Chemical Composition and Biological Activities of Essential Oils of Curcuma Species
Members of the genus Curcuma L. have been used in traditional medicine for centuries for treating gastrointestinal disorders, pain, inflammatory conditions, wounds, and for cancer prevention and antiaging, among others. Many of the biological activities of Curcuma species can be attributed to nonvolatile curcuminoids, but these plants also produce volatile chemicals. Essential oils, in general, have shown numerous beneficial effects for health maintenance and treatment of diseases. Essential oils from Curcuma spp., particularly C. longa, have demonstrated various health-related biological activities and several essential oil companies have recently marketed Curcuma oils. This review summarizes the volatile components of various Curcuma species, the biological activities of Curcuma essential oils, and potential safety concerns of Curcuma essential oils and their components.
Keywords: Curcuma aeruginosa, Curcuma glans, Curcuma longa, Curcuma mangga, Curcuma zanthorrhiza, Curcuma zedoaria
The genus Curcuma L. (Zingiberaceae) represents a group of perennial rhizomatous herbs native to tropical and subtropical regions. Curcuma is extensively cultivated in tropical and subtropical regions of Asia, Australia, and South America . There are approximately 93–100 accepted Curcuma species, however the exact number of species is still controversial . The genus is best known for being an essential source of coloring and flavoring agents in the Asian cuisines, traditional medicines, spices, dyes, perfumes, cosmetics, and ornamental plants . Several Curcuma species are used medicinally in Bangladesh, Malaysia, India, Nepal, and Thailand  for treating pneumonia, bronchial complaints, leucorrhea, diarrhea, dysentery, infectious wounds or abscesses, and insect bites [2,4,5]. The rhizome is the most commonly used part of the plant. The main active components of the rhizome are the nonvolatile curcuminoids and the volatile oil [6,7,8]. Curcuminoids (curcumin, demethoxycurcumin, and bisdemethoxycurcumin) are nontoxic polyphenolic derivatives of curcumin that exert a wide range of biological activities . Several phytochemical studies on Curcuma oils led to the identification of sesquiterpenoids and monoterpenoids as the major components . The essential oil (EO) of Curcuma species possesses a wide variety of pharmacological properties, including anti-inflammatory, anticancerous, antiproliferative, hypocholesterolemic, antidiabetic, antihepatotoxic, antidiarrheal, carminative, diuretic, antirheumatic, hypotensive, antioxidant, antimicrobial, antiviral, insecticidal, larvicidal, antivenomous, antithrombotic, antityrosinase, and cyclooxygenase-1 (COX-1) inhibitory activities, among others [7,10,11,12,13,14,15,16,17]. Curcuma oils are also known to enhance immune function, promote blood circulation, accelerate toxin elimination, and stimulate digestion [18,19]. C. longa (turmeric) and C. zedoaria (zedoary) are the most extensively studied species of Curcuma due to their high commercial value. Other Curcuma species have been studied to a lesser degree. This review provides an update on recent studies performed on the chemical composition and biological studies on genus Curcuma. The search engines Google Scholar, PubMed, ScienceDirect, and ResearchGate were used to access the literature.
2. Volatile Components of Curcuma spp.
Generally, essential oils of Curcuma species are obtained by hydro- or steam distillation of the fresh or dry rhizome . Alternatively, Curcuma volatiles have also been obtained by solvent extraction or supercritical fluid extraction of the powdered rhizome . More recently, solid-phase microextraction (SPME) has been employed as a solvent-free technique to capture and concentrate volatiles from different plant parts. Industrially, Curcuma oil is produced during oleoresin processing as a byproduct of curcumin extraction . After curcumin is isolated from the oleoresin, the mother liquor (about 70–80%) is known as “curcumin-removed turmeric oleoresin” (CRTO) . The oil is then extracted from CRTO by hexane or other organic solvent, a process that could result in the loss of the highly volatile components during solvent evaporation . The use of alcohols as the solvent for oil extraction might cause esterification, etherification, and acetal formation . The volatile components of different Curcuma species, typically identified by gas chromatography mass spectrometry, are summarized in Table 1. In general, Curcuma species produce a wide variety of volatile sesquiterpenes, monoterpenes, and other aromatic compounds [17,23]. The chemical structures of key volatile components are presented in Figure 1. There is a tremendous variation in the composition of Curcuma essential oils (EOs). Differences in the oil chemical profile might be due to genotype, variety, differential geography, climate, season, cultivation practices, fertilizer application, stress during growth or maturity, harvesting time, stage of maturity, storage, extraction, and analysis methods [24,25,26,27]. However, some of the variation could be due to misidentification of the plant species or some of the components.
