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rearrangement (AR), undergoes reductive amination and is con- verted into 2-amino ketone (3), instead of its amino acid analog. (4) as in the case of Amadori ...
Food Sci. Technol. Res., 9 (1), 1–6, 2003

Review Recent Advances in the Chemistry of Strecker Degradation and Amadori Rearrangement: Implications to Aroma and Color Formation Varoujan A. YAYLAYAN McGill University, Department of Food Science and Agricultural Chemistry 21,111 Lakeshore, Ste. Anne de Bellevue, Quebec, Canada, H9X 3V9

Received July 18, 2002; Accepted September 5, 2002 The importance of Strecker degradation lies in its ability to produce Strecker aldehydes and 2-aminocarbonyl compounds, both are critical intermediates in the generation of aromas during Maillard reaction, however, they can also be formed independently of the pathways established for Strecker degradation. Strecker aldehyde can be formed directly either from free amino acids or from Amadori products. Several pathways have been proposed in the literature for the mechanism of this transformation. On the other hand, Amadori or Heyns rearrangements of ammonia with reducing sugars can also generate 2-aminocarbonyl compounds without the formation of Strecker aldehyde. In addition, isomerization of the imine bond of the Schiff base formed between a reducing sugar and an amino acid, can initiate a transamination reaction and convert the amino acid into the corresponding -keto acid and the sugar into its -amino alcohol derivative. The reverse of this reaction, has been documented to produce Amadori products. The keto acids can either decarboxylate to produce Strecker aldehydes or undergo Strecker degradation (as a -dicarbonyl compound) with amino acids to also produce Strecker aldehydes. This review will examine the role of Strecker degradation and Amadori rearrangement, under the light of recent findings, in controlling the balance among four critically important key intermediates: -dicarbonyl, -hydroxycarbonyl, 2-amino carbonyls and 2-(amino acid)-carbonyl compounds, during the Maillard reaction and hence control relative importance of aromagenic versus chromogenic pathways. Keywords: Maillard reaction, Strecker degradation, Amadori rearrangement, aroma, browning, melanoidin, mechanisms

Although Strecker degradation (SD) has been delegated to a “sub-reaction” category in the Maillard reaction scheme, however, in its broader definition, it may play a more critical role, than what currently is assumed, in switching the Maillard reaction towards the direction of aromagenic rather than chromogenic pathways. Similar to other sub-reactions occurring during the Maillard reaction, it has been discovered (Strecker, 1862) before the establishment of the mechanism of Amadori rearrangement itself (Amadori, 1925) by Kuhn and Weygand (1937). Historically, however, sugar amine reactions were investigated as early as 1866 by H. Schiff and later by E. Fischer (Wrodnigg & Eder, 2001) before the reaction was elevated to the status of independent research field by Maillard (Maillard, 1912). Strecker degradation is part of oxidative decarboxylation reactions of amino acids that can be effected by variety of reagents and reaction conditions. It is particularly referred to as Strecker degradation, when -dicarbonyl compounds (1 in Fig. 1) act as oxidizing agents to effect decarboxylation of amino acids which is usually followed by hydrolysis of the resulting imine to produce free ammonia (if inorganic oxidizing agents are used) or a primary amine such as -keto amine and an aldehyde-referred to as Strecker aldehyde. Although the amino acid itself undergoes oxidative decarboxylation, the -dicarbonyl compound however, E-mail: [email protected] Abbreviations: SD, Strecker degradation; AR, Amadori rearrangement; ARP, Amadori rearrangement product; SPME/GC-MS; Solid Phase Micro-extraction/Gas Chromatography-Mass spectrometry

similar to the -hydroxy carbonyl compound (2) during Amadori rearrangement (AR), undergoes reductive amination and is converted into 2-amino ketone (3), instead of its amino acid analog (4) as in the case of Amadori rearrangement process (see Fig. 1). In addition, Amadori and Heyns rearrangements of free ammonia with -hydroxy carbonyl compounds also produce identical intermediates to that of the SD of amino acids with corresponding -dicarbonyl compound, without, of course, the benefit of formation of Strecker aldehyde (Fig. 1). Therefore, both processes (SD and AR) serve the same purpose of reductively aminating different sugar fragments (-dicarbonyls vs -hydroxy carbonyls) by the action of the amino acid (see Fig. 1). The question arises then as to the subsequent over-all effect on the future direction of the Maillard reaction induced by 2-amino ketones (3) generated by SD and their amino acid counterparts (4) generated through Amadori rearrangement reaction. 1. Different pathways of formation of Strecker aldehyde Although the mechanism of Strecker degradation has been established for almost half a century ago (McCaldin, 1960), however, Strecker aldehyde and -amino carbonyl compounds can also be formed independently of each other and by pathways other than that of Strecker degradation. Four different such pathways have been proposed for the formation of Strecker aldehyde (A, B, D & E in Fig. 2). From amino acids through oxidative decarboxylation and thermal reactions Mild oxidizing agents such as sodium hy-

