Characterization of the alkali-induced protein cross-linking in buckwheat sourdough steamed bread

This study elucidated the alkali-induced protein cross-linking behaviors of buckwheat sourdough steamed bread (BSSB). When the alkali concentration increased to 1.5% (on a total flour weight basis), the protein extractability in sodium dodecyl sulfate (SDS) decreased by 2.29% while the disulfide (SS) bond content increased by 5.97 μmol/g protein, indicating that the alkali promoted the SS cross-linking of the proteins. The formation of dehydroalanine (DHA) and lanthionine (LAN) at alkali concentrations of 2.0% and 2.5% demonstrated the occurrence of DHA-derived cross-linking. To explore the role of SS cross-linking in the quality of BSSB, the thiol-blocking agent N-ethylmaleimide (NEM) was also employed. It was determined that the BSSB with NEM had almost no gas cells and that the specific volume was 48.41% lower than the control. The alkali increased the specific volume by up to 26.75% at 2.0% concentration and ameliorated the crumb structure of the BSSB. Dynamic thermo-mechanical analysis revealed that the alkali also delayed the starch gelatinization and gas cell opening, which may have contributed to the gas holding capacity of the BSSB. The amino acid residues involved in the DHA-derived cross-linking and the potential cross-linking sites were identified via liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. It was found that both the Cys197 from wheat peroxidase and the Cys105 from the wheat alpha-amylase/trypsin inhibitor CM16 were crucial to the formation of the DHA-derived cross-links and that the LAN between two cysteines was inter-chain.