Two-dimensional correlation spectroscopy indicates the infrared spectral markers of the optimum scorching degree of rhubarb (Rhei Radix et Rhizoma) to enhance the anti-inflammatory activity

Rhubarb has been used as herbal purgative with a long and worldwide history. But purgation is a side effect of rhubarb in many cases. In traditional Chinese medicine, rhubarb can be stir-baked to scorch to attenuate the purgative function while enhance other bioactivities such as the anti-inflammatory effect. However, the over-scorched rhubarb will lose necessary active ingredients and even generate toxic ingredients. Nowadays, the scorching degree of rhubarb is still determined by the intuitive observation of color changes in production. Therefore, this research was designed to develop more reasonable and objective criteria to evaluate the scorching degree to ensure the efficacy, safety and consistency of the scorched rhubarb. Taking the example of the rhubarb baked at 200 °C for different times, the combination of trait indicator (color), chemical indicator (combined and free AQs, sennosides, gallic acid, 5-HMF) and biological indicator (anti-inflammatory) showed that the rhubarb baked for 30 min can be treated as the right scorched. Two-dimensional correlation spectroscopy helped to reveal the infrared spectral markers of the water extract near 1694 cm^-1, 1442 cm^-1 and 825 cm^-1, as well as the relative strength of the absorption bands of the powder near 1610 cm^-1 and 1020 cm^-1, to discriminate the right-scorched and over-scorched rhubarb. In conclusion, the combination of trait, chemical and biological indicators can provide reasonable and objective criteria for the optimum scorching degree of rhubarb, while FTIR spectroscopy can assess the right endpoint of the scorching process in a rapid, cheap and green way.