Philippine Yam (Dioscorea spp.) Tubers Phenolic Content and

Transcription

Philippine Journal of Science
140 (2): 145-152, December 2011
ISSN 0031 - 7683
Date Received: 04 Jun 2010
Philippine Yam (Dioscorea spp.) Tubers Phenolic
Content and Antioxidant Capacity
Djanna F. Cornago1*, Rowena Grace O. Rumbaoa2, and Inacrist M. Geronimo1
1
Institute of Chemistry, College of Science,
University of the Philippines, Diliman, Quezon City
2
Department of Food Science and Nutrition, College of Home Economics,
University of the Philippines, Diliman, Quezon City
Five Philippine varieties of purple yam or ube (Dioscorea alata) — Daking, Kimabajo, Rapangrapang, Sampero, and Shiket, and two varieties of lesser yam or tugui (Dioscorea esculenta)—
Highland and Lowland, were analyzed in the study for phenolic content and antioxidant
activity. The total phenolic content of the samples ranged from 69.9 to 421.8 mg gallic acid
equivalent (GAE)/100 g dry weight. EC50 values were 1.7-14.8, 6.2-31.7, and 17.5-35.1 mg/mL
for radical scavenging activity, reducing power and iron chelating capacity, respectively. Total
antioxidant activity by ferric thiocyanate method at 50 mg/mL was between 92.0-95.6%. All
samples had better radical scavenging activity and reducing power on a µg analyte basis than
α-tocopherol. Significant correlation was observed between total phenolic content and DPPH
radical scavenging activity (R=-0.7664, p<0.05) and reducing power (R=-0.8083, p<0.05) but
none between total antioxidant activity and phenolic content (0.1378, p>0.05), for both purple
yam and tugui. Significant correlation between total phenolic content and iron-chelating capacity
was observed only for the tugui varieties(R= -0.9859, p<0.05).
Key Words: antioxidant, phenolic content, radical scavenging activity, reducing power, yam
INTRODUCTION
Researches have established that root crop extracts,
specifically yam (Bhandari & Kawabata 2004; Chung
et al. 2008; Hsu et al. 2003), potato (Al-Saikhan et al.
1995; Reyes 2005; Rumbaoa et al. 2009b), sweet potato
(Huang et al. 2005a; Rumbaoa et al. 2009a; Teow et al.
2007), yacon (Yan et al. 1999), cassava and taro (Lako
et al. 2007), exhibit antioxidant activity. Antioxidant
activity of root crops has been attributed to well-known
phytochemicals such as α-tocopherol, ascorbic acid and
β-carotene (Kalt 2005). However, recent researches have
focused on polyphenolic compounds, which are mainly
responsible for antioxidant activity as shown in studies
of in vitro models of lipid oxidation (Vinson et al. 1998).
*Corresponding author: [email protected]
[email protected]
Yam (Dioscorea spp.) belongs to the high antioxidant
activity but low phenolic content group in the study by
Kaur & Kapoor (2002) on Asian vegetables. Halvorsen et
al. (2002) ranks yam as having the 7th highest antioxidant
concentration among 11 roots and tubers analyzed
using Ferric Ion Reducing Antioxidant Power (FRAP)
assay. Ozo et al. (1984) identified cyanidin-3-glucoside,
(+)-catechin and the procyanidin dimers ‘B-1’ and
‘B-3’ as the phenolic constituents of yam. In addition,
Bhandari & Kawabata (2004) reported that yam contains
chlorogenic acid.
There are approximately 600 species of yam (Ozo et al.
1984) but the species cultivated in the Philippines are
Dioscorea alata or ubi and Dioscorea esculenta or tugui
(BAS 2006). Annual production of ubi in the Philippines
is 26,464 metric tons for the period 2000-2005, while
145
Vol. 140 No. 2, December 2011
that for tugui is 2,702 metric tons (BAS 2006). However,
consumption for the same period averaged only 5.0% for
ube and 14.0% for tugui (BAS 2006).
According to Huang et al. (2007), yam contains reasonably
substantial amount of protein, starch and essential amino
acids relative to other root and tuber crops. Further,
Wanasundera & Ravindran (1994) indicated that yam is
a good source of minerals. Aside from its nutritive value,
yam is a possible source of alternative food antioxidant
to synthetic ones such as butylated hydroxytoluene
(BHT), butylated hydroxyanisole (BHA) and tertiary
butylhydroquinone (TBHQ). It is the objective of the
present study to provide information on the phenolic
content and antioxidant activity of locally available ube
and tugui varieties to producers and consumers in order
to promote yam consumption. The antioxidant activity,
which includes radical scavenging, electron donation
and metal chelation, of the methanolic yam extracts
toward reactive species were compared to a commercially
available antioxidant, α-tocopherol, and a metal chelating
agent, ethylenediamine tetraacetic acid (EDTA).
