Effects of Gamma Radiation on Essential Oils: A Review

γ -Radiation provides an effective alternative method to reduce or eliminate microbial con - tamination of medicinal herbs and other plant materials. However, a search in the literature is important to describe the effects of γ -radiation on the content and integrity of secondary metabolites from plants. The present work provides a review of the effects of γ -radiation on extraction yields and chemical composition of essential oils isolated from roots, rhizome and cortex, leaves, fruits, seeds, flowers, and whole plant. In addition, this review describes the effects of γ -radiation on terpenes. The informations in the present work may assist in research about essential oils and dose of γ -radiation that is able to biologically decontaminate without causing chemical changes in secondary metabolites. These reports in the literature can describe the behavior of many of these metabolites when subjected to various doses of radiation. submitted to γ -radiation at doses of 5.0, 10.0, 25.0, and 50.0 kGy. The total content of alcohols, aldehydes, and hydrocarbons increased in irradiated samples (average values considering all irradiated materials = 2.0, 21.9, and 35.2%, respectively) in relation to nonirradiated sample (1.83, 19.77, and 32.72%, respectively). The higher total content of sesquiterpene hydrocar bons was verified for irradiated samples at 5.0 and 25.0 kGy (32.5% for both the samples) and at 10.0 and 50.0 kGy (30.0% for both the samples) in relation to nonirradiated sample


Introduction
Essential oils (EOs) are plant secondary metabolites, mainly constituted by a mixture of terpenes and terpenoid derivatives (Figure 1). Monoterpenes and sesquiterpenes are usually their major constituents [1]. These components are volatile, usually exuding characteristic and pleasant odors. The attraction of insects caused by smell of these essential oils is one of the main factors responsible for the pollination of plants. EOs have many other biological functions, such as protection of plants against diseases caused by fungi and bacteria [2]. Moreover, EOs exhibit a broad spectrum of biological properties, such as mucolytic, expectorant (for example, menthol), and antineoplastic actions [1], stimulating blood flow (for example, EO of mountain pine or common juniper), treatment of gastrointestinal diseases (for example, essential oils of anise, caraway, or fennel), and used in aromatherapy [2]. EOs are also used for production of perfumes and other cosmetic products and are added to foods to improve the flavor [2].
Chemical composition of EOs from plants usually provides important information to its taxonomic identification [3]. However, some environmental factors, such as temperature variation, photoperiod, and light intensity, can influence the biosynthesis of volatile compounds and as a consequence, change its quality and chemical composition [4]. Hydrodistillation is a largely used method in laboratory to obtain EOs from vegetal species. On the other hand, the most usual and popular method to obtain these chemical constituents is a simple decoction. Pressure, temperature, time, dynamic of extraction, and solvent volume are experimental parameters that influence the extraction efficiency and quality of EOs isolated from natural products [5].
Although EOs obtained from plant materials have important medicinal and industrial applications, herbs rich in EOs are often contaminated with microorganisms. Fungal and bacterial contamination is generally caused by the presence of these microorganisms in soil, water, air, or dust, during harvesting, storage, or processing of herbs [6]. The most usual methods for the microbial decontamination of vegetal material are based on applications of ethylene oxide or methyl bromide. However, both methods promote formation of toxic products and have been banned in many countries, such as Japan and those of the European Union [7].
On the other hand, γ-radiation provides an effective alternative method for reducing or eliminating microbial contamination of medicinal herbs and other vegetal products. This type of radiation of high energy usually passes through skin and soft tissue. A small percentage of γ-radiation is absorbed by cells. Once absorbed by a biological material, γ-radiation can provoke direct and indirect effects at molecular level. Direct effects are responsible for DNA double-strand breaks (DSBs), highly toxic lesions that can cause genetic instability and cell death [8].
Indirect effects are more frequent than direct ones. These effects are caused by the interaction of ionizing radiation with water molecules, generating free radicals. In turn, these free radicals are highly reactive with different cell components, such as DNA, enzymes, and secondary metabolites (including EOs) [7]. Literature describes the effects of γ-radiation on the chemical composition of many vegetal species. Chromatographic analyses of EOs isolated from plant indicated that γ-radiation changes their extraction total yields and chemical compositions. Both the parameters (that is, yields and chemical composition) are mainly influenced by vegetal species, radiation dose, and chemical constituents of the plant material [3]. The present review describes the effects of γ-radiation on the chemical composition of EOs isolated from different plant parts: roots, rhizome and cortex, leaves, fruits, seeds, flowers, and whole plant. One entire section is dedicated for description of the effects of γ-radiation on terpenes.
