Evaluation of the Antioxidant Potential of Various Plant Essential Oils

Chapter 22

Evaluation of the Antioxidant Potential of Various Plant Essential Oils Downloaded by UNIV OF MISSOURI COLUMBIA on April 29, 2013 | http://pubs.acs.org Publication Date: September 30, 2008 | doi: 10.1021/bk-2008-0988.ch022

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Alfreda Wei and Takayuki Shibamoto

Department of Environmental Toxicology, University of California, Davis, CA 95616

The antioxidant activities of extracts from medicinal plant essential oils were evaluated using an aldehyde/carboxylic acid assay. The extracts from basil and thyme inhibited the oxidation of hexanal by 100% at a level of 500 µg/mL over 40 days. Rosemary (58%) and chamomile (44%) extracts exhibited moderate antioxidant activities, whereas cinnamon and lavender extracts did not show any appreciable activity. Clove bud extract inhibited hexanal oxidation by 100% at the levels of 200 and 500 µg/mL. Among three different species, E. polyanthemon showed the strongest antioxidant activity of 100% at the level of 500 µg/mL. Among the essential oils tested, rose essential oil exhibited the highest activity (90%) at the level of 500 µg/mL, followed by ylang (87%), and jasmine (86%). Eugenol, which is one of the major constituents of eucalyptus oil, exhibited potent antioxidative activity. Antioxidants such as eugenol and thymol may play an important role in the pharmaceutical activities of natural plant extracts used for aromatherapy.

© 2008 American Chemical Society In Food Flavor; Tamura, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Downloaded by UNIV OF MISSOURI COLUMBIA on April 29, 2013 | http://pubs.acs.org Publication Date: September 30, 2008 | doi: 10.1021/bk-2008-0988.ch022

258 Since ancient times, people in all cultures have known that many natural plants contained aroma chemicals and have used them in baths for medicinal purposes. Combinations of resins, oils, andfragrantplants were used in ceremonies and medicine in most ancient civilizations. The Chinese may have been one of the first cultures to use aromatic plants for well-being. The Romans are also well-known for their elaborate baths. Later, the Egyptians invented a rudimentary distillation machine that allowed for the crude extraction of cedar wood oil. In many places, the steam bath has been enjoyed for the benefits of total relaxation of mind and body: to ease stress; relieve muscle tension and stiff joints; sweat out body toxins; stimulate circulation; increase body metabolism; keep skin glowing and youthful; and to alleviate sinus congestion due to colds, asthma, or allergies. Lavender oil and chamomile oil were used for the treatment of insomnia. Digestive problems were treated with coriander oil. Chamomile, celery, juniper, and coriander oils were used as anti-inflammatory medicines. Eucalyptus oil was known to relieve muscle pain. Until recently, aroma chemicals have been investigatedfromthe viewpoint of flavor andfragrancechemistry. However, some medicinal activities of aroma chemicals, such as antioxidative activity, have been discovered through using essential oils in aromatherapy. Recently, we reported that aroma chemicals found in brewed coffee possess antioxidative activity (1-3). Also, antioxidant activities of natural plant essences including beans (4\ clove bud (5), Eucalyptus (6), herbs and spices (7), and teas (8) have been reported. In the present study, essential oils obtainedfromvarious natural plants were examined for antioxidant activities.

Materials and Methods Materials Essential oils of cinnamon, lavender, chamomile, rosemary, basil, thyme, ylang, rose, jasmine, and peppermint were giftsfromInternational Flavor and Fragrance Co., Ltd. (Keyport, New Jersey). Hexanal, hexanoic acid, and undecane were purchased from Aldrich Chemical Co. (Milwaukee, WI). Authentic aroma chemicals were obtainedfromreliable commercial sources or as giftsfromTakata Koryo Co., Ltd. (Osaka, Japan).

Isolation of Essential Oils from Medicinal Plants Clove buds (200 g) and eucalyptus leaves (200 g) were placed in a roundbottom flask with 1 L deionized water and then steam distilled at 55 °C for 3 h

In Food Flavor; Tamura, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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under reduced pressure (95 mmHg). The distillate (200 mL) was extracted with 50 mL dichloromethane using a liquid-liquid continuous extractor for 6 h. After the extract was dried over anhydrous sodium sulfate, the solvent was removed by distillation with a Vigreux column. The distillation was stopped when the volume of extract was reduced to approximately 1 mL, and then the solvent was further removed under a purified nitrogen stream. The sample was stored at 5 ° C until the antioxidative tests and analyses.

