Contribution of Phenolic Compounds to Food Flavors: Strecker-Type

Dec 22, 2014 - Instituto de la Grasa, Consejo Superior de Investigaciones Científicas, Avenida Padre García Tejero 4, 41012 Seville, Spain. J. Agric...
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Contribution of Phenolic Compounds to Food Flavors: Strecker-Type Degradation of Amines and Amino Acids Produced by o- and p‑Diphenols Rosa M. Delgado, Rosario Zamora, and Francisco J. Hidalgo* Instituto de la Grasa, Consejo Superior de Investigaciones Científicas, Avenida Padre García Tejero 4, 41012 Seville, Spain ABSTRACT: The ability of 20 phenolic derivatives to produce the Strecker-type degradation of phenylalanine and phenylglycine methyl ester was studied to investigate both the direct degradation of amino acids and amines by phenolic compounds in the absence of added oxidants and the effect of the number and positions of hydroxyl groups in the aromatic ring of the phenolic compound in relation to its ability to produce carbonyl derivatives from amino compounds. The obtained results showed that polyphenols can produce the Strecker degradation of amino acids and amines in the absence of added oxidants. The only requisite for producing the reaction is the presence of two hydroxyl groups in ortho or para positions. However, the presence of two hydroxyl groups in meta position in an additional aromatic ring can inhibit the Strecker-degrading ability of the hydroxyl groups in ortho or para positions. A reaction pathway that explains all of these findings is proposed. In addition, the effect of reaction conditions on the obtained reaction yields was studied. Activation energies (Ea) for phenylacetaldehyde formation from phenylalanine in the presence of hydroquinone, 1,2,4-trihydroxybenzene, and benzoquinone were 32.9, 31.5, and 28.8 kJ/mol, respectively. KEYWORDS: amine degradation, amino acid degradation, phenols, quinones, Strecker-type degradation

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INTRODUCTION Phenolic compounds contribute to food flavors in multiple ways. Thus, and because of their bitterness or astringency, phenolic compounds have a significant role in the flavor of foods and drinks such as beer, wine, tree nuts, chocolate, coffee, tea, fruit-based products, and soy products.1−3 In addition, and because of their antioxidant properties, they participate in the lipid oxidation process that takes place in many foods upon processing. In these foods they play a major role in the inhibition of the production of lipid-derived off-flavors.4−6 Furthermore, because some phenols are able to react with carbonyl compounds, they are able to modify the flavorgenerating reactions in which these carbonyl compounds are involved. This occurs, for example, in the flavor and off-flavor development via Maillard chemistry.7−9 Moreover, phenolic compounds are able to scavenge lipid-derived carbonyl compounds that play a significant role in the flavor of foods.10 In addition to all of these roles, phenolic compounds can also be transformed either enzymatically or nonenzymatically into quinones, and quinones are able to degrade amino acids with major consequences in the flavor of foods.11,12 Thus, for example, Rizzi13 converted catechol moieties into o-quinones by using ferricyanide ion as the oxidant at 22 °C in pH 7.17 phosphate buffer, and these quinones were able to convert amino acids into their corresponding Strecker aldehydes. Although different studies have shown that amino acids can be degraded by quinones, to the best of our knowledge the direct degradation of amino acids by phenolic compounds in the absence of added oxidants has not been described. Furthermore, the effect of the number and positions of hydroxyl groups in the aromatic ring of the phenolic compound in relation to its potential for degrading amino acids is not known. © 2014 American Chemical Society

In an attempt to understand the structural characteristics of phenolic compounds that favor their ability to degrade amino acids, this study analyzes the role of 20 phenolic compounds on phenylalanine degradation in a buffered model system analogous to that employed previously in the study of amino acid degradation by lipid oxidation products.14 Phenylalanine was selected as model amino acid because its Strecker aldehyde phenylacetaldehyde has a high boiling point (195 °C), can be easily determined by gas chromatography−mass spectrometry (GC-MS), and is a very powerful odorant.15

