Novel Strecker degradation products of tyrosine and dihydroxyphenylalanine

Tyrosine was oxidised with either potassium peroxodisulphate or glyoxal. Volatile reaction products were isolated and analysed by GC/FID and GC/MS, derivatised with diazomethane and analysed by the same methods. Eight reaction products were identified. The major products were the expected Strecker aldehyde (4-hydroxyphenylacetaldehyde) and its lower homologue 4-hydroxybenzaldehyde. They were followed by 1-(4-hydroxyphenyl)-3-propionaldehyde, phenylacetaldehyde, benzaldehyde, phenol, 4-hydroxybenzoic, and benzoic acid. Analogously, the oxidation of 3,4-dihydroxyphenylalanine yielded the corresponding Strecker aldehyde (3,4-dihydroxyphenylacetaldehyde), its lower homologue 3,4-dihydroxybenzaldehyde, 3,4-dihydroxybenzoic, 3,4-dihydroxyphenylacetic, and caffeic acid. An identification of these oxidation products of tyrosine and 3,4-dihydroxyphenylalanine assumes homolytic cleavage of the Strecker aldehydes and a recombination of free radicals formed by this cleavage. As minor products, six O- and N-heterocyclic compounds arose in systems containing glyoxal (pyrazine, methyl- and ethylpyrazine, 3-furancarbaldehyde, 5-methyl-2-furancarbaldehyde, 2-pyrrolcarbaldehyde).

Acrylamide Formation in Food: A Mechanistic Perspective

Earliest reports on the origin of acrylamide in food have confirmed asparagine as the main amino acid responsible for its formation. Available evidence suggests that sugars and other carbonyl compounds play a specific role in the decarboxylation process of asparagine, a necessary step in the generation of acrylamide. It has been proposed that Schiff base intermediate formed between asparagine and the sugar provides a low energy alternative to the decarboxylation from the intact Amadori product through generation and decomposition of oxazolidin-5-one intermediate, leading to the formation of a relatively stable azomethine ylide. Literature data indicate the propensity of such protonated ylides to undergo irreversible 1,2-prototropic shift and produce, in this case, decarboxylated Schiff bases which can easily rearrange into E Decarboxylated Amadori products can either undergo the well known β-elimination process initiated by the sugar moiety to produce 3-aminopropanamide and 1-deoxyglucosone or undergo 1,2-elimination initiated by the amino acid moiety to directly generate acrylamide. On the other hand, the Schiff intermediate can either hydrolyze and release 3-aminopropanamide or similarly undergo amino acid initiated 1,2-elimination to directly form acrylamide. Other thermolytic pathways to acrylamide—considered marginal at this stage—via the Strecker aldehyde, acrolein, and acrylic acid, are also addressed. Despite significant progress in the understanding of the mechanistic aspects of acrylamide formation, concrete evidence for the role of the different proposed intermediates in foods is still lacking.