Maillard reaction contributing to chemical changes during the processing of Shu Dihuang, the steamed root of Rehmannia glutinosa

The dried root of Rehmannia glutinosa (Scrophulariaceae), Sheng Dihuang, is a well-known cooling crude drug. After a long period of steaming, it is converted to another form, Shu Dihuang, an antianemic drug. Until now, there has been little information about the chemical basis of this functional transformation. In this work, slices of Sheng Dihuang were steamed for different times (0-60 h). The absorbance at 420 nm of a sample of the aqueous extract increased more than 16 times, while the pH value had decreased from 5.4 to 3.8 by the end of the experiment. The processing was divided into two stages: the early stage (0-12 h) and the later stage (12-60 h). In the early stage, amino acids fell by more than 70%, catalpol, sucrose, raffinose and stachyose decomposed, while 5-hydroxymethyl furfural (HMF), glucose, fructose and manninotriose increased. In the later stage, however, amino acids remained at a low level, while HMF and A420 increased linearly. These drastic chemical changes were directly or indirectly related to the Maillard reaction which took place during the preparation of Shu Dihuang.