MATERIALS AND METHODS
Reagents
Gallic acid (Hi-Media, Hi-Media Laboratories Pvt. Ltd.,
India), ethylenediamine tetraacetic acid (EDTA), disodium
salt (Hi-Media, Hi-Media Laboratories Pvt. Ltd., India)
and ferrozine or 3-(2-pyridyl)-5,6-bis(4-phenylsulfonic
acid)-1,2,4-triazine, monosodium salt (Fluka, SigmaAldrich, U.S.A.) and Folin-Ciocalteu’s phenol reagent
were purchased from Belman Laboratories (Quezon City,
Philippines). Linoleic acid (Cica, Acros Organics, Japan)
was purchased from Just-In-One Marketing (Caloocan
City, Philippines). Alpha-tocopherol (Sigma, SigmaAldrich, U.S.A.) and 1, 1-diphenyl-2-picrylhydrazyl
(DPPH) (Sigma, Sigma-Aldrich, U.S.A.) were purchased
from ChemLine Scientific Industries (Quezon City,
Philippines). All reagents used were analytical grade.
Sample
Samples were provided by Northern Philippines Root Crop
Research and Training Center (Benguet, Philippines). Five
purple yam or ube (D. alata) and two lesser yam or tugui
(D. esculenta) varieties were analyzed in the study. The
ube varieties Kimabajo and Sampero have light purple
flesh while Rapang-rapang and Shiket have dark purple
flesh. Daking has white flesh and purple peel. Both tugui
varieties, Highland and Lowland Tugui, have white flesh but
the former is round-shaped while the latter is elongated. The
samples were weighed, washed and cut into 2-cm slices.
Steaming at 100°C was done for 30 minutes to prevent
146
Cornago et al.: Phil. Yam Tubers Antioxidant Capacity
browning of the flesh, after which the samples were cooled,
peeled and cut into cubes. These were then freeze-dried and
ground to fine powder. The flour was placed in a resealable
bag and stored at 4°C until used.
Extraction of Phenolic Compounds
Methanolic extraction was done using a modified method
of Bhandari & Kawabata (2004). Five grams of yam flour
was mixed with 80 mL methanol and kept overnight.
The suspension was filtered through Whatman No.1
filter paper and the filtrate was diluted to 100 mL with
methanol. Sample solutions were stored at 4°C in amber
bottles and served as the stock solution (50 mg yam flour/
mL methanol) for subsequent analyses.
Determination of Total Phenolic Content
The study employed the total phenolic content
determination by Slinkard & Singleton (1997) using
reduced volumes. The extract (200 µL) was mixed with
1.4 mL distilled water and 100 μL of Folin-Ciocalteu
reagent and incubated at room temperature for 30 s to
8 minutes. Three-hundred microliters (300 μL) of 20%
Na2CO3 solution were added and the mixture was allowed
to stand for 2 hours. The absorbance was measured at 765
nm with Lambda 1 UV-Vis Spectrophotometer (PerkinElmer, U.S.A.). Standard solutions of gallic acid (10-100
ppm) were similarly treated to prepare the calibration
curve. Results were expressed as mg gallic acid per 100
g dry sample and per 100 g fresh sample. Six replicates
of the samples were analyzed and values obtained were
averaged.
DPPH Radical Scavenging Activity
Huang et al. (2005a) described the method for the
DPPH (1,1-diphenyl-2-picrylhydrazyl) assay adopted
in the study. A 1 mL aliquot containing 1-20 μg gallic
acid equivalent (GAE) was mixed with 1 mL of freshly
prepared 80 ppm DPPH in methanol. The mixture was
kept in the dark for 30 minutes. The absorbance was
then measured at 517 nm using Lambda 1 UV-Vis
Spectrophotometer (Perkin-Elmer, U.S.A.). The radical
scavenging activity of α-tocopherol (10-50 ppm) was
also determined. Percent activity was calculated using
the equation
% Activity= (1-(ASx/Ablk)) x 100
[1]
The EC50 value, which is the sample concentration at 50%
activity, was determined by interpolation. The test was
done in six replicates and results were averaged.
