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Increases in the levels of staling aldehydes during beer aging are likely to involve bound aldehydes, already present in fresh beer. In particular, bisulfite adducts and imines have been reported as bound-state aldehyde forms and thus potential sources of off-flavor aldehydes. In this study, using headspace solid-phase microextraction followed by gas chromatography-mass spectrometry, a new type of bound-state aldehyde is proposed, namely 2-substituted 1,3-thiazolidine-4-carboxylic acids, resulting from the reaction between the amino acid cysteine and an aldehyde. The binding behavior of aldehydes toward cysteine appears to be similar to their binding behavior toward bisulfite and can be linked to the electrophilicity of the carbon atom of the carbonyl group of the particular aldehyde. Remarkably, within a pH range of 4.4 to 6.0, relevant to malting and brewing, the pH of the reaction mixture was shown to affect the binding of (E)-2-nonenal to both bisulfite and cysteine (more binding at higher pH) but not the binding of its saturated counterpart, i.e., nonanal. Finally, within the experimental setup employed, imine formation resulting from interaction between amino acids and aldehydes was not confirmed. Keywords: Beer aging, Bisulfite, Imine, Flavor stability, Staling aldehydes, Thiazolidine carboxylic acid