Dried root samples of Glycyrrhiza glabra Radix (Licorice) collected in Korea were submitted to γ-radiation at doses of 5.0, 10.0, 25.0, and 50.0 kGy. Irradiated samples exhibited higher total content of aldehydes and hydrocarbons (average values considering all irradiated materials = 24.8 and 17.3%, respectively) than the corresponding nonirradiated samples (17.11 and 15.88%, respectively). Irradiated samples exhibited lower total content of alcohols, ketones, and ethers (average values considering all irradiated materials = 12.8, 2.7 and 5.3%, respectively) than the corresponding nonirradiated samples (14.68, 16.08, and 14.4%, respectively). The total sesquiterpene content in nonirradiated and irradiated samples was near 10.0%. The total monoterpene content was near 4.0% in irradiated and nonirradiated samples, except in the sample irradiated at 50.0 kGy that showed monoterpene content near 8.0% [9].
The total content of aldehydes and hydrocarbons from other materials of the same species (roots of G. glabra) collected in Korea was higher in irradiated samples at doses of 5.0, 10.0, 25.0, and 50.0 kGy (average values considering all irradiated materials = 42.4 and 26.2%, respectively) than the corresponding nonirradiated sample (28.69 and 11.31%, respectively). The irradiated samples exhibited lower total content of alcohols and ketones (average values considering all irradiated materials = 2.8 and 2.5%, respectively) than the corresponding nonirradiated sample (9.66 and 14.08%, respectively). The total content of monoterpenes in irradiated samples was higher than for a nonirradiated sample. The total content of sesquiterpenes in irradiated samples was lower than for a nonirradiated sample. The identified volatile components were exactly the same in irradiated and nonirradiated samples of G. glabra [9].
Dried and powdered roots of G. glabra collected in Syria were submitted to γ-radiation at doses of 5.0, 10.0, 15.0, and 20.0 kGy. Higher contents of glycyrrhetinic acid were observed for irradiated samples (average values considering all irradiated materials = 6.1%) than the corresponding nonirradiated sample (4.37%). The content of glycyrrhetinic acid in both irradiated and nonirradiated samples decreased after 12 months of storage [10].
Samples of Paeonia albiflora Pallas var. trichocarpa Bunge were submitted to γ-radiation at doses of 1.0, 3.0, 5.0, and 10.0 kGy. Maximum yield of EO was obtained for an irradiated sample at 5 kGy (29.91%). Nonirradiated and irradiated samples at 1.0, 3.0, and 10.0 kGy showed yields of 28.14, 25.89, 26.67, and 25.24%, respectively [1]. Gas chromatography (GC) analysis of volatile compounds obtained from irradiated and nonirradiated samples was similar. A total of 54 compounds was identified in the nonirradiated and irradiated samples at 1.0 kGy. Irradiated samples at 3.0, 5.0, and 10.0 kGy exhibited 55 volatile compounds. This new peak on the GC chromatogram of irradiated samples from 3.0 to 10.0 kGy was attributed to 1,3-bis(1,1-dimethylethyl)-benzene. The highest total contents of alcohols and aldehydes were verified for the irradiated samples (average values considering all irradiated materials = 37.7 and 22.1%, respectively) in relation to the nonirradiated sample (34.26 and 21.44%, respectively). The total contents of acids, esters, furans, ketones, hydrocarbons, and terpenoids were not different among irradiated samples and nonirradiated samples [1].
Rhizome samples of Coptis chinensis purchased from a local market in Korea were submitted to γ-radiation at doses of 5.0, 10.0, 25.0, and 50.0 kGy. Higher total content of aldehydes and hydrocarbons was verified for irradiated samples (average values considering all irradiated materials = 11.4 and 42.7%, respectively) in relation to nonirradiated samples (7.56 and 35.98%, respectively). Higher total content of sesquiterpene hydrocarbons was verified for irradiated samples at 5.0, 10.0, and 25.0 kGy (average values considering irradiated materials = 47.5, 47.5, and 42.5%, respectively) in relation to nonirradiated sample and irradiated sample at 50.0 kGy (35.0% for both the samples) [9].
