Isolation of fennel essential oil by conventional and advanced extraction techniques

Fennel (Foeniculum vulgare Mill.) is a perennial aromatic and medicinal plant belonging to the Apiaceae family. It originates from the Mediterranean area, but it has spread worldwide through naturalization and cultivation. The use of fennel in culinary purposes and traditional medicine is known from ancient times, but nowadays its application has been extended to the pharmaceutical, cosmetic and food industries. Although different parts of fennel (leaves, young shoots, roots and seeds) can be utilized, fennel seeds are mostly exploited due to their abundance of essential oil (up to 5-6%) which is characterized by a strong and pleasant aroma. Fennel essential oil has been recognized to possess antimicrobial, antifungal and antioxidant properties, therefore it is widely used in various areas. The chemical composition of fennel essential oil showed up to 80 volatile compounds, among which trans-anethole, fenchone and estragole are major compounds, being the key compounds of the characteristic fennel flavor. Besides these compounds, the presence of other compounds such as α- and β-pinene, myrcene, α-phellandrene, limonene and p-anisaldehyde have also been reported. The isolation of essential oil can be accomplished by various extraction techniques, such as hydrodistillation (HD) and steam distillation (SD) which are known as conventional ones and still are the most common in use. They are effective for the extraction of essential oils from spices and herbs from which such oils are difficult to isolate and they do not include the use of chemical solvents. On the other hand, their main drawbacks are long duration, difficult regulation of heat and possible loss of thermolabile and light volatiles. Therefore, in order to overcome these deficiencies, advanced extraction techniques have been developed, including microwave-assisted extraction (MAE), enzyme-assisted extraction (EAE), ultrasound-assisted extraction (UAE), subcritical water extraction (SWE) and supercritical fluids extraction (SFE), namely extraction with supercritical carbon dioxide (SC-CO2) which is considered “green” showing the most promising beneficial effects due to being sustainable, environmentally friendly and cost-effective. However, regardless of the selected extraction technique, it is necessary to define optimal process conditions taking into account all process parameters in accordance with the raw material used, and optimize the extraction process with the intention to achieve the maximum yield of essential oil of the best quality and unaltered composition.