Isolation and identification of flavor peptides from douchi (traditional Chinese soybean food)

Typical douchi has a particular sensory quality that makes it distinct and highly appreciated by consumers in China. The present study was performed to isolate, purify, and identify the flavor peptides of douchi by gel filtration chromatography, reversed-phase high-performance liquid chromatography (RP-HPLC), sensory evaluation, and electronic tongue analysis. Peptide fractions (P1, P2, P3, and P4) were purified from an aqueous extract of the water-soluble extracts (WSEs) of douchi using Sephadex G-25 gel filtration chromatography. Among all the fractions, P3 exhibited the highest peptide content of 841 ± 4 mg/g and strongest douchi, bitter, and umami taste, with a taste dilution (TD) value up to 128. P3 was further fractionated into P3a–P3j by RP-HPLC. Sensory evaluations and electronic tongue analysis showed that P3h elicited strong douchi, bitter, and umami taste as well as a taste similar to that of the WSEs. P3h was repurified by RP-HPLC to obtain the pure taste peptide for amino acid sequence analysis. Edman degradation revealed that the amino acid sequence of P3h was Ala-Phe-Asp-Glu-Lys. In addition, the synthetic peptides showed similar sensory characteristic to those of the isolated peptides and umami enhancement in the presence of monosodium glutamate, with threshold value of 0.05 mmol/L. Results showed that the flavor peptide isolated from douchi was a novel peptide.