Exploration on the mechanism of “black as lacquer and sweetness as candy” based on the reactions of “crosslinking coloring and Maillard” and “oligosaccharide hydrolysis” during the processing of Radix Rehmanniae

Radix Rehmanniae (RR) has three processing forms in clinical practice, including fresh Radix Rehmanniae (FRR), raw Radix Rehmanniae (RRR), and processed Radix Rehmanniae (PRR). However, the sensory criterion, "black as lacquer and sweetness as candy", which was traditionally used to evaluate the quality of PRR, have not been fully elucidated. In this study, the mechanism of chemical transformation based on oligosaccharide hydrolysis, crosslinking coloring, and Maillard reactions was explored to clarify the observed changes in appearance and flavor. The physicochemical properties of different processed RR products were characterized. Besides, "sweetness as candy" was explained by oligosaccharides hydrolysis, and "black as lacquer" can attribute to crosslinking coloring and Maillard reactions. Importantly, this study provided the first evidence for the existence of crosslinking coloring reaction during the processing of RR. In conclusion, our results unveiled the scientific basis of the sensory attributes "black as lacquer and sweetness as candy" in PRR, offering valuable insights for enhancing quality control of PRR.