Effects of pH on the Flavor Detection Threshold and DoT of Nine Kokumi Peptides in Soybean Fermented Foods

The purpose of this study was to determine the influences of product pHs on retronasal thresholds of nine kokumi peptides commonly found in fermented soybean foods. The following nine kokumi peptides were used in this study: γ-EV (γ-Glu-Val), γ-EW (γ-Glu-Trp), γ-EY (γ-Glu-Tyr), γ-EF (γ-Glu-Phe), γ-EL (γ-Glu-Leu), γ-EM (γ-Glu-Met), γ-EE (γ-Glu-Glu), γ-EH (H-His-Glu-OH), and γ-EVG (γ-Glu-Val-Gly). Retronasal threshold testing was conducted using five series of the 3-ascending forced-choice (3-AFC) method on water-based matrices with three different pH levels (pH 5.0, 5.3, 5.8). Overall, BET values of kokumi peptides decreased (p < 0.05) with increasing pH. This result suggests that a lower kokumi peptide concentration is required for humans to detect components (kokumi) of fermented food-related kokumi taste at pH 5.8 than the kokumi peptide concentration at pH 5.0–5.3. In other words, human-detected kokumi perception is better recognized at a product pH of 5.8. In addition, the DoT value of each kokumi peptide was only higher than 1 in γ-EF, which affected the peptide involved in the kokumi flavor of doenjang.