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Fig. 2. Different pathways of formation of Strecker aldehyde. ARP Amadori rearrangement product; [O]oxidation.

Fig. 1. Comparison of Strecker degaradation (SD) and Amadori rearrangement (AR).

pochlorite, N-bromosuccinimide, silver (II) picolinate, lead tetra acetate, etc. (Barrett, 1985) can cause oxidative decarboxylation of amino acids at ambient temperatures followed by hydrolysis of the resulting imine to give Strecker aldehyde (pathway A). Amino acids alone (Yaylayan & Keyhani, 2001a) or in the presence of -hydroxy carbonyl compounds (Shu, 1998) can also undergo thermal deamination and produce Strecker aldehydes at temperatures above 200˚C in the absence of oxidizing agents. Yaylayan and Keyhani (2001a) detected the formation of the imine 5 formed between the Strecker aldehyde and the resulting amine from decarboxylation of the amino acid when pyrolyzed alone at 250˚C for 20 s. On the other hand, Shu (1998) detected the formation of tetramethylpyrazine and the Strecker aldehyde of amino acids heated at 200˚C for 7 min in the presence of 3hydroxy-2-butanone and proposed a decarbonylation mechanism followed by deamination (pathway B in Fig. 2) to justify the formation of Strecker aldehyde. From amino acids through -dicrabonyl assisted oxidative decarboxylation Alloxan is the original “-dicarbonyl compound” used by Strecker (1862) to effect decarboxylation/deamination of amino acids and formation of the aldehyde named after him. Similarly, the common ninhydrin reaction (McCaldin, 1960) is also based on the ability of -dicarbonyl compounds to deaminate amino acids. The mechanism of Strecker aldehyde formation through -dicarbonyl-assisted oxidation is shown in Fig. 2 (pathway C). Figure 2 also indicates that Amadori rearrangement products (ARP) in principle, could be oxidized into -imino carbonyl compound (6 common intermediate with that of -dicrabonyl adduct in pathway C) and undergo Strecker degradation as shown in Fig. 2 (pathway D). Alternatively, it can undergo oxidative decarboxylation similar to free amino acid (Pathway E) and produce the common intermediate 7 with that

of -dicrabonyl adduct of pathway C. ARP is therefore one oxidation level below the intermediates formed during Strecker degradation. From Amadori rearrangement products Formation of Strecker aldehyde directly from Amadori rearrangement product was first suggested by Cremer et al. (2000) followed by Hofmann and Schieberle (2000) in the same year. Both groups however proposed different pathways (A and B) as shown in Fig. 3. Cremer et al. (2000) observed the formation of Strecker aldehydes in a dry model system (aw0.75) consisting of synthetic ARP in the absence and presence of a different amino acid than that of ARP. The model systems were incubated for 4 days at 20˚C in a headspace vial and then analyzed by SPME/GC-MS by heating for 60 min at 90˚C. The data indicated the formation of Strecker aldehydes from both amino acids and more interestingly, the molar ratio of these two aldehydes remained constant irrespective of the amount of added amino acid, after 1 : 1 ratio. To eliminate the possibility of Strecker aldehyde formation through generation of a -dicarbonyl intermediates from degradation product of the ARP (pathway C in Fig. 3), the authors performed a control experiment using o-phenylenediamine as a trapping agent for -dicrabonyl compounds, this model system also produced the Strecker aldehyde. To explain the results obtained the authors proposed the reaction of free amino acid (either added or released from ARP) with ARP to form the imine 8 (pathway A in Fig. 3) in equilibrium with the isomeric imine 9. Elimination of the free amino acid initiated by decarboxylation can produce the corresponding Strecker aldehyde in addition to structure 10 (Schiff base of 1-deoxyfructose with ammonia). On the other hand, Hofmann and Schieberle (2000) performed their experiments in aqueous solutions (buffered at pH 7.0, heated at 100˚C for 2 h) of phenylalanine ARP under Argon and air atmospheres. They also identified Strecker aldehyde in the mixtures, however in much higher amounts under air and in the presence of CuSO4 than under Argon atmosphere. This prompted them to

