Characterization of fennel extracts and quantification of estragole: Optimization and comparison of accelerated solvent extraction and Soxhlet techniques

• Accelerated solvent extraction and Soxhlet techniques were optimized to identify essential oils from fennel seeds.
• Methanol resulted to be the most suitable extracting solvent for the target volatile compounds.
• Soxhlet technique allowed a more complete characterization of essential oil from fennel compared to ASE.
• ASE was selected for fennel characterization, due to lower laboratory costs and time.
• Box–Behnken design was successfully applied to optimize technique variables by conducting small number of experiments.

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 GC-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 GC-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 °C, 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 h) and solvents, according to their polarity. Methanol and 4 h of extraction showed the best results both qualitatively and quantitatively. The Soxhlet technique provided 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.