Clinical and experimental evidences have confirmed the significant therapeutic effects of rhubarb on ulcerative colitis (UC), but the strong purgative function of rhubarb also aggravates UC symptoms such as bloody diarrhea. Stir-baking to scorch is a traditional Chinese medicinal processing method that can eliminate the adverse purgative function while keep or even enhance the UC therapeutic function of rhubarb. However, the under-baked rhubarb still have the undesirable purgative function, but the over-baked rhubarb may lose the required medicinal functions. Therefore, the determination of the right endpoint is the primary quality concern about the baking process of rhubarb. In this research, typical anthraquinone compounds and mid-infrared (MIR) spectra were recruited to determine the best baking degree of rhubarb for UC therapy. Raw rhubarb slices were baked at 180 °C with rotation to prepare the rhubarbs with different baking degrees. The right-baked rhubarb was defined according to the UC therapeutic responses as well as the traditional color criterion. Referring to the typical anthraquinone compounds in rhubarb slices and extracts, the baking degree of rhubarb may be assessed by the conversion ratio of anthraquinone glycosides to anthraquinone aglycones. MIR spectra showed the gradual decompositions of organic compounds including anthraquinone glycosides and tannins during the baking process. Rhubarbs with different baking degrees can be distinguished clearly by MIR-based principal component analysis. In conclusion, the ratio of anthraquinone glycosides to anthraquinone aglycones may be a reasonable chemical indicator of the right-baked rhubarb. Meanwhile, MIR spectroscopy can identify the right-baked rhubarb simply and rapidly.
Keywords: Anthraquinone; Chinese medicinal processing; Fourier transform infrared spectroscopy; Quality control; Rheum tanguticum; Ulcerative colitis.
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