The Antioxidant Activities and Antimutagenicity of Raw and Heat Processed Sweet
Potatoes Grown in Thailand
Juthamas Mungdee1*, Kaew Kangsadalampai#, Kalyarat Kruawan, Pongtorn Sungpuag
1
Master of Science Program in Food and Nutritional Toxicology, Institute of Nutrition, Mahidol
University, Salaya, Phutthamonthon, Nakornpathom 73170, Thailand
*e-mail: juthamas.mun@student.mahidol.ac.th, #e-mail: kaew.kan@mahidol.ac.th
Abstract
Thai sweet potatoes are rich in dietary fiber, minerals, vitamins and bioactive
compounds; however, no study has been conducted on the health protective effect for consumers.
White, yellow, orange and purple sweet potatoes (Ipomoea batatas) were homemade steamed or
baked. Each lyophilized processed sample as well as its corresponding lyophilized raw sample
was assayed for DPPH (2, 2- diphenyl-1-picrylhydrazl) free radical scavenging activity, FRAP
(Ferric reducing antioxidant power) and the total phenolic content. Results indicated that
steamed purple sweet potato had the highest antioxidant activities (12.19 µM TEAC/g for DPPH
and 279.57 µM/g for FRAP assay) and the amount of phenolic compound was 18.5 mg GAE
(Gallic acid equivalent)/g. Each sample (appropriate amount) was found negative in the somatic
mutation and recombination test using trans-heterozygous (mwh+/+flr3) Drosophila
melanogaster. The same test was also used to evaluate whether the samples could modulate the
mutagenicity of 20 mM urethane. The wings of the surviving flies were analyzed for the
occurrence of mutant spots. Interestingly, all samples enhanced the mutagenicity of urethane up
to 2 folds. The difference of processing as well as the varieties of sample did influence on the
mutagenicity indexes (MIs) of urethane; however, baked yellow sweet potato and raw orange
sweet potato did not have any significant effect on urethane mutagenicity. This potentiation of
specific samples on urethane mutagenicity might attribute to the pro-oxidant activity of some
phytochemicals. This confirms that such samples are safe for consumers who required
antioxidants in carbohydrate source; however, precaution on the content of dietary mutagens
should be aware since synergistic interaction could occur.
Keywords: sweet potato, antioxidant activities, mutagenicity, heat processed
Introduction
Sweet potatoes (Ipomoea batatas L.) are rich in dietary fiber, minerals, vitamins,
antioxidants, phenolic compounds, anthocyanins, tocopherol and -carotene [1]. Such
phytochemicals also provid the distinctive colors of the flesh (cream, deep yellow, orange and
purple) of sweet potatoes. Previous report indicated that the phytochemicals in sweet potatoes
displayed antioxidative or radical-scavenging activity and antimutagenicity [2]; this gave the
hope of reducing the risk of some degenerative diseases, such as cardiovascular disease, cancer,
and age related neuronal degeneration of the consumer [3]. Interestingly, it was reported that
cyanidin (YGM-3) and peonidin (YGM-6) in purple-colored sweet potato had strong inhibitory
effect on the heterocyclic amines, namely Trp-P-1, Trp-P-2 and IQ in the presence of rat liver
microsomal activation systems in Ames test [4]. Since most sweet potatoes are cooked before
they are eaten and it was demonstrated that thermal processing increased the antioxidant activity
of tomatoes [5]; therefore, the objective of this study was to determine the antioxidant activities
using DPPH and FRAP assays and total phenolic content of raw and heat processed white,
purple, yellow, and orange sweet potatoes grown in Thailand. In addition, the antimutagenicity
of the samples against a known mutagen found in fermented food products (bread, yogurt and
cheese) and alcoholic beverages (white wine and beer) [6], namely urethane also determined in
the somatic mutation and recombination test using Drosophila melanogaster.
Methodology
Chemicals and Reagents
Urethane (URE), 2, 4, 6-tripyridyl-s-triazine (TPTZ), ferric chloride hexahydrate, and
ferrous sulfate heptahydrate were purchased from Sigma Chemical (St. Louis, Mo, USA). Fluka
Chemika (Buchs, Switzerland) supplied 2, 2- diphenyl-1-picrylhydrazl (DPPH), gallic acid and
Folin-Ciocalteu reagent. Trolox was purchased from Aldrich Chemical (Milwaukee, WI,
Germany). All other chemicals and reagents were of analytical grade.