Another rhizome sample of C. chinensis was purchased from a local market in Korea and irradiated at doses of 5.0, 10.0, 25.0, and 50.0 kGy. The total content of aldehydes in the irradiated samples (average values considering all irradiated materials = 23.1%) was similar to the nonirradiated samples (23.33%). Higher content of hydrocarbons was verified for the irradiated samples (average values considering all irradiated materials = 16.8%) in relation to the nonirradiated samples (11.21%). The total content of sesquiterpene hydrocarbons was higher in the irradiated sample at 25.0 and 50.0 kGy (20.0% for both the samples) in relation to irradiated samples at 5.0 and 10.0 kGy (15.0% for both the samples) and nonirradiated sample (12.5%) [9].
Rhizome samples of Curcuma longa (turmeric) purchased from a local market in Kerala (India) were submitted to γ-radiation at doses of 1.0, 3.0, and 5.0 kGy. Similar extract yields of volatile oils were obtained for irradiated (1.54, 1.70, and 1.43%, respectively) and nonirradiated samples (1.52%). A total of 23 constituents was identified in the nonirradiated and irradiated samples. Significant changes in the concentration of their constituents were not observed for irradiated and nonirradiated samples [13].
Another rhizome sample of C. longa that was also purchased from the local market in India was submitted to γ-radiation at 10.0 kGy. The overall yield of volatile oil did not change for the nonirradiated sample (1.71%) and irradiated sample (1.72%). The gas chromatography/mass spectrometry (GC/MS) chromatograms did not indicate significant changes in the concentration of its major constituents: α-phellandrene, p-cymene, 1:8-cineol, β-caryophyllene, ar-curcumene, mixture of zingiberene and β-sesquiphellandrene, nerolidol, mixture of ar-turmerone and turmerone, curlone, and dehydrozingerone [8].
Fresh rhizomes samples of Zingiber officinale var. Bangalore (ginger) purchased from a local market in India were submitted to γ-radiation at a dose of 0.06 kGy. The overall yield of EO was slightly higher for an irradiated sample (0.17%) in relation to a nonirradiated sample (0.14%). The GC/MS chromatograms did not indicate significant changes in the concentration of its major constituents: camphene, β-phellandrene, mixture of linalool and α-terpeniol, neral, geranial, ar-curcumene, nerolidol, mixture of zingiberene and zingiberol, and mixture of β-sesquiphellandrene and β-bisabolene [14].
Leaf samples of Eucalyptus radiata purchased from a local market of Tilman (Belgium) were submitted to γ-radiation at a dose of 25.0 kGy. The overall yield of volatile oil did not change for nonirradiated (0.84%) and irradiated samples (0.85%). Both the samples did not exhibit significant differences in the content of α-pinene, eucalyptol, β-myrcene, terpinen-4-ol, sabinene, neral, α-terpineol, linalyl acetate, and β-bisabolone, which were the major constituents identified in its EO [16].
Leaf samples of M. piperita purchased in Marrakesh (Morocco) were submitted to γ-radiation at a dose of 1.0 kGy. Some differences were observed in the composition of the EO for irradiated and nonirradiated samples. Higher contents of carvone and dihydrocarveol in the irradiated sample (35.88 and 6.95%, respectively) were verified in relation to the nonirradiated sample (31.83 and 3.14%, respectively). Some nonidentified constituents (GC retention times at 5.67, 5.83, and 6.73 min) exhibited a slight increase for the irradiated sample. The content of viridiflorol, carvacrol, carvyl acetate, and D-germacrene was only detected on the GC chromatogram of the nonirradiated sample (5.35, 3.28, 1.30, and 1.25%, respectively). The content of 1,8 cineole, dihydrocarvyl acetate, and a-bourbonene was similar for both the samples [18].
Leaf samples of Ocimum basilicum purchased from the local market in São Paulo (Brazil) were submitted to γ-radiation at doses of 10.0, 20.0, and 30.0 kGy. Chromatographic analysis indicated no significant differences between nonirradiated and irradiated samples [19].
