Characterization of fennel extracts and quantification of estragole : optimization and comparison of Accelerated Solvent Extraction and Soxhlet techniques

Fennel (Foeniculum vulgare Miller) is an aromatic plant used, among other applications, in the production of traditional herbal liqueurs. In this study, essential oils from fennel were extracted applying two techniques, Soxhlet and Accelerated Solvent Extraction (ASE). The extracts obtained were characterized by CG-MS. Taking into account that estragole is the major constituent of fennel and due to recent studies pointed out its possible carcinogenic properties; this compound was also quantified by CG-FID. The quantification method showed good linearity (r2 = 0.998) and precision (RSD < 5%) with low values of detection (LOD) and quantification (LOQ) limits. A Box-Behnken design was used to correlate three independent variables (temperature, contact time sample-solvent and number of cycles) with the amount of estragole extracted. Meanwhile, the response surface methodology was applied to optimize the extraction of estragole by ASE. The optimal conditions were 125 ºC, 7 min and 3 cycles. On the other hand, the Soxhlet technique was studied step-by-step. Two variables were optimized: time (4 and 8 hours) and solvents, according to their polarity. Methanol and 4 hours of extraction showed the best results both qualitatively and quantitatively. Soxhlet technique provides higher performance of extraction and greater amounts of compounds extracted compared to ASE, but similar concentration of estragole. The shorter time of extraction and the lower amount of solvent used justify the ASE technique choice to characterize fennel essential oils.
We are grateful for the financial support of this work to the Spanish Ministry of Science and Innovation (project CTQ2011-28967) which has partial financial support from the FEDER funds of the European Union. Jose Manuel Salgado is grateful for Postdoctoral fellowship (EX-2010-0402) of Education Ministry of Spanish Government.