Also known as plant oils, volatile oils, and aetherolea, essential oils are concentrated hydrophobic liquids containing volatile chemical compound from plants. They are called essential because they contain the 'essence' of the plant fragrance.
FUNCTIONS OF ESSENTIAL OILS
Essential oils are secondary metabolites that lend the characteristic odor to plants. Essential oils can be present in the leaves, flowers, stems, or roots of plants. These natural, aromatic oils are obtained by the distillation of plant material and also come from resin tapping, cold pressing, or absolute extraction. These processes concentrate the essential oil molecules, leading to an intense aroma. Essential oils act as protectors against fungi, bacteria, insects, and other predators, play an important role in attraction pollinators, and prevent other botanicals from growing in a plant's territory.
BIOSYNTHESIS OF PLANTS
In aromatic plants, the biosynthesis of essential oils occurs through two complex pathways. Two compounds are the universal precursor to all essential oils: isopentenyl diphosphate (IPP) and its cousin dimethylallyl diphosphate (DMAPP). Using multiple plastid and enzyme pathways, essential oil precursors IPP and DMAPP are condensed to form prenyl diphosphates. The prenyl diphosphates are then processed by various terpene sythases. Essential oils are the end product of these enzymes.
HOW ESSENTIAL OILS ARE RELEASED FROM PLANTS
Atop the surface of the plant are specialized cells called glandular trichomes. These trichomes are tiny, hair-like formations found on the surface of plant stems and leaves. At the tip of the hairs are glandular cells that produce, store, and eventually secrete exudates such as essential oils. The trichomes squeeze the essential oils out of the plant so they can serve their protective function. The trichomes come in many different shapes and sizes, and in fact may be very specific for individual plants.
The trichomes are not directly connected with the plant's vascular system and are therefore not a part of the primary metabolism. Instead, they are a part of the secondary metabolism that secretes aromas in response to external stressors. The compounds secreted by the plant have antibacterial, antifungal, and antiviral activity that protect the plant from further farm. Thus, the more stress a plant has been under, the more likely it is to have highly developed trichomes. In artificial growing environments, the plants can be encouraged to produce more trichomes and more secondary metabolites with exposure to ultraviolet light or by using specialized fertilizer.
VARIABILITY IN ESSENTIAL OIL CHEMISTRY
Essential oils contain thousands of volatile compounds that evaporate quickly into the air, and the ratio of the molecules imparts the aroma to the oil. The season of the year, geography, and distillation techniques can change the ratio of the molecules resulting in subtle or dramatic differences in the aroma, volatility, and viscosity of the essential oil.
HISTORY OF ESSENTIAL OILS
Humans have used essential oils for many centuries across many cultures for their aromas and flavors and to enhance wellness, spirituality, recipes, and cosmetic products. Many ancient uses of essential oils still exist today.
Evidence from the very ancient world suggests a connection between man and plants. An ancient Mesopotamian grave of a Neanderthal man revealed pollen from eight different medicinal herbs, indicating that plants were likely used as a part of ceremonies. Clay tablets from Mesopotamia have inscriptions with herbal remedies using aromatic plants of pine, fennel, and galbanum.
Moving forward in time, we see that in India ancient Ayurvedic medicine practitioners used a wide variety of plants. Arguably the oldest surviving medical text is the Herbal Classic of Shen Nong, dated 2700 B.C. Shen Nong is regarded as a cultural hero who taught herbal farming practices to others and as the father of Chinese medicine. Because of this rich tradition, China is one of the largest producers of essential oils. Essential oils are also mentioned in the writings of Huangdi in The Yellow Emperor's Classic of Medicine.
As early as 2000 B.C., ancient Egyptians used essential oils alongside fatty oils, clays, and sea salts for beauty practices. In Egyptian temples, essential oil extraction is depicted on the walls, and priests used them alongside herbs in tinctures, unguents, salves, and ointments. Egyptian royalty prized essential oils and they were even discovered inside the tomb of King Tutankhamen.
The Greek physician Hippocrates used essential oils in plasters and to fumigate the city. Romans lavishly used essential oils in baths, during massage, and to scent the hair, body, and bed. At least a dozen essential oils are mentioned in the bible as ceremonial aromas, gifts, and traditional medicine, and these practices are maintained the in the modern Catholic Church.
Though used for many thousands of years, during the Middle Ages essential oils were banned by the Catholic Church as being too decadent, and they fell out of the mainstream into the arena of witchcraft and lore. But by the 1600s, essential oils worked their way back into the mainstream, and by the 1800s were included in pharmacopeias across Europe. The south of France was bursting with lavender, and commercialization of essential oils became extremely popular.
In recents history, the work of Rene-Maurice Gattefosse (the man who coined the term "aromatherapy"), Jean Valnet, Daniel Penoel, Herve Casabianca, and Gary Young have firmly established essential oils as powerful plant products with diverse and extensive uses.
Essential oils are more that just pleasant scents; they have complex active chemical constituents inside the plant, and this chemistry is maintained following extraction from plants.
Source: Essentials, 2nd Edition by Linsey Elmore