Chemical Mechanisms Underlying Sweetness Enhancement During Processing of Rehmanniae Radix: Carbohydrate Hydrolysis, Degradation of Bitter Compounds, and Interaction with Taste Receptors.

Thermal processing is widely applied in food manufacturing to enhance flavor, but the mechanisms underlying taste transformation remain insufficiently understood. Rehmannia Radix, traditionally processed by steaming, undergoes a distinctive shift from bitterness to sweetness, serving as a representative model for flavor modulation during processing. In this study, sensory evaluation (n = 12), electronic tongue analysis, HPLC-based sugar and marker profiling across 17 batches, and molecular docking with representative human taste receptors were combined to investigate the mechanisms of taste transformation. The results showed that steaming markedly increased sweetness while reducing bitterness (p < 0.05). Chemical profiling revealed the hydrolysis of oligosaccharides into higher-sweetness monosaccharides (e.g., fructose (Fru) +15.99%, glucose (Glu) +8.90%) and substantial degradation of bitter iridoid glycosides (e.g., catalpol (Cat) -88%). In addition, the formation of 5-hydroxymethylfurfural (5-HMF) was identified as a processing marker. Molecular docking suggested that bitter glycosides in raw samples may interfere with sweet receptor activation and stimulate bitter receptors, whereas monosaccharide enrichment and Maillard products favored sweet receptor interactions, which may explain the observed sensory changes. Overall, these results clarify the chemical basis and receptor-level mechanisms of the bitterness-to-sweetness transition during steaming and identify markers useful for monitoring flavor changes in Rehmannia Radix.