Further Elucidation of Beer Flavor Instability: The Potential Role of Cysteine-Bound Aldehydes

The potential involvement of 2-substituted 1,3-thiazolidine-4-carboxylic acids in beer flavor stability was further investigated. The binding behavior of beer-aging aldehydes toward both cysteine and bisulfite was confirmed and compared in model solutions of varying pH values that are relevant to malting and brewing (pH 6.0, 5.2, and 4.4). It was found that binding of aldehydes increased with increasing pH, especially for binding to cysteine. Furthermore, a sample preparation approach was developed to release aldehydes from their bound-state. After binding between aldehydes and cysteine, the strong base 4-vinylpyridine (4VP) was added to the samples as a competitor of the aldehydes toward cysteine binding, and subsequent release of aldehydes was clearly observed. The same approach also resulted in a release of aldehydes from preformed bisulfite adducts. Sample treatment with 4VP was also applied to fresh pale lager beer, resulting in increased levels of free beer-aging aldehydes. Moreover, the presence of furfural-derived 1,3-thiazolidine-4-carboxylic acid in fresh pale lager beer was confirmed and quantified by a newly developed ultra-performance liquid chromatography-UV method. The insights gained in this study strengthen the hypothesis that 2-substituted 1,3-thiazolidine-4-carboxylic acids may play an important role in beer flavor stability or instability.