2.1. Curcuma longa L.
Curcuma longa (syn. C. domestica Valeton and C. brog Valeton) is also known as “turmeric” worldwide, “kurkum” in Arabic, and “haldi” in Hindi and Urdu. Turmeric is cultivated extensively worldwide but is native to Southeast Asia . It is a perennial herb grown on a very large scale in India, Pakistan, Bangladesh, China, Taiwan, Thailand, Sri Lanka, East Indies, Burma, Indonesia, and Northern Australia . In the West, it is produced in Costa Rica, Haiti, Jamaica, Peru, and Brazil . Turmeric is commercially available as a whole rhizome (fresh, dried, and cured by cooking in water, drying in shade, and polishing), turmeric powder, extracts, and oleoresins, with the powder being the most commonly consumed form. India is the largest producer and consumer of turmeric [66,117,118]. The plant is famous for its culinary and medicinal uses. Turmeric is the golden spice that gives many Asian dishes their yellow color and pungent earthy flavor. It is an essential ingredient of curry powders, accounting for about 10–30% of the blend . In traditional medicine, turmeric is extensively used as a carminative, digestive aid, stomachic, appetizer, anthelmintic, tonic and laxative . It is also used for treating fever, gastritis, dysentery, infections, chest congestion, cough, hypercholesterolemia, hypertension, rheumatoid arthritis, jaundice, liver and gall bladder problems, urinary tract infections, skin diseases, diabetic wounds, and menstrual discomfort [66,94,121]. Turmeric is used in many religious rituals, as a dye, and as a cosmetic [122,123]. Turmeric rhizome typically contains carbohydrates (69.4%), protein (6.3%), fat (5.1%), and minerals (3.5%) .
Turmeric oleoresin is an orange-red viscous liquid, prepared from the powdered rhizome by solvent extraction with a yield of about 12% . The main active components in the rhizome are essential oil and curcuminoids. The volatile oil is responsible for the turmeric aroma, while the curcuminoids (curcumin and its analogues) are responsible for its bright yellow color [65,119]. It is worth mentioning that curcumin, present in turmeric rhizomes, oleoresin, and CO2 extract, has not been reported in the essential oil . Turmeric chemotypes in the literature vary widely. Hundreds of compounds have been identified from the turmeric EO; however, the major constituents are ar-turmerone, α-turmerone, and β-turmerone, followed by notable amounts of α-zingiberene, curlone, ar-curcumene, α-santalene, santalenone, β-sesquiphellandrene, (Z)-β-ocimene, β-bisabolene, β-caryophyllene, α-phellandrene, (Z)-β-farnesene, humulene oxide, β-selinene, caryophyllene oxide, (E)-γ-atlantone, 1,8-cineole, and terpinolene. Samples from Brazil had (Z)-γ-atlantone, ar-turmerone, and (E)-γ-atlantone . A sample from north-central Nigeria was a mixture of β-bisabolene, (E)-β-ocimene, β-myrcene, 1,8-cineole, α-thujene, α-phellandrene, limonene, zingiberene, and β-sesquiphellandrene . Turmeric EOs from Sri Lanka and São Tomé and Principe had α-phellandrene, α-turmerone, 1,8-cineole, p-cymene, ar-turmerone, β-turmerone, and terpinolene as the main constituents [13,87]. Some turmeric rhizome EOs from India contained 1,8-cineole, α-turmerone, β-caryophyllene, β-elemene, ar-turmerone, β-sesquiphellandrene, camphor, α-farnesene, and (Z, Z)-farnesol [17,86]. Wide variations are also found between the EO obtained from fresh and dry rhizomes. Unfortunately, comparative data on the chemical composition of volatile oil from fresh and dry rhizomes from a single source, single geographical location, and same season are scarce. However, some of the variation could be explained by the rhizome processing. The order of yield is cured > fresh > dried rhizome . Some of the highly volatile low-boiling-point compounds might be lost during rhizome processing that involves grating, heating, drying, and grinding . For example, turmerones are major components in fresh rhizomes, while only minor ones in dry rhizomes, which might be due to oxidation/polymerization of the two conjugated double bonds .
Turmeric root EO from Kerala, India contained ar-turmerone (46.8%) and ar-curcumene (7.0%) as the main components . There are seven different chemotypes of the C. longa leaf EO reported so far : (1) ar-turmerone-rich chemotype ; (2) α-phellandrene-rich chemotype [66,68,70,83,89,91,95,96]; (3) terpinolene-rich chemotype [84,86,93]; (4) β-sesquiphellandrene-rich chemotype ; (5) p-cymene-rich chemotype ; (6) 1,8-cineole-rich chemotype ; and (7) myrcene-rich chemotype . Turmeric flower EO from India contained p-cymen-8-ol (26.0%) and terpinolene (7.4%) , while the floral oil from France had terpinolene (67.4%) as the main component .