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Recent Advances in the Chemistry of Strecker Degradation

propose a mechanism based on oxidation (catalyzed by metal) of the eneaminol moiety of the ARP into imino carbonyl (11) similar to oxidation of enediols into -dicrabonyls (see Fig. 3 pathway B). This intermediate can either get hydrolyzed into glucosone or undergo a very similar decarboxylation reaction (after ring closure) to that of intermediate 8 (Pathway A in Fig. 3), but eliminating water instead of an free amino acid. Intermediate 12 can undergo hydrolysis to produce the Strecker aldehyde. Both proposed mechanisms A & B are feasible but suffer from lack of direct evidence such as detection or isolation of the proposed side products (intermediates 10 and 13 in Fig. 3). In a related study, Hofmann et al. (2000) have demonstrated that during Strecker degradation, the intermediate formed after decarboxylation and hydration steps (structure 16 in Fig. 4) can also undergo air oxidation and form intermediate 17 after an isomerization step. Further hydrolysis of 18 can generate Strecker acid. Alternatively, structure 16 can lose a water molecule and generate Strecker aldehyde as shown in Fig. 4. In addition, if the intermediate 14 is capable of ring closure (such as structure 11 in Fig. 3) then further oxidation and hence Strecker acid formation is prevented. However, the generality of this pathway with other amino acids apart from L-phenylalanine has to be demonstrated.

2. Formation of Amadori rearrangement product through transamination and strecker aldehyde through decarboxylation of -keto carboxylic acids In the absence of a catalyst, -keto carboxylic acids (19 in Fig. 5) acting as -dicrabonyl compounds, can also bring about oxidative decarboxylation of amino acids, being themselves converted into the corresponding amino acids. For example the odor of benzaldehyde could be detected above a boiling solution of phenylglycine and pyruvic acid along with the formation of alanine (Jones, 1979). The isomerization of the initial imine (20) into 21 followed by hydrolysis and decarboxylation can form the Strecker aldehyde as shown in Fig. 5. However, formation of -keto carboxylic acids during Maillard reaction has not been studied in detail and therefore it is difficult to ascertain their importance in the formation of Strecker aldehydes during the Maillard reaction. Theoretically, -keto carboxylic acids could be formed from the isomerization of the Schiff base intermediate (23 in Fig. 5) and formation of imine 22. The latter can undergo hydrolysis and generate an -keto carboxylic acid as shown in Fig. 5. Using 13Clabeled alanines, Yaylayan and Wnorowski (2002) provided evidence for the formation of pyruvic acid in alanine/glycolaldehyde model system. On the other hand, the interaction of -keto

Fig. 3. Proposed mechanisms of Strecker aldehyde formation from Amadori rearrangement product.

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carboxylic acids with -amino alcohols should generate imine 22 which after base catalyzed isomerization can form the Schiff base (23), the immediate precursor of Amadori product. Using 13 C-labeled pyruvic acid, Yaylayan and Wnorowski (2002) provided evidence for the formation of alanine and glycolaldehyde in pyruvic acid/ethanolamine model system. Tressl et al. (1994) using labeled sugars confirmed the conversion of cystein into pyruvic acid through transamination reaction in a model glucose/ cysteine system. Similarly, Yaylayan et al. (2000) confirmed the formation of alanine during thermal decomposition of L-serine, through transamination reaction between pyruvic acid and ethanolamine generated in situ, using 13C-labeled L-serines. If the generality of this remarkable interconversion between reducing sugar/amino acid and their corresponding -keto carboxylic acid/-amino alcohol (Fig. 5) can be verified with different model systems, then mixtures consisting of -keto carboxylic acids and -amino alcohols can be used to mimic the Maillard reaction systems, since they can potentially generate an Amadori product, a reducing sugar and an amino acid as shown in Fig. 5. The Schiff base (23) is the common intermediate between the two pathways leading to Amadori rearrangement product. 3. Implications of Strecker degradation versus Amadori rearrangement in the generation of aroma and color During Strecker degradation (SD), -dicarbonyl compounds (1), similar to the -hydroxy carbonyl compounds (2) during Amadori rearrangement (AR), undergo reductive amination and are converted into 2-amino ketones (3), instead of their amino acid counterparts (4) as in the case of Amadori rearrangement process (see Figs. 1 and 6). The significance of this transformation lies in the fact that the reactive primary amino group in structure 3 allows dimerization and other reactions with different aldehydes or ketones to form neutral and stable N-containing heterocyclic aroma compounds such as pyrazines, pyrroles and oxazoles (Kort, 1970; Yaylayan & Keyhani, 2001b), whereas the