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MATERIALS AND METHODS

Chemicals. Twenty phenolic compounds were employed in this study. They were simple phenolic compounds having two or three hydroxyl groups, phenolic derivatives having also alkyl, methoxy, or carboxylic groups, and complex phenolic compounds having more than one aromatic ring (Figure 1). To facilitate their study, they were classified into five groups: o-diphenols, m-diphenols, p-diphenols, trihydroxy derivatives, and complex phenols. These groups also included some analogues for comparison purposes. All of these phenolic compounds as well as the other employed chemicals were purchased from Aldrich (Milwaukee, WI, USA), Sigma (St. Louis, MO, USA), Fluka (Buchs, Switzerland), or Merck (Darmstadt, Germany) and were of analytical grade. Formation of Phenylacetaldehyde in Phenylalanine/Phenol Reaction Mixtures. Mixtures of phenylalanine (25 μmol) and the phenolic compound (0−30 μmol) in 500 μL of 0.3 M buffer were introduced in closed test tubes and heated in a heater block at 120− 180 °C for 0−1 h. The following buffers were employed: sodium Received: Revised: Accepted: Published: 312

September 30, 2014 December 16, 2014 December 22, 2014 December 22, 2014 DOI: 10.1021/jf5047317 J. Agric. Food Chem. 2015, 63, 312−318

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Journal of Agricultural and Food Chemistry

Figure 1. Phenolic compounds employed in this study. min; transfer line to MSD, 280 °C; ionization EI, 70 eV; ion source temperature, 230 °C; mass range, 28−550 amu. Determination of Phenylacetaldehyde Content. Quantification of phenylacetaldehyde was carried out as described previously14 by preparing standard curves of phenylacetaldehyde in the 1.55 mL of solution prepared for GC-MS injection. Ten different concentration levels of phenylacetaldehyde were used. Phenylacetaldehyde content was directly proportional to the aldehyde/internal standard area ratio (r = 0.999, p < 0.0001). The coefficients of variation were 0.99, p < 0.01) between the amount of phenol added and the phenylacetaldehyde produced at low concentrations of phenol. The maximum concentrations of phenylacetaldehyde were achieved when 25 μmol of phenylalanine was treated with either 10 μmol of hydroquinone, 7.5 μmol of benzoquinone, or 5 μmol of 1,2,4-trihydroxybenzene. Addition

of the amino acid in sodium citrate buffer, pH 3, for 1 h at 180 °C. The obtained results (Table 1) showed that there was a relationship between the structure of the assayed phenol and its ability to produce phenylacetaldehyde. Table 1. Formation of Phenylacetaldehyde in Mixtures of Phenylalanine and Phenolsa phenol tested none catechol 4-methylcatechol 3,4-dihydroxybenzoic acid caffeic acid ferulic acid resorcinol orcinol 2,6-dihydroxybenzoic acid hydroquinone 2,5-dihydroxybenzoic acid benzoquinone pyrogallol gallic acid 1,2,4-trihydroxybenzene phloroglucinol catechin epicatechin epigallocatechin quercetin myricetin

phenylacetaldehyde formed (μmol/mmol of phenylalanine) 10.7 30.3 36.7 19.2

± ± ± ±

2.2 2.4 4.2 5.5

a b,c b,d e,f

22.0 8.5 11.1 11.5 10.6

± ± ± ± ±

1.7 0.8 1.0 1.1 2.1

e,g a a a a

29.5 ± 1.9 b,c 24.9 ± 5.7 c,e,g 66.1 43.2 27.5 46.0 9.2 10.9 11.2 11.9 11.9 12.9

± ± ± ± ± ± ± ± ± ±

2.0 4.5 1.5 1.5 1.5 1.8 1.8 2.4 0.4 1.9

h d,i c,g i a a a a,f a a,f

a

Values with the same letter in the same column are not significantly different (p < 0.05).

