Cascading chemical transitions of rhubarb (Rhei Radix et Rhizoma) during the scorching process revealed by heated ATR-FTIR spectroscopy and two-dimensional correlation analysis

Rhubarb has a long history of medicinal applications in Asia and Europe. For some clinical applications of traditional Chinese medicine, rhubarb has to be stir-baked to scorch to eliminate the purgative activity, but increase the functions of blood-cooling and hemostasis. Understanding the chemical transitions during the stir-baking process is necessary for the quality assessment a quality control of scorched rhubarb. This research used the heated ATR-FTIR spectroscopy to study the thermal transitions of rhubarb and related compounds. Critical points during the heating process of rhubarb, emodin, emodin-8-glucoside, glucose and 5-HMF were recognized from PCA score plots. Two-dimensional correlation spectroscopy (2DCOS) was used to interpret the spectral changes of rhubarb during different thermal stages. It was found that the decomposition of anthraquinone glycosides and the formation of aromatic carbon networks at high temperature (above 170 °C) may be the primary reactions to generate scorched rhubarb. These results are helpful for the reasonable procedure optimization and quality control of the stir-baking process to produce scorched rhubarb.