Strecker degradation products

Strecker Degradation Products from (1-13C)-d-Glucose and Glycine... 

This important flavor compound was identified in the head-space volatiles of beef broth by Brinkman, et al. (43) and although it has the odor of fresh onions, it is believed to contribute to the flavor of meat. This compound can be formed quite easily from Strecker degradation products. Schutte and Koenders (49) concluded that the most probable precursors for its formation were etha-nal, methanethiol and hydrogen sulfide. As shown in Figure 5, these immediate precursors are generated from alanine, methionine and cysteine in the presence of a Strecker degradation dicarbonyl compound such as pyruvaldehyde. These same precursors could also interact under similar conditions to give dimethyl disulfide and 3,5-dimethyl-l,2,4-trithiolane previously discussed. 

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed as a Strecker degradation product of cysteine in the presence of a diketone (37). 

Simple amino acid degradation products Aldehydes, cf. Strecker degradation Gly CH20 Ala CH3CHO... 

Aroma of Strecker Degradation Products of a-Amino Acids with Isatin (Diketodihvdroindole) ... 

Roasting cocoa beans results in the production of volatile and non-volatile compounds which contribute to the total flavor complex. 5-Methyl-2-phenyl-2-hexenal, which exhibited a deep bitter persistant cocoa note, was reported in the volatile fraction (53). It was postulated to be the result of aldol condensation of phenylacetaldehyde and isovaleraldehyde with the subsequent loss of water. The two aldehydes were the principal products of Strecker degradation products of phenylalanine and leucine, respectively. Non-volatiles contained diketopiperazines (dipeptide anhydride) which interact with theobromine and develop the typical bitterness of cocoa (54). Theobromine has a relatively stable metallic bitterness, but cocoa bitterness is rapidly noticed and disappears quickly. 

The premise is to utilise a liquid film to provide a reaction environment which can be dynamically controlled in terms of heat and mass flux (influx/effiux) and to complement this with the on-line monitoring technique of Atmospheric Pressure chemical Ionisation (APcI)-Ion Trap Mass Spectrometry (ITMS). This technique allows the flux of protonated molecular ions (Mlf to be directly monitored (mass spectral dimension 1) and to fragment these species under tailored conditions within the ion trap (Collision Induced Dissociation (CID),mass spectral dimension 2), to produce fragment ions representative of the parent ion. This capability is central to allowing species with a common molecular weight to be quantified, for example butan-2,3-dione (MW=86 MH =87, glucose degradation product) and 3-methylbutanal (MW=86 MH =87, Strecker aldehyde from leucine). 

In the WGH, DWGH, and AWGH model systems we identified and quantified 55, 51, and 53 volatile confounds, respectively (Table I). The major volatiles identified were fiirans, pyrazines, aldehydes, alcohols, ketones, pyrrolizines. These volatile confounds were mainly derived fiom Strecker degradation, sugar degradation, lipid degradation, and the further interactions of these degradation products. 

Using labeled precursors, Blank et al. (1996) explained the formation of homofuraneol in reactions of xylose with alanine (preferentially to glycine). The proposed mechanism suggested the incorporation of the Strecker degradation product, acetaldehyde. This mode of formation is preferred to sugar fragmentation. 

Its formation during roasting is probably due to the reaction of ammonia on 2,3-hexanedione and acetaldehyde, the Strecker degradation product of alanine. 

During Strecker degradation of [l-i CJ-D-glucose with primary a-amino acids, pyrroles and pyridinols are formed as major products (6). 4-Aminobutyric acid and peptide bound lysine are transformed into [i3CHO]-2-formyl-5-hydroxymethylpyrroles (9). Amino acids like Val, He, Leu, Phe and Met are transformed into 2-[i3cjjO]-pyrrole lactones (70). Equimolar amounts of cysteine (methionine) and [l(or 6)- C]-D-glucose were heated for 1,5 h at 160°C in aqueous solution at pH 5. The volatiles were extracted with pentane/ether and analyzed as described (7). In Table I selected (unlabeled) Strecker degradation products from cysteine and methionine are summarized. Pyruvat (1), 2- and 3-mercaptopropionic acids (2, 3) from cysteine as well as 2-oxo-5-thiahexanoic acid (4) and 5-thiahexanoic acid (5) from methionine. 

Table I. Strecker Degradation Products from [l- C]-D-Glucose with Cysteine or Methionine...

Chan, R, Reaction Kinetics for the Formation of Strecker Degradation Products in Model Systems, M.S. Thesis, University of Minnesota, Minneapolis, 1994. 

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