**1. Introduction**

The origin of the distillation methods is an invention attributed to the Arab alchemists and to the Persian scientist Avicenna (980–1037) with the establishment of the steam distillation process. Avicenna invented a setup to prepare essential oils and aromatic waters. Essential oils, sometimes called quintessence, are a very complex mixture of volatile compounds produced by the secondary metabolism in various plant organs (flowers, fruits, seeds, leaves, etc.) and algae. According to ISO and AFNOR standards, essential oils are defined as volatile composition obtained from raw materials by steam distillation and/or by cold expression from citrus peels (known as essences) [1]. The definition of an essential oil excludes other volatile fractions obtained by steam distillation and/or hydrodistillation from the crude extract resulted from solvent extraction, supercritical fluid extraction, solventand water-free microwave extraction, ultrasound-accelerated solvent extraction, solid-phase microextraction, and headspace extraction. The chemical composition of essential oils and volatile fraction could be quite similar. Moreover, it should be pointed out the clear difference between the physical and chemical properties of essential oils and fixed or fatty oils. The fixed oils contain mainly triglycerides, esters composed of three saturated fatty acids linked to glycerol, characterized by

high boiling and low volatility. The chemical composition of essential oils is principally composed of terpenes derived from the mevalonate and methylerythritol pathways [2]. Monoterpenes and sesquiterpenes are commonly the main contributor group of compounds identified in several essential oils [3]. Moreover, some essential oils contain other chemical classes, such as phenols (derived from shikimic acid pathway); the saturated and unsaturated fatty acids, acting as biosynthetic precursors; alkanes; and, more rarely, nitrogen and sulfur derivatives [4]. The essential oils play an important role in the allelopathic interaction of plants. They are involved in defense and signaling processes [5] and attraction of pollinating insects [6]. They constitute an important raw material source for the pharmaceutical, food, cosmetics, and perfume industries [7]. The essential oils of different plants exhibit a broad spectrum of biological activities. They show antibacterial activities attributed, in some cases, to the presence of phenolic compounds [8]. The literature reports also the excellent antioxidant [9], anti-inflammatory [10], and cancer chemoprotective activities [11].