For polyphenols having two hydroxyl groups at the aromatic ring, only those having these groups in ortho or para positions were able to produce the Strecker degradation of the amino acid to a higher extent than the control phenylalanine. For that reason, neither resorcinol, orcinol, nor 2,6-dihydroxybenzoic acid produced more phenylacetaldehyde than the control (Table 1). In addition, the presence of the two hydroxyl groups is needed, and one hydroxyl group cannot be replaced by a methoxy group. For that reason, ferulic acid did not produce more phenylacetaldehyde than the control. On the contrary, caffeic acid was able to increase phenylacetaldehyde formation by 106%. There was not any difference between the Strecker degradation ability of o- and p-diphenols. Thus, catechol and hydroquinone increased the phenylacetaldehyde produced by the control by about 180%. However, the presence of other groups in the ring and the relative positions of all groups determined either an increase or a decrease in the phenylacetaldehyde produced. Thus, a methyl group at position 4 (4methylcatechol) increased the formation of phenylacetaldehyde by 33% with respect to that formed by catechol. On the contrary, the presence of a carboxylic group at the same position (3,4-dihydroxybenzoic acid) reduced the formation of phenylacetaldehyde by 57% with respect to that formed by catechol. Curiously, the same carboxylic group only reduced by 24% the formation of phenylacetaldehyde with respect to that formed by hydroquinone in 2,5-dihydroxybenzoic acid. Finally, 314

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< 0.02) as a function of time at the different assayed temperatures when phenylalanine was heated in the presence of hydroquinone (Figure 4A). Similar behavior was observed in the case of the other two derivatives assayed: 1,2,4trihydroxybenzene (r > 0.97, p < 0.03; Figure 4B) and benzoquinone (r > 0.97, p < 0.002; Figure 4C). Reaction rates at the different assayed temperatures were calculated using the equation [phenylacetaldehyde] = [phenylacetaldehyde]0 + kt

where [phenylacetaldehyde]0 represents the intercept, k is the rate constant, and t is the time. These rate constants were used in an Arrhenius plot (Figure 5) for calculation of the activation

Figure 2. Effect of reaction pH on the phenylacetaldehyde produced in the reaction of phenylalanine with hydroquinone (△), 1,2,4trihydroxybenzene (○), and benzoquinone (▽). Equimolecular mixtures of the amino acid and the phenol were heated at 180 °C for 1 h.

Figure 5. Arrhenius plot for phenylacetaldehyde production in the reaction of phenylalanine with hydroquinone (△), 1,2,4-trihydroxybenzene (○), and benzoquinone (▽). Figure 3. Effect of the concentration of the phenol on the phenylacetaldehyde produced in the reaction of phenylalanine with hydroquinone (△), 1,2,4-trihydroxybenzene (○), and benzoquinone (▽). Mixtures of the phenylalanine (25 μmol) and the phenol in 0.3 M sodium citrate buffer were heated at 180 °C for 1 h.

energy (Ea) of phenylacetaldehyde formation from phenylalanine in the presence of phenolic compounds. The values obtained for Ea were 32.9 kJ/mol for hydroquinone, 31.5 kJ/ mol for 1,2,4-trihydroxybenzene, and 28.8 kJ/mol for benzoquinone. Strecker-Type Degradation of Phenylglycine Methyl Ester. In an attempt to clarify the mechanism of the reaction and to investigate if amines were also able to suffer Streckertype degradation, the reaction of phenylglycine methyl ester with both hydroquinone and benzoquinone was studied under the same reaction conditions above-described for phenylalanine. Phenylglycine methyl ester was selected for this study because it has several substituents at the α-carbon that can favor the exit of the proton at this carbon. Figure 6A shows the chromatogram obtained after 1 h of heating at 180 °C of an equimolecular mixture of hydroquinone and phenylglycine methyl ester. As observed in the figure, the initial phenylglycine methyl ester was absent after 1 h of heating, but a significant amount of hydroquinone was still present. The main reaction product was benzaldehyde, although other degradation products of phenylglycine methyl ester could be easily identified. These were the Strecker-type degradation products methyl 2-oxo-2-phenylacetate, benzoic acid, benzoic acid methyl ester, and benzaldehyde dimethyl acetal. Products derived from hydroquinone were 4-methoxyphenol and trace amounts of benzoquinone, which was unambiguously identified. 4-Aminophenol could also be present in trace amounts, but it appeared under the hydroquinone peak and could not be unambiguously identified. When the reaction was carried out between benzoquinone and phenylglycine methyl ester, the same products as in the reaction with hydroquinone were identified in the reaction mixture, although proportions among them were slightly

of higher amounts of phenolic compound did not produce significant increases in the phenylacetaldehyde produced. Phenylacetaldehyde formation also depended on the incubation time and temperature (Figure 4). Thus, the amount of phenylacetaldehyde produced increased linearly (r > 0.93, p

