Standardization of Rehmannia glutinosa (Gaertn.) DC. steam processing and evaluation of its chemical, anti-oxidant, and anti-inflammatory properties

Rehmannia glutinosa (Gaertn.) DC., belonging to the family Scrophulariaceae, is an important medicinal herb cultivated in East Asia. Traditionally, R. glutinosa is steam processed to increase its efficacy in treating various ailments such as diabetes, hematinic deficiencies and adrenal disorder. However, standardization of processed R. glutinosa is highly needed to increase its quality to fulfill global market demand that is safe and possess high level of efficacy. Therefore, this study aimed to optimize the R. glutinosa steam processing methods by evaluating some key parameters such as steaming temperature, number of steaming times, steaming duration, and additive supplementation. R. glutinosa samples were steam processed at different temperatures (100 °C, 110 °C, and 120 °C), various steaming times (1 to 5 times), several steaming duration (1 to 4 h), and additives supplementation (rice wine, 5% EtOH, 10% EtOH, 20% EtOH, 30% EtOH, and 40% EtOH). As the result, 2 h, 3 replications, and supplementation with 20% EtOH at 120 °C were identified as the optimal conditions for R. glutinosa steam processing. Optimized processed R. glutinosa (SPRR 20%EtOH) resulted in significantly higher content of 5-HMF (7648.60 ± 150.08 µg/g) and iso-verbacoside (203.80 ± 10.72 µg/g) compared with unprocessed R. glutinosa (UPR). Compared to those of other samples, SPRR 20% EtOH samples had higher total flavonoid (55.36 ± 1.68 mg/g) and phenolic (69.24 ± 4.56 mg/g) contents and stronger DPPH antioxidant activity (56%). Furthermore, SPRR 20% EtOH had excellent anti-inflammatory activity, as evidenced by the suppression of inducible nitric oxide synthase (iNOS) caused by activation of nuclear factor-κB (NF-κB) through p-p65 pathway in LPS-stimulated RAW 264.7 cells. These findings will provide a basis towards industrialization of R. glutinosa processing technology that will be very helpful for oriental medication field.