Fourier transform infrared spectroscopic markers of the scorching degree of Scutellariae Radix related to the variation patterns of flavonoids and carbohydrates

Scutellariae Radix (SR) is a common herb in Asia and Europe. In the clinical practice of traditional Chinese medicine, the raw SR is often stir-baked to partly scorch to reduce the side effects (stomach discomfort, diarrhea, etc.) but enhance some desired effects (such as the hemostatic activity). The scorching degree is crucial to assure the safety and efficacy of the scorched SR. Under-scorching is insufficient to adapt the adverse and favorable activities, while over-scorching can destroy all activities. Up to now, the scorching degree of SR is still determined by the manual observation of colors. Since the visual judgement is vulnerable to personal knowledge and experience, it is difficult to control the optimization and consistence of the scorching degree of SR. This research was designed to explore the potential indicators that can exactly reflect the scorching degree of SR and be measured objectively and quantitatively. A total of 15 morphological and chemical properties as well as Fourier transform infrared (FTIR) spectroscopy were included to profile the variations of the SR slices that were stir-baked at different temperatures for different times. According to the principal component analysis and statistical tests, 10 properties showed close relationships with the scorching degree of SR. As the increase of the heating time at the same heating temperature, the yield, L*, a*, b*, sucrose, baicalin and wogonoside decreased continuously and showed the monotone variation patterns. Meanwhile, 5-HMF, baicalein and wogonin showed the inflection variation patterns, which means these properties increased in the earlier stage but decreased in the later stage during the scorching process. FTIR spectra also revealed the variations of flavonoids and carbohydrates. The scorching degree of SR can be determined by the intuitive observation of FTIR spectral peaks at 1738 cm−1, 1705 cm−1, 1611 cm−1, 1586 cm−1, 1450 cm−1, 1410 cm−1 and 1023 cm−1. In summary, FTIR spectroscopy can be used for the rapid assessment of the scorching degree of SR, while the contents of flavonoid glycosides and aglycones may be used as the quality criteria of the scorched SR.