Figure 4. Time course of phenylacetaldehyde produced in the reaction of phenylalanine with (A) hydroquinone, (B) 1,2,4-trihydroxybenzene, and (C) benzoquinone. Equimolecular mixtures of phenylalanine and the phenol in 0.3 M sodium citrate buffer were heated at the indicated times and temperatures. The temperatures assayed were 180 (○), 160 (△), 140 (▽), 120 (◇), and 100 °C (□). 315

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having at least two hydroxyl groups in ortho or para positions were able to degrade phenylalanine. However, the presence in the same molecule of other aromatic ring with two hydroxyl groups in meta position canceled the ability of these complex phenols to degrade phenylalanine. This behavior should be a consequence of the mechanism of the reaction. Thus, the fact that two hydroxyl groups in ortho or para positions are needed suggests that the polyphenol has to be converted into the corresponding quinone to produce the reaction. This was observed when the reaction between hydroquinone and phenylglycine methyl ester was studied by GC-MS (Figure 6A). The presence of trace amounts of benzoquinone was observed in the chromatogram, although no oxidant was added. These oxidations of o- and p-dihydroxy derivatives to the corresponding quinones have been described by different authors.22,23 According to these findings, the reaction pathway shown in Figure 7 is proposed for the

Figure 6. Total ion chromatograms obtained for reaction mixtures of phenylglycine methyl ester with (A) hydroquinone and (B) benzoquinone. Samples were heated for 1 h at 180 °C in sodium citrate buffer, pH 3. Identified compounds are indicated in the figure. Peaks marked with ∗ corresponded to the buffer.

different (Figure 6B). Thus, phenylglycine methyl ester disappeared completely after 1 h of heating at 180 °C in the presence of benzoquinone, and a part of benzoquinone was converted into hydroquinone. In addition, the residual presence of benzoquinone was detected to a higher extent than when hydroquinone was the reactant (see Figure 6A). Analogously to the hydroquinone/phenylglycine methyl ester reaction mixture, the main reaction product was benzaldehyde. Other reaction products were methyl 2-oxo-2-phenylacetate, benzoic acid, benzoic acid methyl ester, and benzaldehyde dimethyl acetal. Analogously to hydroquinone/phenylglycine reaction mixtures, the products derived from benzoquinone were 4-methoxyphenol and hydroquinone. 4-Aminophenol could also be present in trace amounts, but it appeared under the hydroquinone peak and could not be unambiguously identified.

Figure 7. Proposed pathway for the reaction of phenylglycine methyl ester with hydroquinone and benzoquinone.

phenylglycine methyl ester degradation in the presence of hydroquinone. In the presence of amino compounds and air, oand p-diphenols are able to produce the corresponding quinones to some extent. These quinones form then the corresponding imines with the amino group of the amino compound. Finally, tautomerism converts the ketoimine into the corresponding α-imino ester, the driving force being conjugation. The hydrolysis of the α-ketoimine is the origin of the Strecker-type carbonyl compound (methyl 2-oxo-2-phenylacetate) detected in the reaction and of 4-aminophenol, which was not unambiguously identified. This 4-aminophenol might be the origin of the detected 4-methoxyphenol. Methyl 2-oxo2-phenylacetate was not stable under the employed reaction conditions as it degraded to benzaldehyde, which was the major reaction product. Benzaldehyde oxidation is likely the origin of benzoic acid and its methyl ester, although benzoic acid can also be produced directly in the cleavage of methyl 2-oxo-2phenylacetate. A similar pathway is likely taking place with phenylalanine (Figure 8). In this case, the produced α-iminocarboxylic acid intermediate decarboxylates to the corresponding N-(4-

