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What are terpenes ?

Terpenes are one of the most widespread group of natural products. They have many different functions in plants and animals, but for food they are mainly important as aroma components. The aroma of for example citrus, cinnamon and many other spices is characterised by several terpenes. Common terpenes (and terpenoids, see below) are limonene and citral (both in lemons), camphor, pinene (pine trees), eugenol (cloves), anethol (fennel, anise), thymol (thyme, oregano), geraniol (roses) and menthol.

As terpenes are largely found in essential oils, they were used in the Ancient Egypt for various religions aims. Camphor was introduced in Europe from the East by the Arabs around the 11th century.
The process of obtaining plant essential oils by fatty extraction was known by the early Middle Ages, both in the West as in China . In the 12th century, Arnaud de Villanosa described distillation of oils from rosemary and sage. He made an " oleum mirabile " from oils of turpentine and rosemary. It is noticeable that some 60 oils were described in the Nuremberg edition of " Dispensatorium valerii cordi " written in 1592.

Chemically terpenes may be defined as a group of molecules whose structure is based on a various but definite number of isoprene units (methylbuta-1,3-diene, named hemiterpene, with 5 carbon atoms). 

This definition leads to a rational classification of the terpenes depending upon the number of such isoprene units incorporated in the basic molecular skeleton.

Mono-, sesqui-, di-, and sesterpenes contain the isoprene units linked in a head to tail fashion. The triterpenes and carotenoids (tetraterpenes) contain two C15 and C20 units respectively linked head to head.
Many terpenes are hydrocarbons, but oxygen-containing compounds such as alcohols, aldehydes or ketones are also found. These derivatives are frequently named terpenoids .
Mono- and sesquiterpenes are the chief constituents of the essential oils while the other terpenes are constituents of balsams, resins, waxes, and rubber.