Two-step mass spectrometric approach for the identification of diketopiperazines in chicken essence

Diketopiperazines (DKPs) have been found in a variety of foods and beverages that impart a metallic bitter taste. The presence of DKPs in chicken essence (CE) was first postulated by a two-step monitoring technique, with mass precursor ion scanning from the immonium ion of a specific amino acid first, followed by mass matching with a DKP database. The postulated DKPs were further verified by tandem mass spectrometric (MS/MS) analysis. It was found that this technique performed well, with about 20 DKPs tentatively identified. Proline was an important constituent of the DKPs in CE, together with other non-polar amino acids, such as glycine, alanine, valine, leucine/isoleucine and phenylalanine, which also showed great magnitude. Some proline-based DKPs in CE were quantified before and after thermal treatment at 130degreesC for 1 h. The concentrations of the DKPs were determined not only by the relative ease of formation and the stability of the DKPs, but also by past thermal processings.