Limonene is basically a colorless fluid hydrocarbon that is categorized as a cyclic terpene. This substance has a fresh, light sweet citrus aroma and is generally employed for flavoring and making fragrances. Limonene is found in nature and is present is several citrus fruits. Limonene is used in several types of cosmetics as well as personal care products, including formulation of bubble baths, bath products, aftershave lotions, moisturizers, lipsticks, eye shadows, mascara, hair care products, cleansing products, colognes, perfumes, several skin care products as well as suntan products.
Limonene belongs to a family of hydrocarbons that are made by plants. This substance is called "terpene" and possesses a potent fragrance, similar to that of oranges. Precisely speaking, this is an orange terpene. Present in essential oils, terpenes are especially found in citrus oils. They can be used for several purposes, including flavoring food, in fragrances as well as in aromatherapy. Limonene is employed in chemical synthesis in the form of a pioneer of carvone, which belongs to a family of chemicals known as "terpenoids". In addition, this terpene is also used in the form of a solvent that is renewably-based n cleansing products. The word limonene has its origin in lemon, since the peel of lemon, similar to that in several other citrus fruits, encloses significant amounts of this terpene. It is worth noting here that the odour of these citrus fruits is attributed to this compound.
A chemical compound, limonene can manifest in two different ways - as D-limonene and L-limonene. Chemically, D-limonene and L-limonene are mirror images of each other and possess similar properties. However, they have their individual aromas. The smell of D-limonene is similar to that of a citrus fruit and it is used in food grade products, beauty products and cleaning products. On the other hand, L-limonene has a somewhat sour scent, something similar to sour turpentine blended with pine. This version of limonene is mostly used as a substitute solvent for cleansing products. Both, D-limonene and L-limonene can be produced in the same way and it is possible to produce the technical grade of both versions. Though they are made in the same manner, they are separated chemically by scientists. In fact, cleaning products have been containing technical grade limonene since the 1990s. As discussed above, the name limonene has been derived from "lemon". This is because this natural compound is commonly found in citrus fruits' rinds.
Most commonly, limonene is extracted from the rind of oranges. First, the juice of the rinds is extracted to produce a food grade limonene. Subsequently, the product is placed in a steam extractor, which brings out more oil. When the steam is compressed down, it allows the oil to float on the surface, from where it is collected. The oil that is collected in this way is technical grade.
Since the 1990s, people have been using technical grade limonene in cleaning products. The orange fragrance imparts a pleasant aroma to the cleaning product. These days, several consumers prefer natural products compared to man-made or synthetic alternatives. However, people who are keen on conservation, find using chemicals tempting. The orange rinds that are currently used to flavour foods were discarded earlier. Now, these rinds are used differently for different purposes. It has been found that limonene does not have any caustic effect on metallic surfaces. However, this compound can debase plastics. This aspect of limonene does not make this terpene an effectual adhesive remover. This colorless liquid is not soluble in water.
The boiling range of limonene varies between 154°C and 177°C (310°F and 352°F). The flash point of limonene is 48°C (119°F), which actually classifies this natural compound as a combustible product. In other words, limonene has also been classified as a harmful material for purpose of disposals.
In its unadulterated form, limonene has been classified as a substance that causes light skin irritations. However, compared to other alternative cleaning products, limonene is much safer. Nevertheless, as of now, no significant studies have been undertaken to ascertain the safety or hazardousness of limonene to humans on a informal basis. It has, however, been found that using this chemical on a long-term basis may result in mild respiratory irritations.
Currently, researchers have undertaken some studies to examine whether or not the food grade or D-limonene can also be used for treating cancer. Initial results of a number of studies have shown that rats with mammary tumours that were given D-limonene experienced a decrease in the size of their tumours. Now, scientists are carrying on tests to find whether the results in animals can be repeated in humans.
D-limonene is mainly used in the form of a carvone precursor.
Limonene is widely used in cosmetic products. D-limonene is the primary aroma constituent of citrus, belonging to family Rutaceae. This substance is employed in food manufacturing as well as several medicines, for instance, as a flavoring agent to conceal the bitter flavour of alkaloids. In addition, limonene is also used in the form of a fragrant in perfumery. Moreover, many botanical insecticides also contain limonene.
It has been found that the (R)-(+) version of limonene is especially most active when used as an insecticide. In addition, this enantiomer is included in cleaning products like hand cleaners with a view to impart a lemon-orange scent.
On the other hand, L-limonene has an odour that is akin to a blend of pine and turpentine.
Currently, people are using limonene more and more in the form of a solvent in cleaning products. It is being used for removing oil from machinery, since it is made from a renewable source - citrus oil, in the form of a derivate of orange juice manufacture.
At the same time, limonene is also used in the form of a paint stripper, especially for removing paint from wood. It also serves as an aromatic substitute for turpentine.
In addition, limonene is also employed in the form of a solvent in a number of airplane glue models.
Philatelists use limonene in order to detach postage stamps that are self-adhesive from the envelopes. For the uninitiated, philately involves the study of postal stamps, postal history as well as other related things.
Since limonene is flammable, its use as a biofuel is also being considered.
Apart from the above mentioned uses of limonene, this compound can also be employed to dissolve an organic compound called polystyrene. Compared to its substitute, acetone, limonene is more environment friendly.
D-limonene is regularly used while preparing tissues for histology - which involves the study of microscopic anatomy of plant and animal cells and tissues. In such cases, D-limonene is used in the form of a less toxic alternative to xylene to clean up dehydrated specimens. In fact, cleaning agents are fluids that can be blended with alcohol (for instance ethanol or isopropanol). They can also be used with liquefied paraffin wax. Specimens are implanted in the blend to make cutting the thin sections easier for microscopy.
Limonene as well as its oxidation products cause irritation to the skin and the respiratory tract. Limonene-1.2-oxide, which comes into existence owing to aerial oxidation, is also known to be a skin sensitizer. Most cases of irritation that are reported involve prolonged industrial exposure to the unadulterated compound, for instance during making paints or degreasing.
A study undertaken on patients suffering from dermatitis revealed that about 3 percent of the subjects were sensitive to limonene.
There was a time when it was believed that limonene was responsible for renal cancer in rodents, but now some researchers believe that this compound is an important chemo preventive agent having the potential to be used in the form of a dietary anti-cancer agent in humans. As of now, there is no proof that limonene has geno toxicity or carcinogenicity vis-à-vis humans.
However, the Carcinogenic Potency Project approximates that limonene is responsible for cancer in humans on a level that is roughly equal to cancer caused by coming in contact with caffeic acid through coffee consumption. Till now, there is no information regarding the health effects of D-limonene inhalation exposure in humans. If fact, scientists have also not undertaken any long-term inhalation research on laboratory animals.
As discussed earlier, though D-limonene is biodegradable, it has a very low flash point. Hence, this natural compound ought to be essentially treated as a harmful waste for disposal purposes.