Sample Preparation
White, purple, yellow and orange sweet potatoes were washed, sliced into pieces and
treated as raw, steamed (for 30 min) and baked (for 50 min at 200 °C) as suggested by Kim et al
[7]. Total twelve samples were freeze-dried and grinded to be powder. All dried samples were
stored in a refrigerator for further investigation.
Antioxidant Activities and Total Phenolic Content
Each sample (1g) was extracted with 80% methanol (10 ml) at 37 ºC for 1 h. The solution
was filtered through Whatman filter paper No. 1 and collected into a glass bottle. Each
methanolic extract was assayed for DPPH free radical scavenging activity, ferric reducing
antioxidant power or FRAP and the total phenolic content as suggested by Kruawan and
Kangsadalampai [8].
Somatic Mutation and Recombination Test
Each freeze-dried sample was substituted (25, 50 or 100%) for corn flour and sugar in the
standard Drosophila medium [9] in order to obtain the experimental medium for mutagenicity
evaluation as described by Graf [10]. The standard medium containing 20 mM urethane was
used for the positive control group. Toxicity of each sample was determined from the data of
survival rate of adult flies. The wing spots data were evaluated using the statistical procedure and
a multiple decision procedure as described by Frei and Wurgler [11]. Urethane (20 mM) was
substituted for water in the experimental medium; the medium was used to evaluate the
modulating effect expressed as mutagenicity index (MI) of each sample.
MI =
Spot per wing induced by urethane in the presence of sample
Spots per wing induced by urethane only
It is proposed that the MI < 0.4, 0.4-0.6, 0.6–0.8 or 0.8–1 indicates strong, moderate,
weak or negligible antimutagenicity, respectively. On the other hand, the MI higher than 1 means
negligible, weak, moderate or strong potentiating effect when it is 1-1.2, 1.2-1.4, 1.4-1.6 or >1.6,
respectively.
A
GAE (mg/g dw)
20
15
10
5
B
White sweet
potato
Yellow sweet
potato
Orange sweet
potato
Purple sweet
potato
White sweet
potato
Yellow sweet
potato
Orange sweet
potato
Purple sweet
potato
Yellow sweet
potato
Orange sweet
potato
Purple sweet
potato
14
12
10
8
6
4
2
0
300
FRAP values (µM/g dw)
C
TEAC (µM /g dw)
0
250
200
150
100
50
0
White sweet
potato
Figure 1. The total phenolic contents assay (A) and antioxidant activities measured with DPPH assay (B) and
FRAP assay (C) of
raw,
steamed or
baked sweet potatoes grown in Thailand.
Figure 2. The modulating effect on 20 mM urethane induced wing spots in Drosophila melanogaster of
raw,
steamed or
baked sweet potatoes.
Results
Antioxidant Activities and Total Phenolic Contents
Figure 1 shows that different household processing methods (steaming and baking)
affected on the antioxidant activities and total phenolic contents of potatoes; the effect was
clearly seen on purple sweet potato; the steamed one had the highest total phenolic content (18.5
mg GAE/g dry sample). The same trend was also observed in the FRAP assay (279.57 FRAP
values µM/g dry sample). White potato also displayed the same effect of household processing
on all parameter determined; it seemed that baking had the highest reducing ability.
Mutagenicity and Antimutagenicity Evaluations
The survival rates of adult flies obtained from larvae fed on medium containing each
sample were not much different from those of the negative control group. This indicated that no
sample was toxic to the tester organism. None of the samples was mutagenic (data not shown)
since they did not significantly induce the frequencies of mutant spots to be higher than that of
the negative control group. The results confirm that such samples are safe for all consumers.
However, Figure 2 demonstrates that none of the samples substituted for corn starch in the
medium could inhibit the mutagenicity of 20 mM urethane; on the other hand, they tended to
potentiate the notorious effect posed by urethane to the tester organism. Yellow and orange
sweet potatoes seemed to have lesser effect on the mutagenicity of urethane than others did.
Discussion and Conclusion
All samples in this study contained antioxidants and phenolic compounds; it seemed that
darker sweet potato contained higher phytochemicals. This was not surprised since similar trend
of total phenolic contents was previously reported in commercial sweet potato cultivars. Furuta
et al. [12] indicated purple varieties had higher phenolic content than that of orange, yellow and
white varieties; the same information also confirmed by Huang et al. [13] and Teow et al. [14].
In addition, Rowena Grace et al. [15] suggested that different genotypes of sweet potato
determined the amount of phenolic compounds.
