model roasting experiments have proven that ARPs and amino acids are adequate precursors of the Strecker aldehydes, the question whether the ARPs and the amino acids can act as suitable precursors of the Strecker aldehydes in real cocoa products should be clarified by means of an isotope enrichment analysis. For this purpose, 3-day fermented unroasted cocoa beans were spiked with four labeled amino acids in the concentration prior to the determined amount of unlabeled amino acid in cocoa. The same procedure was carried out for four ARPs. These precursors bearing a labeled amino acid moiety were added in the same concentration to the fermented, unroasted cocoa beans as measured for the respective unlabeled precursor. As described previously (23), unlabeled and labeled precursors are similarly degraded while roasting cocoa. By measuring the concentrations of the newly formed unlabeled and labeled Strecker aldehydes it could be shown if the respective precursor was able to generate an aldehyde in the same ratio, because if the precursor was the only compound to generate the odorant, the concentration of unlabeled and labeled Strecker aldehyde should be identical. The spiked cocoa samples were roasted in a roasting block and the concentrations of unlabeled as well as labeled Strecker aldehydes originating from either ARPs or amino acids were determined (Table 3). In both spiking experiments, the same amount of cocoa was used. After roasting of the two different spiked cocoa samples, the unlabeled Strecker aldehydes were generated 9 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

in the same concentration within the limit of error for both samples. Because of this, for a clearer presentation only one concentration representing the amount of unlabeled Strecker aldehyde is illustrated.

Table 3. Isotope enrichment analysis of Amadori compounds and amino acids. Concentrations (μg/kg) of unlabeled Strecker aldehydes in unroasted cocoa beans as well as of unlabeled and labeled Strecker aldehydes in roasted cocoa beans. conc. of the Strecker aldehydea

Strecker aldehyde Downloaded by 80.82.78.170 on December 11, 2016 | http://pubs.acs.org Publication Date (Web): November 18, 2016 | doi: 10.1021/bk-2016-1237.ch001

unroasted

roasted unlabeled

labeledb (ARP)

labeledc (amino acid)

3-methylbutanal

1161 ±17%

41290 ±13%

3250 ±3%

22110 ±12%

2-methylbutanal

674 ±4%

14990 ±9%

4417 ±5%

5677 ±10%


13 ±1%

1110 ±11%

94 ±15%

1024 ±7%

phenylacetaldehyde

58 ±20%

2843 ±15%

50 ±20%

705 ±20%

a

Mean value out of at least four work-ups of one batch and root mean square deviation. Labeled Strecker aldehyde originating from ARP. c Labeled Strecker aldehyde originating from amino acid. b

The concentrations of unlabeled Strecker aldehydes after roasting were much higher compared to the ones in commercial roasted cocoa beans (cf. Table 1). The highest amount in roasted cocoa samples was noted for 2-methylbutanal (41290 µg/kg) which was by a factor of 105 higher compared to the commercial roasted beans (395 µg/kg). Also 3-methylbutanal (14990 µg/kg), phenylacetaldehyde (2843 µg/kg), and 3-(methylthio)propanal (1110 µg/kg) were significantly generated after roasting. [2H3]-3-Methylbutanal was mainly formed from [2H3]-L-leucine (22110 µg/kg), but also from [2H3]-Fru-Leu. [13C5]-2-Methylbutanal originating from the respective ARP and amino acid were generated in similar concentrations with 4417 and 5677 µg/kg, respectively. The labeled [13C,2H3]-3-(methylthio)propanal was almost completely generated from the respective amino acid. [2H5]Phenylacetaldehyde was also more effectively formed from the amino acid (705 µg/kg). Only 50 µg/kg of [2H5]-phenylacetaldehyde was formed from the respective ARP. There are, however, other yet unknown precursors next to L-phenylalanine and Fru-Phe responsible for the generation of the respective Strecker aldehyde. It could be shown that high amounts of Strecker aldehydes were formed from the respective amino acids and ARPs. Including the concentration of each precursor in cocoa and their conversion rate, the results of Table 4 demonstrate that both precursors showed high activity in the formation of Strecker aldehydes. The labeled ARPs Fru-Leu and Fru-Ile generated the respective labeled Strecker 10 Granvogl et al.; Browned Flavors: Analysis, Formation, and Physiology ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

aldehydes with 10.88 and 13.82 mol%, respectively. In the model roasting experiments, the unlabeled Fru-Leu and Fru-Ile formed 3-methylbutanal with 4.00 mol% and 2-methylbutanal with 6.16 mol%. Fru-Met and Fru-Phe generated higher amounts of 3-(methylthio)propanal and phenylacetaldehyde in the model roasting experiments.

Table 4. Yields of Strecker aldehydes generated in cocoa from either the labeled amino acid or the respective labeled Amadori product, respectively, correlated to the concentration of the respective precursor. yielda [mol%]


Strecker aldehyde

a

ARP

amino acid

[2H3]-3-methylbutanal

10.88

2.73

[13C5]-2-methylbutanal

13.82

9.48

[13C, 2H3]-3-(methylthio)propanal

3.04

1.72

[2H5-]-phenylacetaldehyde

0.27

0.10

Mean value out of at least four work-ups.

Related to their applied amount of amino acids, the labeled precursors [2H3]L-leucine, [13C6,15N]-L-isoleucine, and [13C,2H3]-L-methionine formed more of the respective Strecker aldehydes in real cocoa samples compared to the ones in the model roasting experiments in coconut fat. To sum up the results, the amino acids generated in most of the cases a higher amount of the respective Strecker aldehyde after roasting of cocoa. This is due to the higher concentration of amino acids in 3-day fermented cocoa compared to the respective ARPs and, thus, in total releases the major part of the odorants after thermal treatment.

Conclusion Systematic studies on the generation/degradation of the Strecker aldehydes as well as the respective precursors were conducted during controlled fermentation and roasting. It could be shown that ARPs as well as Strecker aldehydes were formed during maturation and fermentation, i.e., are present in unroasted cocoa beans. By applying an isotope enrichment analysis, amino acids and ARPs were unequivocally evidenced to be effective precursors of Strecker aldehydes.

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