Fig. 4. Proposed mechanism of Strecker acid formation (based on Hofmann et al., 2000). [O]oxidation.

Fig. 5. Proposed mechanism of formation of Strecker aldehyde and Amadori rearrangement product through -keto carboxylic acids.

secondary amino group in structure 4 prevents such amino-carbonyl type reactions to proceed to the extent of formation of stable aromatic moieties, for example in the case of dimerization, it leads to the formation of N,N¢-dialkyl-dihydropyrazines that are unable to aromatize and eventually form, through a single electron transfer process very unstable pyrazinium radical cations (Hofmann et al., 1999). These cations being unstable, they further undergo disproportionation (Hofmann et al., 1999) to regenerate dihydropyrazine and form doubly charged pyrazinium diquaternary salts (24 in Fig. 6) considered to be the precursors of colored melanoidins. According to Hofmann (1999), such intermediates, formed specifically from glycolaldehyde play an important role in the early melanoidization of Maillard mixtures compared with the longer chain analogs of -hydroxycarbonyl intermediates. Apart from this free radical based browning pathway, Hofmann (1998b) also identified an ionic pathway that leads to the formation of low molecular weight non-melanoidin type colored compounds at the later stages of Maillard reaction. This pathway is mainly initiated by the further interactions of furan moieties such as furan aldehydes and acetylformoin (2,4dihydroxy-2,5-dimethyl-3(H)-furanone), the latter compound is described as a chemical switch activated in the presence of excess of either primary or secondary amino acids to direct the formation of amino acid specific chromophores (Hofmann, 1998c). Can aromagenic and chromogenic pathways be traced back to -aminocarbonyls and their amino acid counterparts as their principle initiators? According to the pathways described in Fig. 6, it can be proposed that Strecker degradation reactions and Amadori rearrangement of ammonia may direct the Maillard

Recent Advances in the Chemistry of Strecker Degradation

reaction mainly towards aromagenic pathways through intermediate 3 and Amadori rearrangement of amino acids with hydroxycarbonyl compounds may lead the Maillard reaction mainly towards chromogenic pathways and melanoidization through intermediate 4 and to chromogenic pathways through intermediate 1. Although there are no studies yet on the relative ability of structures such as 1, 3 and 4 in the generation of aroma and browning, however, Hofmann (1999) investigated the role of the different -dicarbonyl and -hydroxycarbonyl intermediates (such as 1 and 2 in Fig. 6) as browning precursors, in addition to Amadori products (4) of intact carbon chain. In this landmark study, Hoffman calculated relative browning activity of various carbohydrate precursors such as glyoxal, glycolaldehyde, pyruvaldehyde, hydroxyacetone, etc. in the presence of alanine, when refluxed in phosphate buffer (pH 7.0) for 15 min. Glycolaldehyde (a C2 -hydroxycarbonyl), showed the highest browning activity among all the intermediates studied and compared with its dicarbonyl counterpart glyoxal, it showed 8 fold higher browning activity. On the other hand, 2-oxopropanal (a C3 dicarbonyl) exhibited 256 fold higher browning activity than its -hydroxycarbonyl counterpart hydroxy-2-propanone. Furthermore, 2,3-butanedione (a C4 -dicarbonyl) exhibited 8 fold higher browning activity than its -hydroxycarbonyl counterpart 2-hydroxy-3-butanone. This results are consistent with the proposed concept in Fig. 6. Furthermore, the results indicate that in addition to the nature of the chemical moieties involved as key precursors (1, 2, 3, and 4) their carbon chain lengths (C2, C3 or C4) can also play a decisive role in promoting aldol condensations that lead to formation of furanoid species and hence induce browning through ionic pathway (see Fig. 6). In the case of glycolaldehyde/glyoxal couple, it is statistically less likely to generate a furan moiety through repeated aldol condensations, hence there is only one pathway of browning through ARP (free radical). With C3 and C4 precursors, aldol condensation can genarate

