Limonene is a monoterpene with a bitter taste, which is found in many different plants and herbs, including cannabis. The terpene, which consists of a colorless liquid, is also known under the names dipentene, carvene or citrene. Limonene is recognizable by the typical citric scent and is formed in the sticky trichomes (resin glands) of plants.
The name limonene is derived from lemon
The name limonene, which this monoterpene carries, originates from the English word lemon. This refers to the heavily fragrant peel of lemons, which, like other citrus fruits, have large concentrations of limonene. Though this terpene is only formed in the oil glands (trichomes) of the shell, limonene also occurs to a lesser extent on flowers, leaves, stems and other parts of the plant.
The molecule of limonene is subdivided into two stereo-isomers. These are the same molecules, but with a different structure. Both molecules have their own distinctive flavour:
D-limonene (orange flavor)
L-limonene (pine and turpentine aroma)
Especially the d-limonene molecule is found in large concentrations in the essential oil of citrus fruit. Well-known examples are lemons, oranges, tangerines and grapefruit. Because limonene can be extracted cheaply from orange peels (waste product of the juice industry), this terpene is hardly obtained through a synthetic way.
How is limonene obtained?
The monoterpene can be obtained from the peel of oranges through two easy methods:
By means of centrifugal separation
By means of steam distillation
The manner, in which the essential oils are obtained, influences the terpenes amount. When oil is obtained by expression, it will be composed of about 90% limonene. But by steam distillation, this level will be much lower. In a solvent extraction percentages are even lower and sometimes it can occur that no terpenes are present whatsoever.
How does the biosynthesis of limonene go?
Like all other terpenes, limonene has evolved from geranyl pyrophosphate (GPP) and is one of the two main components of α-pinene (alpha-pinene). Geranyl pyrophosphate is also known under the name geranyl diphosphate (GDP).
The biosynthesis is as follows:
Isopentenyl diphosphate (isopentenyl pyrophosphate or IPP) and dimethylallyl diphosphate (dimethylallyl pyrophosphate or DMAPP) are combined in order to form geranyl diphosphate. Hereinafter, geranyl diphosphate is cyclised to limonene via a three-step route. The conversion enzyme is known under the name monoterpene synthase.
The beneficial terpene limonene has many medicinal properties that can be a boost to health. For example, limonene is known as sedative and causing sleepiness. It is also an antispasmodic and can suppress stress and anxiety.
Because of the powerful effect of d-limonene, it is medicinally used to increase the gall function. It ensures that fats are digested better and is able to destroy substances accumulated in the gallbladder. The terpene has a beneficial effect on the healthy functioning of the gallbladder and prevents the harmful effects of fat accumulation.
Besides medicinal applications limonene has also many other useful applications. For example, it is used as a raw material for certain products, such as paint remover and as a solvent for fats. It is also used as an alternate means of turpentine and as a botanical insecticide. Because orange oil consists for a large part of limonene it can be used to destroy both ants and the scent of the ants.
Limonene is also widely used as a fragrance in the perfume industry and serves as a fragrant addition in aftershave and bath products. Because of the refreshing lemon-like odor and because of the property that it is capable to break down fats and oils, the monoterpene is also added to cleaning products and hand cleaners. After a long-term exposure of pure limonene on the skin, skin irritations may occur.
Another well-known and useful property of limonene is that it is able to decrease the psychoactivity of THC in weed. This can be useful for people who need high concentrations of THC in supporting certain diseases. Through the combination of limonene the feeling of being stoned, is toned.
Chemical data D-Limonene
The chemical formula of the terpene limonene is: C10H16
The molecular mass is: 136.1252 g / mol
The systematic name is: (4R) -1-methyl-4-prop-1-en-2-yl-cyclohexene
Boiling point: 176 ° C (349 ° F)
Vapor pressure: 1.50 mm Hg (25 ° C)
Melting point: -95 ° C
Flashpoint: 48 ° C
Density: 0.8411 g / cm³
Limonene dissolves poorly in water but blends well in alcohol
Limonene is sensitive to light, air, heat, alkalis and acids
Auto-oxidised to carvone and carveol
Oxidation with sulfur, forms p-cymene
In which plants and herbs does limonene occur?
The terpene limonene occurs in a large number of plants and herbs, including hemp and marijuana plants. In addition it is found in citrus fruits, rosemary, red pepper, chamomile, ginger and turmeric. It also occurs in St. John's wort, valerian, hops, anise and fennel. Limonene can also be found in bay leaves, mint, basil, lavender, parsley and sage.