Strecker degradations

12 The Maillard Reaction Source of Flavour in Thermally Processed Foods o 

---

Strecker Degradation (Oxidative Deamination), Mild oxidizing agents such as aqueous sodium hypochlorite or aqueous A-bromosuccinimide, cause decarboxylation and concurrent deamination of amino acids to give aldehydes. 

Strecker aldehydes are produced by the Strecker degradation of the initial Schiff base (Figure 5). An a-amino carbonyl compound and... 

Strecker aldehyde are generated by rearrangement, decarboxylation and hydrolysis. Thus the Strecker degradation is the oxidative de-amination and de-carboxylation of an a-amino acid in the presence of a dicarbonyl compound. An aldehyde with one fewer carbon atoms than the original amino acid is produced. The other class of product is an a-aminoketone. These are important as they are intermediates in the formation of heterocyclic compounds such as pyrazines, oxazoles and thiazoles, which are important in flavours. 

Scheme 18.—Mechanism for Acylpyrrole formation from 3-Deoxyhexos-2-ulose by the Strecker Degradation.

It has been found that AAs do not react with 3,4-disubstituted o-benzo-quinones in the expected Strecker degradation reaction instead, a decar-boxylative condensation reaction afforded the corresponding benzoxazoles. A mechanistic explanation has been advanced for this transformation. It should be mentioned, however, that other quinones or diones did not react in the described manner (78JOC509). 

Amino acids may also undergo thermal degradation, which is almost always coupled with some other food components, particular sugars. The major types of volatile compounds formed from amino-sugar interactions include Strecker degradation aldehydes, alkyl pyrazines, alkyl thiazolines and thiazoles and other heterocycles [35, 36]. As the subject has mainly relevance for baked and roasted vegetable food products, this subject will not be discussed in further detail. 

The first group contains compounds produced in the early stages of the reaction by the breakdown of the Amadori or Heynes intermediates, and includes similar compounds to those found in the caramelisation of sugars. Many of these compounds possess aromas that could contribute to food flavour, but they are also important intermediates for other compounds. The second group comprises simple aldehydes, hydrogen sulphide or amino compounds that result from the Strecker degradation occurring between amino acids and dicarbonyl compounds. 

Table 12.1 Aldehydes and some other related intermediates formed in by Strecker degradation...

Several mechanisms have been proposed for the formation of pyrazines in food flavours [18, 23, 25], but the major route is from a-aminoketones, which are products of the condensation of a dicarbonyl with an amino compound via Strecker degradation (Scheme 12.3). Self-condensation of the aminoketones, or condensation with other aminoketones, affords a dihydropyrazine that is oxidised to the pyrazine. 

Hydrogen sulphide is a key intermediate in the formation of many heterocyclic sulphur compounds. It is produced from cysteine by hydrolysis or by Strecker degradation ammonia, acetaldehyde and mercaptoacetaldehyde are also formed (Scheme 12.4). All of these are reactive compounds, providing an important source of reactants for a wide range of flavour compounds. Scheme 12.6 summarises the reactions between hydrogen sulphide and other simple intermediates formed in other parts of the Maillard reaction. 

The Amadori compound may be degraded via either of two pathways, depending on pH, to a variety of active alcohol, carbonyl and dicarbonyl compounds and ultimately to brown-coloured polymers called melanoidins (Figure 2.31). Many of the intermediates are (off-) flavoured. The dicarbonyls can react with amino acids via the Strecker degradation pathway (Figure 2.32) to yield another family of highly flavoured compounds. 

Figure 232 Strecker degradation of L-valine by reaction with 2,3-butadione.

The formation of aldehydes (flavorings) in roasting of cocoa beans, for example, is caused by Strecker degradation of amino acids. 

Isatin has been used in the Strecker degradation of a-amino acids to aldehydes,434-437 and in the formation of benzaldehydes from benzyl-amines.431,435,438-440 These conversions have been the subject of a review, and mechanisms have been proposed.441 This formation of aldehydes from primary amines may, in part, explain some of the... 

Both the ninhydrin reaction and pyridoxal phosphate-catalyzed decarboxylation of amino acids (Chapter 14) are examples of the Strecker degradation. Strecker reported in 1862 that alloxan causes the decarboxylation of alanine to acetaldehyde, C02, and ammonia.c... 

STM image of glycogen 174 Stoichiometric dissociation constant 305 Stokes-Einstein equation 461 Stokes radius 109 Stomach ulcer 187 Stomata 30 Stop codons 236 Stopped flow technique 468 Stratum corneum 439 Strecker degradation 121 Strength of binding. See also Formation constants, Dissociation constants, Gibbs in metal complexes 307 Streptavidin 728 Stromelysins 627 Strontium 19... 

---