Essential oils in plants: Plant physiology, the chemical composition of the oil, and natural variation of the oils (chemotaxonomy and environmental effects, etc.)

Essential oils are complex fragrant volatile liquids that constitute a compact percentage in aromatic plants but exhibit significant contributions in the food, pharmaceutical, and cosmetic industry. Different methods (conventional and innovative)are employed to extract them effectively with diverse complexity in chemical constituents. These essential oils are predominantly composed of a terpenoid family with isomers possessing positional, geometric, spatial, and optical isomerism. Different essential oils also have various therapeutic properties including antioxidant, antiinflammatory, anticancer, antidiabetic, antimicrobial, antiviral, antihyperpigmentation, analgesic, neuroprotective, cardioprotective, and hepatoprotective attributes. Owing to their medicinal properties, many are used not only as a flavoring and aromatic agent but also in the enhancement of shelf-life in food commodities. In this chapter, the physiology and chemical compositional profile of the major essential oils along with factors causing variation in their quality and quantity will be discussed. Furthermore, a summarized overview of both conventional and innovative extraction methods will also be evaluated.