Chinese medicinal materials (CMMs) often have to be processed by using physical or chemical treatment before prescription or clinical usage. The aims of processing are to alter the clinical efficacy and/or reduce the toxicity of CMMs to fulfill the different requirements of therapy [1]. There is a close relationship between the efficacy, safety and processing of CMMs. Improper processing methods may produce poor clinical effects or even result in poisoning [2]. Characteristics of CMMs such as color are usually used to assess the degree of processing. For example, when dry Zingiberis Rhizoma is roasted with sand until it becomes brown externally, it is called Zingiberis Rhizoma Praeparatum. Cirsii Japonici Herba is called Cirsii Japonici Herba Carbonisata when it is stir-fried until the surface becomes black. However, this approach often depends to a certain extent on the practitioner’s experience, which is strongly subjective and lacks objective criteria, so modern scientific analytical techniques are needed to confirm the validity of associations between the color and processing degree of CMMs.
Schizonepetae Spica (SS) is the dried spike of Schizonepeta tenuifolia Briq. (Chinese Pharmacopoeia, 2020 edition) [3]. It was first recorded in Shen Nong Ben Cao Jing (Shen Nong’s herbal classic), a book written 2000 years ago [4]. Chemical studies revealed that SS contains volatile oils, flavonoids, organic acids and so on [5–7]. Pharmacological analyses have shown that SS has three main biological properties, namely, anti-inflammatory [8], antiviral [9, 10], and hemostatic activity [11–13]. The essential oils of SS are recognized as the major constituents responsible for its anti-inflammatory and antiviral effects [8, 14]. In addition, the nonvolatile compounds such as flavonoids and organic acids from SS exhibit various biological activities. For example, luteolin-7-O-β-D-glucoside has been demonstrated to have good biological activity in terms of anti-inflammatory and antiviral effects [15, 16]. Apigenin-7-O-β-D-glucoside, hesperidin and luteolin have also been documented to possess significant antiviral activity [5, 17, 18]. Moreover, luteolin has been shown to be a potent hemostatic drug candidate [19]. Rosmarinic acid exhibits many biological properties, including antioxidant, antibacterial, anti-inflammatory and antiallergic effects [20]. Apigenin has notable anticancer activity in vitro and in vivo [21]. Thus, the contents of flavonoids and organic acids could be used as evaluation markers for SS. The herb is commonly used in traditional Chinese medicine (TCM) prescriptions to treat the common cold, fever, bloody stool and metrorrhagia [22]. To treat the common cold and fever, TCM practitioners often prescribe raw SS, while to treat bloody stool and metrorrhagia, they usually use Schizonepetae Spica Carbonisata (SSC; raw SS processed by stir-frying until the surface becomes blackish-brown and the interior turns dark yellow) [3]. However, the quality control standards of processing practices are inconsistent in terms of the national standard and various local standards. For example, the processing method “stir-frying until the surface becomes blackish-brown and the interior turns dark yellow” is recorded in the Chinese Pharmacopoeia (2020 edition), while stir-frying until the surface becomes dark brown and the interior turns brown is recorded in “The Practices of Processing Chinese Crude Drugs in Jiangsu Province” [23]. In addition, color discrimination of SSC is mainly based on the experience of individual operators. To guarantee the quality of SSC, a standardized processing method is critical. Therefore, the above problems of “SS with several processing methods and differences in different areas” should be harmonized, and consistent practices based on modern scientific studies need to be established.
In recent years, advances in sensor technologies such as colorimeters, electronic noses and electronic tongues have provided signals related to sensory attributes, making it easier to objectively characterize the color, aroma and taste of various products. Some studies have reported the use of a colorimeter to substitute the traditional color discrimination of CMMs [24, 25]. The colorimeter technique can be used to measure processed CMM products, with the additional advantage of controlling the technological parameters. Both colorimetry techniques and modern analytical techniques should be combined to map marker components in order to evaluate the scientific elucidation of color discrimination behind processing procedures and the quality of processing.
Therefore, the main purposes of this study were to investigate the relationship between chromatic aberrations and content variations in the marker nonvolatile substances of SS during processing in order to clarify the core scientific elucidation of color as an important indicator of the degree of processing. For this purpose, first, HPLC coupled with a diode array detector (DAD) was used to evaluate five flavonoids and one organic acid of SS during stir-fry processing, and the powder color was simultaneously determined using a colorimeter. Second, the correlations between marker constituents and chromatism were investigated by Pearson correlation analysis. Finally, polynomial regression models were established with the color parameters to rapidly predict the contents of the marker constituents. This was the first study to develop a method for the quantitative analysis of nonvolatile ingredients of SS using color parameters during stir-fry processing.