The formation of pyrazines from l-serine and l-threonine has been studied. l-Serine and l-threonine, either alone or combined, were heated at 120 °C as low temperature for 4 h or at 300 °C as high temperature for 7 min. The pyrazines formed from each reaction were identified by GC/MS, and the yields (to the amino acid used, as parts per million) were determined by GC/FID. It was found that pyrazine, methylpyrazine, ethylpyrazine, 2-ethyl-6-methylpyrazine, and 2,6-diethylpyrazine were formed from serine, whereas 2,5-dimethylpyrazine, 2,6-dimethylpyrazine, trimethylpyrazine, 2-ethyl-3,6-dimethylpyrazine, and 2-ethyl-3,5-dimethylpyrazine were formed from threonine. Mechanistically, it is proposed that the thermal degradation of serine or threonine is composed of various complex reactions. Among these reactions, decarbonylation followed by dehydration is the main pathway to generate the α-aminocarbonyl intermediates leading to the formation of the main product, such as pyrazine from serine or 2,5-dimethylpyrazine from threonine. Also, deamination after decarbonylation generates more reactive intermediates, α-hydroxycarbonyls. Furthermore, aldol condensation of these reactive intermediates provides α-dicarbonyls. Subsequently, these α-dicarbonyls react with the remaining serine or threonine by Strecker degradation to form additional α-aminocarbonyl intermediates, which then form additional pyrazines. In addition, decarboxylation and retroaldol reaction may also involve the generation of the intermediates. Keywords: Serine; threonine; pyrazine formation; decarbonylation; decarboxylation; deamination; α-aminocarbonyls; Strecker degradation; aldol condensation

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Because the reaction of acyloins with ammonia generates pyrazines and acyloins are not involved with Strecker degradation, this study uses acetoin (a simple acyloin, from sugar degradation) to react with amino acids to determine whether ammonia is released, leading to the formation of tetramethylpyrazine. The results show that all reactions between R-amino acids and acetoin generate not only tetramethylpyrazine but also the corresponding Strecker aldehydes. However, the reactions between -, γ-, or -amino acid and acetoin generate only tetramethylpyrazine. Quantitative analysis shows that among such reactions, both R-and -amino acids generate significantly higher amounts of tetramethylpyrazine than γ-and -amino acids. On the basis of these data, it is proposed mechanistically that R-amino acids decarbonylate to generate the reactive intermediates, 1-hydroxyamines, deamination of which leads to the formation of the Strecker aldehydes. This study demonstrates that pyrazines and Strecker aldehydes are also formed from R-amino acids and reducing sugar via deamination in addition to their formation via the well-known Strecker degradation.

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An aqueous solution of glucosamine hydrochloride was heated to 150 °C for 5 min under different pH conditions. The reaction product mixture obtained was analyzed by GC/MS. It was found that the major products formed were furfurals, especially at pH = 4 and 7. At pH = 8.5, additional flavor components were generated, including pyrazines, 3-hydroxypyridines, pyrrole-2-carboxaldehyde, furans, acetol, and several other compounds. Of the components identified, it is worthwhile to note the formation of pyrazine and methylpyrazine as major components at pH = 8.5. It is proposed that a retro-aldol condensation plays an important role in the formation of the intermediates, α-aminoacetaldehyde (I) and α-amino propanal (II). As a result, self-condensation of I generates pyrazine and combination of I and II generates methylpyrazine. In addition, it is also interesting to note the formation of 3-hydroxypyridines and pyrrole-2-carboxaldehyde. It is suggested that both groups of compounds are derived from furfurals. As the ammonia is liberated from glucosamine, it initiates the ring-opening of furfurals to form 5-amino-2-keto-3-pentenals. Intramolecular condensations of these intermediates between the amino group and the carbonyl groups lead to the formation of 3-hydroxypyridines and pyrrole-2-carboxaldehyde. Keywords: Glucosamine; retro-aldol condensation; α-amino acetaldehyde; α-amino propanal; GC/MS