Optimization of Ultrasonic-Assisted Extraction and Purification of Rhein from Cassia fistula Pod Pulp

Rhein is used as an active ingredient in laxatives in medicinal herbal products and is a chemical marker for quality control purposes. Thus, a simple and effective method for the optimized extraction of a high amount of rhein from the fruit pulp of Cassia fistula was investigated using ultrasonic-assisted extraction (UAE). The response surface methodology was applied to find the most suitable parameters for optimizing the extraction process and to study the factors’ relationships with each other. The best conditions for ultrasonic extraction were the application of 1:40 g/mL solid-to-liquid ratio and 10% EtOH–H2O as a solvent at 75 °C for 40 min. This method was compared to a conventional decoction in two variations. In these experiments, it was confirmed that the UAE was more favorable than the decoction methods. The resulting crude extract was further purified by liquid–liquid extraction with a basic pH adjustment, followed by recrystallization. High-purity rhein was obtained by using chromatographic techniques and nuclear magnetic resonance spectroscopy. Therefore, this study suggests that UAE is an efficient alternative method for the extraction of rhein from C. fistula pod pulp. The resulting optimized conditions can be applied as a useful tool for the large-scale industrial production of a rhein-rich plant extract.