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DISCUSSION Since its discovery more than 150 years ago,17 the Strecker degradation of amino acids has emerged as an almost endless source of flavor-significant compounds.12,18 Although it was considered for many years as a part of Maillard chemistry produced by carbohydrates, nowadays Strecker-type reactions are known to be also produced in the course of nonenzymatic browning reactions by other food components such as lipid oxidation products,19,20 amines,21 and polyphenol-derived quinones,13 among others. The results obtained in the present study extend the number of food components able to produce the Strecker degradation of amino acids also to polyphenols in the absence of added oxidants. Nevertheless, not all polyphenols were able to produce the Strecker degradation of amino acids. Only polyphenols having some structural characteristics produced this reaction. Thus, only polyphenols 316

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All of these results confirm that polyphenols can produce the Strecker degradation of amino acids in the absence of added oxidants. In addition, analogous reactions can also be produced in amines, therefore demonstrating that the presence of the carboxylic group might be desirable but not essential. The only requisite for polyphenols to produce Strecker-type degradations is the presence of two hydroxyl groups in ortho or para positions. On the other hand, the presence of two hydroxyl groups in meta position in another part of the molecule can inhibit the Strecker-degrading ability of the hydroxyl groups in ortho or para positions.

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AUTHOR INFORMATION

Corresponding Author

*(F.J.H.) Phone: +34 954 611 550. Fax: +34 954 616 790. Email: [email protected].

Figure 8. Proposed pathway for the reaction of phenylalanine with hydroquinone and benzoquinone.

Funding

This study was supported in part by the European Union (FEDER funds) and the Plan Nacional de I + D of the Ministerio de Economı ́a y Competitividad of Spain (Project AGL2012-35627).

hydroxyphenyl)aldimine. After hydrolysis of the latter aldimine, phenylacetaldehyde is produced. These pathways explain the relative activities of simple diand trihydroxy derivatives, which are related to their relative facility to be converted into quinones. In addition, the electronic effects of other substituents in the ring will also play a role in the overall reaction yield. On the contrary, it does not explain the low yields of phenylacetaldehyde obtained with complex phenols. These last results are likely a consequence of two opposite effects. On the one hand, the above results demonstrate that o- and p-diphenols are easily transformed into quinones, and these derivatives are able to convert amino acids and amines into their corresponding carbonyl derivatives. On the other, m-diphenols have been shown to be very efficient carbonyl scavengers.10,24,25 The obtained results seem to suggest that the carbonyl-scavenging ability of the A-ring is higher than the Strecker-inducer capacity of the B-ring. Only myricetin, which has a pyrogallol moiety in the B-ring, seemed to increase slightly the phenylacetaldehyde produced in comparison to control, although this increase was not significant. A further proof of this pathway was obtained when the reaction between benzoquinone and phenylglycine methyl ester was studied. The reaction not only produced compounds identical to those of the hydroquinone/phenylglycine methyl ester reaction mixtures but significant amounts of hydroquinone were found, therefore confirming the existence of redox processes in these mixtures that would facilitate the conversion of benzoquinone into hydroquinone and vice versa. For the same reason, the appearance of benzoic acid as an oxidation product of benzaldehyde is not surprising. The only differences between benzoquinone and hydroquinone for these reactions are the reaction yield, which was higher for benzoquinone (Table 1 and Figure 3), and the lower reaction time needed for benzoquinone to produce phenylacetaldehyde (Figure 4). Both results are in agreement with the conversion of hydroquinone into benzoquinone as a previous step for the Strecker degradation. Nevertheless, Ea for phenylacetaldehyde formation in benzoquinone/phenylalanine reaction mixtures was only slightly lower than Ea for phenylacetaldehyde formation in hydroquinone/phenylalanine reaction mixtures, therefore indicating that conversion of hydroquinone into benzoquinone is not an energetically difficult process, although it does not seem to be very favored kinetically.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to José L. Navarro for technical assistance. REFERENCES

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