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furanoid species that lead to browning (Hofmann, 1998b). As to the variation in the differences of browning abilities (8 fold versus 256) among the precursors, this could be related to their relative ability to be reduced or to be oxidized in the reaction system. In the above experiment, the large difference (8 versus 256) in browning ability between glyoxal/glycoladehyde redox couple and that of 2-oxopropanal/hydroxy-2-propanone could be related to the much higher tendency of glyoxal to be reduced to glycolaldehyde then the tendancy of hydroxy-2-propanone to be oxi-

Fig. 7. Interconversion of key precursors of aroma and color during Maillard reaction. [O]oxidation, [H]reduction. Double headed arrows ( ) indicate reversibility.

Fig. 6. Relationship of Strecker degradation (SR) and Amadori rearrangement (AR) to aromagenic and chromogenic pathways of Maillard reaction. [O]oxidation, [H]reduction.

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dized to 2-oxopropanal. As a result both -dicrabonyl and hydroxycarbonyl species, due to their ease of interconversion through redox reactions, can produce relatively similar browning (8 fold difference), for example through free radical pathway, as in the case of glyoxal/glycolaldehyde couple. On the other hand, due to difficulty of redox interconversion between 2-oxopropanal and hydroxy-2-propanone, a larger difference (256 fold) in browning ability was detected, indicating the importance of ionic pathway when the carbon chain of the precursors exceeds C2. According to Hofmann (1998a) in a typical Maillard reaction, most of the browning is due to low molecular weight (1000 amu) fraction of the product which constitiutes 78.4% by weight of the mixture in the case of glycine/glucose system. When the browning activity of the precursors produced in situ in a refluxing glucose/alanine and xylose/alanine mixtures at various time intervals were also analyzed (through repeated derivatization at the end of each time interval) it was shown that relative browning activities of the intermediates changed with the time of the reaction, some decreased and some increased over a period of 120 min (Hofmann, 1999). These observations indicate that some chromophores are produced early (through pyrazinium radical cation, formed mainly through glycolaldehyde) in the course of the reaction and others later (further reactions of furanoid species). In both model systems glyoxal contributed to the overall browning at the early phase of the reaction and pyruvaldehyde and 3-deoxyosons contributed towards the end of the reaction. The two types of chromophores (early and late) could be therefore distinguished structurally. Conclusion In the context of Fig. 6, the contribution of Strecker degradation to the concentration of 2-amino carbonyl species (3) and its role in depleting -dicarbonyl species (1), not to mention, generation of Strecker aldehydes, verifies its known importance in aromagenesis during the Maillard reaction. On the other hand, Amadori rearrangement plays a dual role during the reaction, it can contribute to aromagenesis through production of -dicarbonyls both oxidatively and non-oxidatively (see Fig. 7) and through formation of 2-amino carbonyls (3) in the presence of ammonia. Simultaneously, it can contribute to the browning and melanoidization of the reaction mixture through ionic and free radical pathways. The balance among different key precursor moities (1, 2, 3, and 4 in Fig. 7) which controls relative aromatization versus melanoidization can be easily disrupted through initiation of redox reactions (Fig. 7) that are affected by the amount of dissolved oxygen, and by the amount and timing of the release of reducing species produced by the reaction (reductones), disproportionation and dehydration reactions and concentration of metal ions. The role of Amadori rearrangement and Strecker degradation is crucial in controlling this balance. Knowledge of concentration changes of these species (1, 2, 3, and 4 in Fig. 7) over time, during the reaction is therefore critical in fully understanding and controlling melanoidization, aromagenesis and different browning pathways. References Amadori, M. (1925). Products of condensation between glucose and pphenetidine I. Atti Accad. Lincei, 2, 337–342. Barrett, G.C. (1985). Reactions of amino acids. In “Chemistry and