The other leaf sample of O. basilicum purchased from a local market in Copenhagen (Denmark) was irradiated using doses of 3.0, 10.0, and 30.0 kGy. The content of 1,8 cineole, β-caryophyllene, methylchavicol, methyleugenol, and linalool also did not exhibit significant differences between nonirradiated and irradiated samples [20].
Leaf samples of Origanum vulgare collected in Turkey were submitted to γ-radiation at doses of 5.0, 7.5, 10.0, and 30.0 kGy. The majority of the identified volatile constituents was only slightly affected by the radiation. Irradiated sample at 5.0 kGy did not exhibit changes in relation to nonirradiated sample. Irradiated sample at 10.0 kGy exhibited a significant increase of the content of linalool, hotrienol, sabinen hydrate, p-methoxypyridine, α-terpinolene, and two linalool oxide derivatives. Irradiated sample at 30.0 kGy exhibited a significant increase of p-methoxypyridine, α-terpinolene, and both the linalool oxide derivatives. On the other hand, a decrease of bicyclogermacrene was observed for irradiated samples [22].

Essential oils from fruits
Fruit samples of Carica papaya were submitted to γ-radiation at doses from 0.05 to 3.0 kGy. The GC chromatogram of the irradiated samples exhibited a new peak that was identified as phenol. The content of phenol in different irradiated samples exhibited a dose-dependent increase with radiation, being linear in the dose range of 0.1-3.0 kGy [27].
Fruit samples of Citrus paradise (grape fruits) were collected in Texas (USA), and 3 days after harvest, the samples were submitted to γ-radiation at doses of 0.15 and 0.30 kGy. Pulp of nonirradiated fresh grape fruits exhibited higher content of D-limonene and myrcene (10.00 and 0.27%, respectively) than fruits exposed to radiation at 0.15 kGy (6.00 and 0.17%, respectively). However, irradiated sample at 0.3 kGy did not exhibit significant changes of D-limonene and myrcene in relation to nonirradiated sample [28].
A sample of grape fruit variety (Rio Red) collected in Texas (USA) was submitted to γ-radiation at doses of 0.00, 0.07, 0.20, 0.40, and 0.70 kGy. The content of β-carotene, limonin-β-Dglucopyranoside, and total carotenoids did not exhibit changes between irradiated and nonirradiated samples. A total of 35 days after harvest, fruit samples exposed to radiation at 0.07 kGy exhibited higher content of lycopene (1.53%) than fruits exposed to 0.70 kGy (1.32%) [29]. Volatile extract of Maroc late (Mature oranges) harvested in Morocco was submitted to γ-irradiation at doses of 1.0 and 2.0 kGy. GC analysis did not indicate significant differences between nonirradiated and irradiated samples at 1.0 kGy. Irradiated fruits at 2.0 kGy exhibited lower content of linalool, citral, and methyl anthranilate (0.59, 0.16, and 0.08%, respectively) in relation to the corresponding content of nonirradiated sample (0.80, 0.24, and 0.13%, respectively). On the other hand, the content of D-limonene (94.17%) was higher than the corresponding content for nonirradiated samples (93.70%) [30].
Seed samples of Monodora myristica were submitted to γ-radiation at 15.0 kGy. The effects of γ-radiation on the EO were not significant. The most remarkable change was observed for α-thujene and β-cymene. The content of these monoterpenes in the irradiate sample (16.76 and 9.29%, respectively) was higher than to the nonirradiated sample (7.14 and 7.14%, respectively). On the other hand, terpinolene, α-terpineol, α-cubebene, and caryophyllene were only detected in small amounts in the nonirradiated sample [36].
Seed samples of Linum usitatissimum (linseed) were submitted to γ-radiation at doses of 2.5, 4.0, 5.5, and 7.0 kGy. Irradiated samples at 2.5 and 4.0 kGy decreased the content of p-xylene, limonene, and styrene in relation to the nonirradiated sample. The content of 1-hexanol, p-xylene, and limonene increased in the irradiated sample at 5.5 kGy. Compounds p-cymene, benzaldehyde, and nonanol were not detected in the irradiated samples [37].

