4160 J. Agric. Food Chem., Vol. 48, No. 9, 2000
Choi et al.
F igu r e 3. Scavenging effects of Trolox, R-tocopherol, ascorbic acid, and BHT on the DPPH radical. a-c Means of remaining DPPH
%. Values with the same superscripts are not significantly different (p < 0.05).
stronger scavenging activities than Iyokan, Hyuga-
natsu, and ujukitsu (p < 0.05). Daidai, Valencia, and
Tarocco oranges, ozu, and kumquat, in which γ-ter-
pinene and terpinolene were almost 0%, form a group
with the lowest DPPH radical-scavenging activity level.
As those oils were mainly composed of limonene in a
proportion of >90%, it is considered that limonene would
not play the principal role in determining the scaveng-
ing activity for the radical. Kiyookadaidai, banhakuyu,
ujukitsu, and mochiyuzu were exceedingly abundant in
myrcene compared with others, 63.7, 37.2, 28.3, and
20.6%, respectively. However, there was no direct cor-
relation between the myrcene content and the radical-
scavenging activity. In each citrus essential oil, geraniol
content was low; nevertheless, geraniol showed the
highest radical-scavenging activity.
The relative effectiveness of antioxidants is dependent
on the action type of antioxidants (free radical inhibitor,
peroxide decomposer, metal inactivator, or oxygen scav-
enger) (Yagi, 1970, Yen and Duh, 1994), test system,
emulsion system, concentration, oxidation time, method
used (Frankel et al., 1994; Huang et al., 1996), and
growth stage of plant samples (Baldwin, 1993). Under
a wide range of conditions and test systems, the anti-
oxidant activity of Trolox proved to be superior to that
of R-tocopherol (Frankel et al., 1996). For example,
Trolox is significantly more active than R-tocopherol in
a bulk oil system, whereas in an oil-in-water emulsion
R-tocopherol is significantly more active than Trolox
(Frankel et al., 1994). Yamaguchi et al. (1998) reported,
however, that R-tocopherol and Trolox had almost the
same free radical-scavenging activities when deter-
mined by the DPPH-HPLC method, which means that
neither lipophilicity nor hydrophilicity of the antioxidant
affects the reaction with DPPH. This result is in
agreement with our data, shown in Figure 3. As shown
in Figure 3, ascorbic acid and BHT acted as more potent
radical scavengers than R-tocopherol and Trolox at 25
µM. No significant difference was found between the
radical-scavenging activities of 25 µM R-tocopherol and
25 µM Trolox. Moreover, the use of both R-tocopherol
and Trolox did not show any synergistic effect. There
was no significant difference among 25 µM ascorbic acid,
25 µM BHT, and 50 µM Trolox.
eases. Crowell (1997) and Gould (1997) reported that
terpenoids such as carveol, limonene, sobrerol, and
perillyl alcohol, which are found in plant essential oils,
are effective in the treatment of breast, liver, and/or
other cancers. These facts support our idea that citrus
essential oils will protect against damaging free radicals
and may be natural protectors against such diseases.
Further work is required to determine the mechanism
involved in the radical-scavenging activity of citrus
essential oils. The present results support the view that
citrus essential oils are highly bioavailable and may be
active in the body as antioxidants and free radical
scavengers.
ACKNOWLEDGMENT
We thank Mr. Y. Higuchi at the Kochi Prefectural
Fruit Experimental Station and Dr. T. Yoshida at the
Okitsu Branch Experimental Station, the Ministry of
Agriculture, Forestry and Fisheries, for kindly providing
citrus samples. H.S.C. is grateful for the Postdoctoral
Fellowship offered by the Korea Science and Engineer-
ing Foundation.
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Antioxidants rein in free radicals by offering their own
electrons. Free radicals can be produced from a number
of sources such as cigarette smoke, pollution, pesticides,
herbicides, and overexposure to sunlight. Free radicals
are responsible for inflammation, compromised immune
systems, and degenerative and other age-related dis-