Antioxidative Tests of Samples Using the Aldehyde/Carboxylic Acid Assay Various concentrations of testing samples were added to a 2 mL dichloromethane solution of hexanal containing undecane as a gas chromatographic internal standard. Oxidation of the sample solution was initiated by heating at 60 ° C for 10 min in a sealed vial; it was then stored at room temperature. The headspace of each vial was purged with pure air every 24 h for the first 10 days. Hexanal concentrations were monitored at 5-day time intervals. BHT and oc-tocopherol were used as controls to compare the antioxidative activity of the plant extracts. All sample vials were wrapped with aluminum foil to avoid UV oxidation. The quantitative analysis of hexanal was conducted according to a previously reported internal standard method (9). A gas chromatograph equipped with a 30 m * 0.25 mm i.d. (d = 0.1 (im) DB-1 bonded-phase fused-silica capillary column and a flame ionization detector (FID) was used. f

Results and Discussion The most common method used to determine the antioxidative activity of a chemical or a group of chemicals is the thiobarbituric acid assay (TBA). The TBA assay involves measurement of so-called thiobarbituric acid reactive substances (TBARS), including malonaldehyde (MA), formedfroma lipid upon oxidation. A capillary gas chromatographic method for specific determination of MA has been also used (10-13). Methods involved in lipid/MA assay are useful for a rapid analysis of samples, but occur under artificially strong oxidative conditions. Therefore, these assays may not accurately represent oxidative processes, which are often associated with food systems. Generally, in food systems, oxidation processes occur slowly over a period of 40 days. Because the lipid/MA assays measure short-term oxidation, the aldehyde/carboxylic acid assay was developed for determining the long-term antioxidant potential of a chemical or a group of chemicals (14). This method is based on the autooxidation of aldehydes to carboxylic acids with active oxygen species such as a



260 hydroxyl radical (75). Fatty aldehydes are readily converted to the corresponding fatty acid in an oxygen-rich dichloromethane solution through a radical-type reaction as shown in Figure 1 (16). Although the aldehyde/carboxylic acid assay requires prolonged time periods, it offers a better measure of the oxidation process that occurs in foods and beverages. Figure 2 shows the antioxidative activities of extractsfromherbs and spices, and the standards (BHT and a-tocopherol) obtained using a hexanal/hexanoic acid assay throughout a storage period of 40 days at the level of 500 ng/mL. The antioxidant activity of samples was directly related to the remaining amount (%) of hexanal. The extractsfrombasil and thyme inhibited the oxidation of hexanal by 100% at a level of 500 |*g/mL over 40 days. Their activities were comparable to those of standard antioxidants BHT and a-tocopherol. Rosemary (58%) and chamomile (44%) extracts exhibited moderate antioxidant activities, whereas cinnamon and lavender extracts did not show any appreciable activity. Thyme, basil, rosemary, and chamomile extracts showed the dose response activity. All extracts exhibited dose-related activity and Figure 3 show a typical example of dose-related activity (basil extract). This extract inhibited hexanal oxidation over 40 days at levels of 200 and 400 ng/mL. On the other hand, it inhibited only 10% and 35% over 40 days at the levels of 10 ng/mL and 20 ^ig/mL, respectively. Figure 4 shows antioxidant activities of various essential oils over 40 days. Rose oil exhibited the highest activity (90%) among the oils tested, at the level of 500 jLig/mL, followed by ylang (87%), and jasmine (86%). Rose and jasmine essential oils inhibited hexanal oxidation by over 80% at the level of 100 mg/mL, whereas ylang oil did not exhibit appreciable activity at the same level. Lavender oil exhibited slight antioxidant activity at the level of 500 mg/mL. However, chamomile and peppermint oils did not show appreciable activity at any level tested. Figure 5 shows the antioxidant activities of medicinal plant extracts examined over 40 days at various levels. Clove bud extract exhibited the highest antioxidant activity with dose-response. It inhibited hexanal oxidation 100% at the levels of 200 and 500 jxg/mL. Its activity was consistent with that of atocopherol at the level of 50 ng/mL. Among three different species of eucalyptus oils, the oilfromE. polyanthemos showed the strongest antioxidant activity. It inhibited hexanal oxidation by 100% at the level of 500 jug/mL. The oilfromE. globulous exhibited moderate dose-response activity. The oil of E. perhniana inhibited hexanal oxidation by 30% at the level of 100 mg/mL, but it showed less activity at the higher levels. It is interesting that the well-known medicinal plant, aloe vera, did not exhibit appreciable antioxidant activity at any of the levels tested.


