Aldehydes Strecker

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. 

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

Strecker aldehydes (Figs. 3 and 4). All 5 aldehydes increased during storage, but none exceeded more than 4.3% of their respective flavor thresholds. 

The aldehydes 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, methional, and phenylacetaldehyde are so-called Strecker aldehydes, formed as a result of a reaction between dicarbonyl products of the Amadori pathway and amino acids, having one less carbon atom than the amino acid (i). 

Martin, F. L., Ames, J. M. (2001). Formation of Strecker aldehydes and pyrazines in a fried potato model system. 

Proline and hydroxyproline differ from the other amino acids in that they contain a secondary amino group in a pyrrolidine ring therefore, they do not produce aminoketones and Strecker aldehydes in the reaction with dicarbonyls, ffowever, nitrogen heterocyclics are produced, including 1-pyrroline, pyrrolidine, 2-acetyl-1-pyrroline and 2-acetyltetrahydropyridine (Scheme 12.5) [19]. 

Aliphatic carbonyl compounds, such as diacetyl, which has a butter-like odour, also may contribute to the aromas derived from the MaiUard reaction, and many of the Strecker aldehydes also have characteristic aromas (Table 12.1). 

Amino acid Strecker aldehyde Odom description... 

Proline Pyrrolidine, 1-pyrroline. No Strecker aldehyde Important intermediates for bread-like aromas... 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

Two branches of the above reaction pathways provide active reagents for the degradation of a-amino acids to aldehydes and ketones of one less carbon atom (Strecker degradation), which is another arm of the Maillard reaction. Strecker aldehydes from these reactions are important flavor compounds (18). 

It becomes clear that analytical methods based on the evaluation of the end products of deteriorative reactions will not be satisfactory. Therefore in our own experiments amino acid analysis of Amadori compounds and gas chromatography of volatile Strecker aldehydes were applied to detect the onset of the Maillard reaction well before detrimental sensory changes occurred. 

Oxidation of carbohydrate or Strecker aldehyde by S-MnOz to form carboxylic acid, or formation of carboxylic acid during Strecker degradation... 

Also acetic acid may arise from a reaction of this type. Most important compounds of this pathway are pyruvic aldehyde, diacetyl, hydro-oxyacetone and hydroxydiacetyl which can easily react with amino acids. The Strecker degradation is a reaction where the amino acid is de-carboxylated and loses its amino group. Reaction products are the Strecker aldehyde and - as an intermediate - an aminoketone which forms a pyrazine by dimerization. This pathway is considered to be most important for the origin of pyrazines in thermal aromas. However, only limited knowledge is available about the fate of the Strecker aldehydes. As we will demonstrate they are very reactive. 

Dicarbonyl compounds react with amino acids to yield the Strecker aldehydes and aminoketones which can be converted via dimerization to yield pyrazines. 

The question of the fate of the "Strecker" aldehydes requires an answer. By converting the amino acid phenylalanine to yield aroma compounds, phenyl acetaldehyde is liberated. Because of its phenyl ring it is a good detector compound. We were able to establish some of its reaction products. For example, we have identified, among others, phenylethylpyrazine, phenylfuran, phenylethylpyrrole and phenylpyri-dine. We assume that aldol condensations are responsible for the formation of these compounds. Figure 5 illustrates our assumption.We have identified several compounds the structures of which make probable an aldol condensation (3-(2 -furyl)-2-phenyl-2-propenal, phenyl hydroxyketones) likely. This assumption is supported by the identification of pyrazines with up to 5 carbon atom side chains in other experiments. 

Maillard products which were identified in methionine/ reducing sugar model experiments, result predominantly from the Strecker aldehyde (methional) and methylmer-captan, respectively. Figure 10 summarizes compounds... 

Rothe (5, , 1 5) calculated the aroma values of some volatiles identified in the crumb of wheat bread and the crust of rye bread. The data listed in Table I indicate that ethanol, isobutanal, iso-pentanal, diacetyl and isopentanol contribute with high aroma values to the aroma of the wheat bread crumb. During baking of rye bread, the two Strecker aldehydes, isobutanal and isopentanal, increased so much in the crust that they showed the highest aroma values of the volatiles investigated. 

Preparation of Microwave and Conventional Cakes. Diacetyl and acetoin were added at 200 ppm to a commercially available cake mix. The conventional cake was baked 35 minutes at 250 degrees F. in a standard General Electric electric oven. The microwave cake was baked 6.5 minutes in a 600 Watt G.E. Space Maker Microwave Oven. Diacetyl and acetoin concentrations were determined by gas chromatographic headspace analysis as previously described for quantitation of the Strecker aldehydes. 

Strecker Aldehyde Formation. Formaldehyde, acetaldehyde and iso-butyraldehyde were formed by Strecker degradation of glycine, alanine and valine, respectively. Relative concentrations of aldehydes produced by microwave and conventional heating to comparable temperature is shown in Table I. Significantly higher concentrations were observed for microwave heated samples. 

Table I. Strecker-Aldehydes Produced by Microwave and Conventional Heating of Amino Acids and Diacetyl...

Tressl et al7r J1 designated the linear polymers as Type I and the branched ones as Type II. In most melanoidins, they would represent domains (or substructures), unsubstituted pyrroles and Strecker aldehydes, for example, being integrated into the melanoidin backbone, giving a complex macromolecular structure overall. Tressl et aV1 consider the oligomerisation/polycondensation reactions described as the only experimentally established pathways by which simple Maillard products generated from hexoses and pentoses are easily and irreversibly converted into macromolecules. 

Table 3.5 Arrhenius rates used to fit the experimental concentrations of pyrroles, furans, Strecker aldehydes, and pyrazines from a glucose-alanine system as a function of time and temperature130...

For the thiophen mentioned above, all the carbon atoms stemmed from cysteine, carbohydrate apparently not having a role. On the other hand, theoretically, thiophens can be derived from furans simply by reaction with H2S. Belitz and Grosch251 postulate derivation from 2-mercaptoethanal (Strecker aldehyde from cysteine) and acrolein or butenal (aldol condensation product from acetaldehyde). Vemin and Parkanyi216... 

Strecker aldehydes, the main volatiles of Group 2, tend to have low threshold values (phenylethanal, 4 /j.g L 1 in water - thresholds will be given in these units unless stated otherwise and are taken from Rychlik et al.104 and Maarse276 3-methylbutanal, 0.35 methional, 0.2). Some Group 1 volatiles have comparable olfactory power (HDMF, 0.6 butanedione, 4), but others do not (norfuraneol, 2100 maltol, 9000 acetic acid, 22000). The threshold value of acetaldehyde (15), although a Strecker aldehyde, is only moderately low. 

It is worth noting that, in being converted into acrylamide, the Strecker aldehyde from asparagine needs to have a hydroxy group reduced as well as to lose a molecule of water. 

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