Effect of 2-alkenals on the Maillard reaction of cysteine-glucose: Initial stage intermediate formation and flavor compound generation pathways

To explore the effect of (E)-2-hexenal, (E)-2-heptenal, and (E)-2-octenal on the initial Maillard reaction pathways and formation of volatile compounds, cystine-Amadori and thiazolidine derivatives were synthesized, identified by UPLC‒TOF/MS and NMR, and quantified by UPLC‒MS/MS. Depending on the concentration of the intermediates, the reactivity of the 2-alkenals in the reaction became weaker with the extension of the carbon chain. According to the content of 87 volatile compounds detected by GC‒MS‒O, the Maillard reaction was inhibited by decreasing the number of carbon atoms of 2-alkenal. The carbohydrate module labeling method demonstrated that the 2-ethylthiophene, 2-propylthiophene, and 2-butylthiophene carbon backbones were derived from 2-alkenal. Particularly, it was found for the first time that 2,3-dihydro-1 H-indole could be generated from both the intact skeleton of glucose and the degradation product of 2-alkenal. Notably, we proposed a route for the formation of 2-butanoylthiophene starting from (E)-2-octenal which was further converted to 2,4-octdienal. This study can be used as guidance in improving the flavor of meat and savory flavorings.