261

Aldehyde R-CHO

2

*W RCO

P ^ -

o-o-


RH OH

R-CHO

R- CO

R" CO ?

2R~COOH * Carboxylic acid

H

RCHO

R~P-H

—J-

o

R-C O-OH

O— O-^-R O

Figure 1. Oxidative conversion mechanisms of aldehyde to carboxylic acid.

BHT a-Tocopherol Cinnamon Lavender Chamomile Rosemary

i

Basil Thyme 0

20

40

60

80

100

Activity (% of remaining hexanal) Figure 2. Antioxidative activities of extractsfromherbs and spices.



262

Time (days) Figure 3. Typical example ofdose-related activity obtainedfrom basil extract.

Figure 4. Antioxidant activities of various essential oils over 40 days.



263

Figure 5. Antioxidant activities of medicinal plant extracts over 40 days.

Figure 6. Antioxidant activities of chemicals found in clove buds and eucalyptus leaves over 40 days at the level of 160 ug/mL.



264 Figure 6 shows the antioxidant activities of chemicals found in clove buds and eucalyptus leaves over 40 days at the level of 160 jug/mL. Eugenol exhibited the highest activity (90%), followed by benzyl alcohol (80%), thymol (80%), eugenyl acetate (62%), and terpinene-4-ol (57%). The results indicate that eugenol, eugenyl acetate, and benzyl alcohol contribute significantly to the antioxidant activity of eucalyptus oils. Also, the strong antioxidant activity of clove buds is due to the presence of thymol and terpinene-4-ol. Our preliminary experiments indicate that the antioxidative activities of aroma constituents—including maltol, eugenol, l-octen-3-ol, and benzaldehyde—are not as potent as those of known antioxidants, BHT and a-tocopherol. However, large amounts of these aroma chemicals are present in natural plants. Therefore, their total activity may be comparable to those of known antioxidants. Investigation of the antioxidative activity of each aroma chemical found in natural plants is currently underway.

References 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Singhara, A.; Macku, C.; Shibamoto, T. In Functional Foods for Disease Prevention II: Medicinal Plants and Other Foods; T. Shibamoto, J. Terao and T. Osawa, eds, ACS Symposium Series 701, ACS, Washington, DC, 1998, pp. 101-109. Fuster, M. D.; Mitchell, A. E.; Ochi, H.; Shibamoto, T. J. Agric. Food Chem. 2000, 48, 5600-5603. Yanagimoto, K.; Ochi, H.; Lee, K.-G.; Shibamoto, T. J. Agric. Food Chem. 2004, 52, 592-596. Lee, K. -G.; Mitchell, A. E; Shibamoto, T. J. Agric. FoodChem. 2000, 48, 4817-4820. Lee, K.-G.; Shibamoto, T. J. Sci. Food Agric. 2001, 81, 1573-1579. Lee, K.-G.; Shibamoto, T. J. Sci. Food Agric. 2001, 81, 1573-1579. Lee, K.-G.; Shibamoto, T. J. Agric. Food Chem. 2002, 50, 4947-4952. Yanagimoto, K.; Ochi, H.; Lee, K.-G.; Shibamoto, T. J. Agric. Food Chem. 2003,51,7396-7401. Ettre, L. S. In The Practice of Gas Chromatography, L. S. Ettre and A. Zlatikis, eds, Interscience Publishers: New York, NY, 1967, pp. 402-440. Umano, K.; Dennis K. J.; Shibamoto, T. Lipids 1988, 23, 811-814. Osawa, T.; Katsuzaki, H.; Hagiwara, Y; Hagiwara, H.; Shibamoto, T. J. Agric. FoodChem. 1992, 40, 1135-1138. Nishiyama, T.; Hagiwara, Y; Hagiwara, H.; Shibamoto, T. J. Agric. Food Chem. 1994,42, 1728-1731. Ogata, J.; Hagiwara, Y; Hagiwara, H.; Shibamoto, T. J. Am. Oil Chem. Soc. 1996, 73, 653-656.


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14. Macku, C.; Shibamoto, T. J. Agric. FoodChem. 1991, 39,1990-1993. 15. Homer, L. In Auto-oxidation and Antioxidants, W. O. Lundberg, ed, John Wiley & Sons: New York, NY, 1961, pp. 197-202. 16. Nonhebel, D. C.; Tedder, J. M.; Walton, J. C. In Radicals, Cambridge University Press, London, 1979, pp. 155-157.


Evaluation of the Antioxidant Potential of Various Plant Essential Oils

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