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Biochemistry of amino acids.” ed. by G.C. Barrett, pp 355–375. Cremer, D.R., Vollenbroeker and Eichner, K. (2000). Investigation of the formation of Strecker aldehydes from the reaction of Amadori rearrangement products with -amino acids in low moisture model systems. Eur. Food Res. Technol., 211, 400–403. Hofmann, T. (1998a). Studies on the relationship between molecular weight and the color potency of fractions obtained by thermal treatment of glucose/amino acid and glucose/protein solutions by using ultracentrifugation and color dilution technique. J. Agric. Food Chem., 46, 3891–3895. Hofmann, T. (1998b). Identifiaction of novel colored compounds containing pyrrole and pyrrolinone structures formed by Maillard reactions of pentoses and primary amino acids. J. Agric. Food Chem., 46, 3902–3911. Hofmann, T. (1998c). Acetylformoin -A chemical switch in the formation of colored Maillard reaction products from hexoses and primary and secondary amino acids. J. Agric. Food Chem., 46, 3918– 3928. Hofmann, T. (1999). Quantitative studies on the role of browning precursors in the Maillard reaction of pentoses and hexoses with L-alanine. Eur. Food Res. Technol., 209, 113–121. Hofmann, T., Bors, W. and Stettmaier, K. (1999). Studies on radical intermediates in the early stage of the nonenzymatic browning reaction of carbohydrates and amino acids. J. Agric. Food Chem., 47, 379–390. Hofmann, T., Münch, P. and Schieberle, P. (2000). Quantitative model studies on the formation of aroma-active aldehydes and acids by Strecker-type reactions. J. Agric. Food Chem., 48, 434–440. Hofmann, T. and Schieberle, P. (2000). Formation of aroma-active Strecker-aldehydes by a direct oxidative degradation of Amadori compounds. J. Agric. Food Chem., 48, 4301–4305. Jones, J.H. (1979). Amino acids. In “Comprehensive Organic Chemistry.” ed. by D.H.R. Barton and W.D. Ollis, Vol. 5, p 825. Kort, M.J. (1970). Reactions of free sugars with aqueous ammonia. Adv. Carbohydr. Chem., 25, 311–349. Kuhn, R. and Weygand, F. (1937). The Amadori rearrangement. Ber. Dtsch. Chem. Ges., 70B, 769–772. Maillard, L.C. (1912). Acrion des acides amines sur les sucres; formation des melanoidines par voie methodique. Compt. Rend. Acad. Sci., 154, 66–68. McCaldin, D.J. (1960). The chemistry of ninhydrin. Chem. Rev., 60, 39–51. Shu, C-K. (1998). Pyrazine formation from amino acids and reducing sugars—a pathway other than Strecker degradation. J. Agric. Food Chem., 46, 1129–1131. Strecker, A. (1862). On a peculiar oxidation by alloxan. Justus Liebigs Ann Chem., 123, 363–367. Tressl, R., Kersten, E., Nittka, C. and Rewicki, D. (1994). Formation of sulfur-containing flavor compounds from [13C]-labeled sugars, cysteine, and methionine. In “Sulfur compounds in Foods.” ed. by C.J. Mussinan and M.E. Keelan, ACS symposium series 564, pp 224–235. Wrodnigg, T.M. and Eder, B. (2001). The Amadori and Heyns rearrangements: Landmarks in the history of carbohydrate chemistry or unrecognized synthetic opportunities? Topics Curr. Chem., 215, 115–152. Yaylayan, V.A. and Wnorowski, A. (2002). The role of -hydroxyamino acids in the Maillard reaction: Transamination route to Amadori products. In “Maillard Reaction in Food Chemistry and Medical Sciences: Update for the Postgenomic Era.” ed. by S. Horiuchi, N. Taniguchi, F. Hayase, T. Kurata, and T. Osawa. International Congress Series 1245. Elsevier Science. Amsterdam, The Netherlands, pp. 195–200. Yaylayan, V.A. and Keyhani, A. (2001a). Carbohydrate and amino acid degradation pathways in L-methionine/D-[13-C]glucose model systems. J. Agric. Food Chem., 49, 800–803. Yaylayan, V.A. and Keyhani, A. (2001b). Elucidation of the mechanism of pyrrole formation during thermal degradation of 13C-labelled L-serines. Food Chem., 74, 1–9. Yaylayan, V.A., Keyhani, A. and Wnorowski, A. (2000). Formation of sugar-specific reactive intermediates from 13C-labeled L-serine. J. Agric. Food Chem., 48, 636–641.