Strecker degradation

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Strecker degradation of amino acids: Amino acids can react with α -dicarbonyl compounds, such as the intermediates in browning reactions. The amino acid is converted into an aldehyde with one fewer carbon atom (Blockmans et al., 1975; Hashimoto and Kuroiwa, 1975 ). Polyphenols may have a catalytic role ( Blockmans et al., 1979 ).

Strecker degradation of amino acids has been suggested as pathways for the formation of carbonyl compounds.

Strecker aldehydes are produced as a consequence of Maillard reaction.

The Strecker aldehyde phenylacetaldehyde is a primary odorant in beer.

Transamination can take place between an amino acid and an α-dicarbonyl in a reaction called “Strecker degradation” (Figure 8). The nucleophilic addition of the unprotonated amino group to the carbonyl group initiates the reaction, forming an unstable hemiaminal. This readily undergoes reversible loss of water, followed by irreversible decarboxylation, yielding an imine zwitterion. The addition of water results in an unstable amino alcohol, which decomposes into an α-ketoamine and a “Strecker aldehyde”, containing one carbon atom less than the amino acid from which it is derived.21,123,127 In principle, the large number of different amino acids can give rise to different Strecker aldehydes. However, when the difference in concentration of individual amino acids is considered, in combination with the flavor threshold of the respective Strecker aldehydes, only a few Strecker degradation reactions are of interest in beer flavor: 2-methylpropanal from valine, 2-methylbutanal from isoleucine, 3-methylbutanal from leucine, methional from methionine, and phenylacetaldehyde from phenylalanine. Although benzaldehyde is thought to be formed indirectly from phenylalanine with phenylacetaldehyde as intermediate, it is still considered to be a Strecker aldehyde. Several pathways have been proposed, of which many involve the presence of oxygen.128,129 An example is the free radical initiated oxidation, as described by Chu and Yaylayan128 (Figure 9). The Strecker degradation is often categorized under “Maillard reactions”, because various α-dicarbonyls can be produced by Maillard reactions as shown before.127 However, these compounds can originate from more diverse sources, such as oxidation of polyphenols or the transformation of 2,3-butanedione (diacetyl) and 2,3-pentanedione precursors, excreted by fermenting yeast.130

Strecker-like Reactions: The reaction of an amino acid with an α-unsaturated carbonyl compound, replacing the α-dicarbonyl in the Strecker degradation strictly speaking, is termed a “Strecker-like” reaction. An example of such an α-unsaturated carbonyl is (E)-2-nonenal, derived from lipid degradation. Furfural, derived from Maillard reactions, is an option as well,123 as is benzaldehyde. The initiation of this Strecker-like reaction, the loss of water, and subsequent decarboxylation are similar to Strecker degradation, forming an imine zwitterion (Figure 10). Addition of water and degradation of the unstable amino alcohol can result in, among others, a Strecker aldehyde and, in some cases, a dihydro derivative of the initial unsaturated aldehyde (e.g., nonanal from (E)-2-nonenal) after release of ammonia. This pathway is, however, based on nonaqueous model systems and has not been confirmed in aqueous solutions, but it is likely that it comprises a Strecker aldehyde source in food products.123 Other similar Strecker-like reactions have been identified as well, involving α-cyclopropylcarbonyls, α-epoxycarbonyls, α-epoxyenals, α-epoxyenones, and 4-hydroxy-2-alkenals.123 This fact illustrates an overlap in reaction mechanisms that were considered separately in the past, because some of these compounds can be found among lipid degradation products.

Direct Strecker Aldehyde Formation from Amadori Compounds: Strecker aldehydes are also thought to be formed from Amadori compounds by direct reaction with amino acids123,127,132 or via transition metal ion-catalyzed oxidation of the Amadori compound123,127,133 (Figure 11). However, research on these reactions was performed in model systems and it is, therefore, not clear yet as to what extent these reactions are relevant in beer production processes. Nevertheless, the observation that more Strecker aldehydes are generated during beer aging in the presence of oxygen, and less in the absence, supports this hypothesis.

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