To investigate the dynamic change of chroma and viscosity during the nine times steamed and processed Dihuang (Rehmanniae Radix) and the correlation with its intrinsic quality, and to explore the optimal processing end point by establishing a comprehensive quality index (QI), so as to provide reference for the quality control and processing end point judgment of Dihuang (Rehmanniae Radix).

The chroma and viscosity of Dihuang (Rehmanniae Radix) samples were quantified objectively by thin layer imager, MATLAB and viscometer. The polysaccharide content was determined by ultraviolet spectrophotometry, and the contents of Rehmannium D, veronin, iso-veronin, motheronin and 5-hydroxymethylfurfural (5-HMF) were determined by high performance liquid chromatography (HPLC) . SPSS 26.0 and SIMCA software were used to conduct multivariate statistical analysis of the results, and the correlation and change rule of chroma, viscosity and their internal chemical composition were analyzed. Then, the QI of nine times steamed and processed Dihuang (Rehmanniae Radix) was established by triangle area method to predict the optimal processing end point.

During the process of steaming process of nine times steamed and processed Dihuang (Rehmanniae Radix), the color of the prepared product turned black and the viscosity increased. There were correlations between chroma and viscosity and different processing degrees, indicating that chroma and viscosity could be used as the evaluation indexes for processing end points. There was a correlation between the color of decoction pieces and the color of powder, and the first or second steaming may be the main stage of the color change of Dihuang (Rehmanniae Radix). The correlation between chroma, viscosity and intrinsic chemical composition can be established, and a better regression model can be established, and the change of chemical composition content in the processing process can be objectively predicted by measuring chroma value I and viscosity to a certain extent. The principal component analysis of intrinsic chemical composition showed that the first to fourth steaming may be the main stage of the change of chemical composition content of Dihuang (Rehmanniae Radix) . The quality index QI value was the highest when the seven-steaming system was used, which suggested that the seven-steaming system might be the best processing end point of Dihuang (Rehmanniae Radix) .

The color and appearance of Dihuang (Rehmanniae Radix) can be quantified objectively based on chroma and viscosity, and the correlation between them and component content is analyzed and a regression model is established, which can objectively predict the changes of internal chemical component content in the processing of Dihuang (Rehmanniae Radix) to a certain extent. The establishment of comprehensive quality index QI can predict the optimal processing endpoint. It provided reference for the quality control and processing end point determination of Dihuang (Rehmanniae Radix) processing process.