The total phenolic content and antioxidant activity (determined with FRAP assay) of
white and purple sweet potatoes in this study varied with household processing methods i.e.
baking caused higher loss of antioxidant than streaming did. This confirmed Padda and Picha
[16] who found that heat-processing methods resulted in a significant loss in total phenolic
content and antioxidant capacity of the skin tissue of sweet potatoes which contained the highest
concentrations of total phenolic compounds; baking resulted in greater losses in antioxidant
capacity of skin tissue. It is not known whether high temperature during cooking breaks down
phenolic compounds or they form larger macromolecules with other compounds present in the
tissue of sweet potato. Padda and picha [16] suggested that heating might disrupt the intracellular
separation of the phenolic acids and oxidative enzymes (polyphenoloxidases), resulting in
degradation of the phenolic acids. On the other hand, Kim et al. [7] reported that the total content
of anthocyanin in purple sweet potato decreased by nearly half upon steaming (for 10 min at 121
˚C) but only slightly upon baking (for 40-50 min at 200 ˚C). Further studies should address the
bioavailability of phenolic compounds of sweet potato after various cooking methods.
The samples of this investigation were demonstrated to be good sources of antioxidant
which might be appreciated by health concerning consumers. It might suggest that high
consumption of both raw and processed sweet potatoes should be done with caution because the
data of the present investigation suggested that lyophilized raw, steamed or baked sweet potatoes
substituted for all corn flour and sugar of Drosophila medium enhanced the mutagenicity of 20
mM urethane. It was proposed that when each sample was substituted for the whole amount of
corn flour in the Drosophila medium, it led to the over consumption of some phytochemicals.
Some fruits and their processed products, namely orange and pamelo [17], durian and
mangosteen [18] increased the mutagenicity of urethane in the somatic mutation and
recombination test. The enhancing effect of each sample might be due to some natural
compounds could induce the catalytic activities of cytochrome P-450 enzyme system (phase 1)
or inhibited glutathione-S-transferase as well as decreased the amounts of glutathione of phase 2
detoxifying system in Drosophila melanogaster [19]. It was found that myristicin (500 µmol/kg
body weight, i.p) was an inducer of rat liver cytochrome-P450s, namely 1A1/2, 2B1/2, and 2E1;
this compound caused 2-20 folds increase the activities of liver cytochrome-P450s with respect
to those of the control animal [20]. The effects of the five major constituents in the Ginkgo
biloba extract, namely bilobalide, ginkogolides A, ginkogolides B, quercitin and keampferol on
the expression of the major cytochrome-P450s in rat were also investigated by Deng et al. [21].
They found that quercetin (250 mg/kg body weight) significantly increased cytochrome-P450
2E1 activity (1.31-fold) and cytochrome-P450s 2E1 protein expression (1.43-fold); however,
cytochrome-P450 2E1 activity and cytochrome-P450 2E1 protein expression were not
significantly altered by keampferol. Paolini et al. [22] demonstrated that high intakes of vitamin
C (500 mg/kg body weight) for 4 days in rat markedly induced hepatic cytochrome-P450 2E1linked mono-oxygenases measured as p-nitrophenol hydroxylase activity and corroborated by
mean of western blot analyses. Literature review on the effect of natural constituents of fruit on
phase 2 modification enzymes is scarce. Only the works of Van Zanden et al. [23] and Cermak
[24] revealed that the flavonoids, namely galangin, kaempferol and quercetin and the flavones,
namely eriodyctiol had inhibitory effects on glutathione-S-transferase activity of subfamily
GSTP1-1 in transfected human MCF7 breast cancer cells. Therefore, there is a need to determine
and/or quantify the compounds that pose such activity in the sample.
Conclusively, it is suggested that consumption of some color sweet potatoes seems to be
good practice for health concerning consumer since it was demonstrated in this investigation that they
were good sources of antioxidants. However, it is cautioned that high consumption of sweet potatoes
might lead to the over consumption of antioxidant that may potentiate the activity of some mutagens.
For instance, Chen et al. [25] revealed that butylatedhydroxytoluene (a synthetic antioxidant) had little
effect on the mutagenicity of 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) and 2-amino-3,8dimethyl-imidazo[4,5-f]quinoxaline (MeIQx) at low concentrations, but significantly increased their
mutagenicity at high concentrations. The other demonstrated that an inappropriate amount of antioxidant
caused harmful effect; the consumption of high doses of beta-carotene in the form of dietary supplement
in an attempt to prevent lung and other cancers revealed the association between the antioxidant
supplement and a higher risk of lung cancer in cigarette smokers [26]. Therefore; the present investigation
has confirmed that the amount of any food items consumed by consumers should be neither high nor low
but appropriate to support their good health.
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