Essential oils from buds
Bud-fermented samples of Camellia sinensis (oolong tea) purchased from a local market in São Paulo (Brazil) were exposed to γ-radiation at doses of 5.0, 10.0, 15.0, and 20.0 kGy. Principal component analysis indicated that volatile constituents from samples irradiated at 15.0 and 20.0 kGy showed a chromatographic profile more similar to nonirradiated sample than samples exposed to other applied doses [40].
Samples of Trifolium pratense L. (clove) purchased from a local market in Mumbai (India) were exposed to γ-radiation at a dose of 10.0 kGy. Extraction yield of EO increased for the irradiated sample (18.88%) in relation to nonirradiated sample (15.25%). The content of benzylalcohol, eugenol, β-caryophyllene, humulene, eugenol acetate, and vanillin did not change for the irradiated sample [42].
Samples of Elettaria cardamomum (cardamom) purchased from a local market in Mumbai (India) were exposed to a dose of 10.0 kGy. The overall yield of EO was not changed for irradiated and nonirradiated samples (5.80 and 5.78%, respectively). The higher content of  [42].
Aerial parts of Zataria multiflora purchased from a local market in Shiraz (Iran) were submit-

Discussion
Internal and external factors can determine yield and composition of EOs from plant materials. Internal factors are usually genetic, physiological, and evolutionary (stage of maturity). On the other hand, external factors are usually seasonality, circadian rhythm, temperature, water availability, nutrient availability, air pollution, altitude, mechanical stimuli, attack pathogens, or extraction conditions [13].
The effects of gamma radiation on EOs constituents also depend on the different factors, such as radiation dose, dose rate, vegetal species, temperature, and sample state. Exposition to γ-radiation may increase or decrease the extraction yield of EO and the content of their constituents [35].
The higher yield of EO extraction verified for the irradiated samples has been usually attributed to radiation-induced disruption of the cell wall structure, providing a higher extractability of oil from the plant tissues. Moreover, changes on EO extraction yield can be due to a recombination of the radiolytic products with time. Specific effects can be observed on a secondary metabolite in different essential oils even though submitted to the same radiation conditions. The content of a constituent upon radiation is presumably due to its radiation sensitivity at different doses [14].
Detection and identification of radiolytic products are very important in the study of the effect of radiation in plant materials because changes in the chemical structure of some compounds may lead to formation of toxic radical species. However, the identification of these structures on essential oils is not easy. Radiolytic products are in trace amounts and usually undetected by GC/MS or coelute with other oil constituents. Studies of the effect of γ-radiation on pure compounds are useful to understand these degradation processes, but the response of these compounds could be different when they are a part of a vegetal material.
Volatile compounds, such as terpenes and terpene derivatives, are usually majoritary components in EOs from plant material and contain different chemical functional groups. A volatile compound in different EOs is under specific reactional environments and the conditions of each EO submitted to different ways of isomerization, oxidation, and hydroxylation when exposed to γ-radiation provide new compounds [24].
The effects of radiation on the volatile compounds are different when a constituent is contained in different vegetal specie. For example, the monoterpene linalool showed a great sensitivity to γ-radiation in leaf EO from O. basilicum [20]. However, this compound was radiation-resistant in leaf EO from T. vulgaris [16]. In the same context, the content of carvacrol significantly decreased in aerial part EO from Z. multiflora [43] and increased at doses up to 5.0 kGy in the leaf EO from O. vulgare, whereas it was not affected by γ-radiation in leaf EO from T. vulgaris [16].
Moreover, the content of α-pinene significantly decreased in the EO from A. gigas Nakai [3]. However, the content of this volatile compound increased after exposition to γ-radiation in the aerial part EO from Z. multiflora [16]. Another example is the menthone, which increased according to the γ-radiation dose increase in the EO from M. piperita [17], but its content increased after γ-radiation at 10.0 kGy in the EO from M. pulegium and decreased at 20.0 and 30.0 kGy [44].
In spite of the exposition to radiation on secondary metabolites studied a long time ago, new studies are necessary to better understand its effect on cell structure and chemical structure of the constituents of the EOs from vegetal material. In addition, it seems to be interesting to study whether a modification of the structure of some pure compounds (even in trace) could lead to the formation of toxic, long-lived radicals [16].