N-Propionyl aromatic Amino acids restore taste balance in reduced-salt soy sauce

Reduced-salt soy sauce often exhibits excessive acidity and residual bitterness. To mitigate these defects, N-propionyl-L-tryptophan (NPr-Trp), N-propionyl-L-tyrosine (NPr-Tyr), and N-propionyl-L-phenylalanine (NPr-Phe) were synthesized using an optimized thermal route. NPr-Trp achieved the highest yield (25.36%), consistent with its lowest calculated activation barrier for thermal synthesis (ΔG^‡ = 102.9 kJ·mol^-1). Sensory evaluation showed that NPr-Trp reduced sourness (38%) and bitterness (31%) in reduced-salt soy sauce while enhancing umami and kokumi; similar effects were observed for NPr-Tyr and NPr-Phe. Docking simulations suggest that these derivatives may suppress sourness by stabilizing OTOP1 in a closed state and may reduce bitterness by occluding TAS2R binding pockets. Additionally, computational screening suggests that the propionyl (C₃) chain provides an optimized hydrophobic motif for receptor anchoring. Collectively, these findings support the use of N-propionyl aromatic amino acids as effective taste modulators to improve the flavor quality of low-sodium fermented foods.