Assessment of conventional and novel extraction techniques on extraction efficiency of five anthraquinones from Rheum emodi

Rheum emodi is principally known to consist 1,8-dihydroxyanthraquinones (DHAQs) that find immense use in the chemical, pharmaceutical, cosmetic industries and in herbal medication and food sector. The aim of this study was to compare non-conventional and classical methods for extraction of anthraquinones from R. emodi. Optimisation of the extraction parameters for various methods was done and their extraction efficiency was evaluated. In preliminary screening experiments, choice of solvent and solid : solvent ratio was optimised. Comparison of extraction efficiency for classical methods like maceration, heat-reflux, soxhletion and non-conventional methods like ultra-sonication and sublimation was done for five DHAQs - aloe emodin, rhein, emodin, chrysophanol and physcion using HPLC-UV and fluorescence detection in native and acid hydrolysed samples. It was observed that ethanol was the best solvent for extraction of anthraquinones with a solid : solvent ratio of 1:20. A prior acid hydrolysis led to significant increase in anthraquinone extraction. Among the extraction methods heat reflux for 45 min was the most prominent extraction method with highest recovery of the DHAQs. In ultrasonic assisted extraction, an increase in the anthraquinone extraction was seen till 45 min after which the concentration declined. A novel, solvent-free, green and selective method of extraction by sublimation was found to be effective for extraction of anthraquinones.