Due to the small amounts of essential oils in the plant and the significant importance of them, many methods have been developed to determine the amount of these compounds. Two common and important techniques for the analysis of essential oils are chromatography and mass spectrometry. gas chromatography with mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC) and are the most widely used [7, 15, 16]. Gas chromatography-mass spectrometry (GC-MS) has added to the development and improvement in the analysis of essential oils which could be distinguished compounds with very similar chemical structures of each other [17]. In general, Gas chromatography (GC) for the analysis of volatile compounds and liquid chromatography (LC) for Analysis of non-volatile compounds in essential oils used [18].
In addition to essential oils, other aromatic compounds extracted from plants include: concrete (obtained by extraction with non-polar solvents), absolute (obtained by washing concretes in ethanol or methanol and vacuum distilling alcohol away ), pomade (obtained by a process known as enfleurage (described in later sections), resinoids (resinoids are extracted from the natural resin material by hydrocarbon solvent. Resinoids are usually derived from dry matter) [17, 19].
Essential oils can be extracted from different plants by various extraction methods. The manufacturing of essential oils, and essential oil extraction method are dependent on botanical material used. The another factor for extraction essential oil is state and form of material. One of the important factors that have a great impact on the quality of essential oil is the extraction method. Inappropriate extraction procedure can lead damage the chemical structure of the essential oil, which in turn changes the quality and natural properties of the essential oil [4]. In generally, extraction methods can be divided into two groups: conventional methods (distillation, cold pressing and etc.) and novel methods (supercritical fluid, microwave and etc.). Investigation in novel technologies, in the last decades has led to more efficient extraction processes for example reduction of energy consumption and extraction time, high yield and improvement of essential oils quality. Recently, due to the limitations and disadvantages of methods of extraction from intact plants, large companies are trying to move their way to biotechnology methods and make a variety of plant compounds by microorganisms. By doing this, they have no stress about problems such as climate conditions, supply shortages, natural disasters, plant diseases etc., and a product with very high efficiency and quality is obtained. Production of natural products and volatile through biotechnology can be an appropriate response to consumer demand in relation to the consumption of natural products and move towards ”green processes” and environmentally friendly.
In this review, we have tried to explain the production methods of essential oils, including conventional, novel and biotechnology methods, and make a general comparison between them.