Reducing Power
The reducing power assay used in the study was
based on the study by Singh & Rajini (2004). Sample
solutions containing 5-50 μg GAE were prepared from
the stock solution. A mixture of 1 mL sample, 1 mL of
0.2 M phosphate buffer (pH 6.6) and 1 mL of 1% (w/v)
K3Fe(CN)6 was placed in a test tube and incubated at 50°C
for 20 minutes. The reaction was stopped with the addition
of 1 mL of 10% w/v trichloroacetic acid. The resulting
mixture was centrifuged at 3000 rpm for 10 minutes and
1 mL of the supernatant was taken. One mL of distilled
water and 0.2 mL of 0.1% (w/v) FeCl3 solution was added
and absorbance was measured at 700 nm using Lambda 1
UV-Vis Spectrophotometer (Perkin-Elmer, U.S.A.). The
reducing power of α-tocopherol (20-100 ppm) was also
determined. The EC50 value, the concentration at which
the absorbance is 0.500, was determined by interpolation.
The analysis of the samples was done in six replicates and
results were averaged.
Iron-Chelating Capacity
The iron-chelating capacity assay by Hsu et al. (2003)
was applied to the present study with some modifications.
One mL aliquot of the sample, with concentration range
of 10-50 mg yam flour mL-1 methanol, was mixed
with 1 mL of methanol, 0.1 mL of 2 mM FeCl2· 4H2O,
and 0.2 mL of 5 mM ferrozine. After 10 minutes, the
absorbance was measured at 562 nm using Lambda 1
UV-Vis Spectrophotometer (Perkin-Elmer, U.S.A.).
EDTA, with concentration ranging from 30 to 50 ppm,
was used as positive control. Percent activity was
calculated using equation [1]. The EC50 value, which is
the sample concentration at 50% activity, was determined
by interpolation. The test was done in six replicates and
values obtained were averaged.
Total Antioxidant Activity
The ferric thiocyanate method as outlined by Huang et al.
(2006) was used to determine the total antioxidant activity
of the yam extracts. A mixture comprised of 1 mL sample
(50 mg yam flour mL-1 methanol), 1 mL of 2.51% (v/v)
linoleic acid solution in 99.5% (w/v) ethanol, 2 mL of
0.05 M phosphate buffer pH 7.0, and 1 mL distilled H2O
was incubated in the dark at 40°C. Blank and control
solutions were prepared by substituting the sample with
methanol for the blank and 100 µg/mL α-tocopherol for
the control. A 0.1 mL aliquot of the mixture and 0.1 mL
of 30% (w/v) NH4SCN was diluted with 9.7 mL of 75%
(v/v) ethanol. One hundred microliters of 20 mM FeCl2 in
3.5% (v/v) HCl was added and absorbance was measured
after 3 minutes at 500 nm using UVPC-3101 UV-Vis-NIR
Spectrophotometer (Shimadzu, Japan). The process was
repeated every 24 hours until the absorbance of the control
solution reached the maximum value. Percent inhibition
was calculated as follows:
% Activity= (1-(ΔASx/ΔAblk)) x 100
[2]
where ΔA is the absorbance increase. The test was run
in in six replicates and values obtained were averaged.
Statistical Analysis
Data were analyzed using univariate analysis of variance
(ANOVA) and means were compared using Duncan’s
Multiple Range Test. The Statistical Analysis Software
for Windows (v. 6.12) was used. Means were considered
to be significantly different when the P-value is less
than 0.05 (*P<0.05). Correlation tests were done using
Microsoft Excel 2007.
RESULTS AND DISCUSSION
Total Phenolic Content
The total phenolic content of ube (D. alata) varieties,
Daking, Kimabajo, Rapang-rapang, Sampero and Shiket,
and tugui (D. esculenta) varieties, Highland and Lowland,
were expressed as mg gallic acid equivalent (GAE) per
100 g sample, wet and dry basis, and are listed in Table 1.
Phenolic content for the ube samples analyzed in the study
ranged from 69.9 to 421.8 mg GAE/100 g dry sample,
while, that for tugui samples is between 112.4 and 156.5
mg GAE/100 g dry sample. The results of the present
study are higher than that obtained by Chung et al. (2008)
of 25 mg GAE/100 g dry sample for the 50% ethanolic
extract of D. alata flesh and Lako et al. (2007) of 8 mg
GAE/100 g fresh sample for white-fleshed D. alata and
26 mg GAE/100 g fresh weight red-fleshed D. alata. The
higher phenolic content may be attributed to the sample
preparation employed in the study. In the literature, the
samples were boiled for about 20-25 minutes with water
prior to freeze-drying (Lako et al. 2007), while the present
study used only steam blanching. Heating may result in
degradation of phenolic components and leaching from
the plant tissue (Kalt 2005). The use of different extraction
Table 1. Total Phenolic Content of Yam Samples.
Variety
Moisture
Content (%)
Total Phenolic Content
(mg Gallic acid/100g sample)*
Dry Basis
Daking
69.7±0.7
Wet Basis
a
127.8 ± 5.3a
f
421.8 ± 17.6
Kimabajo
76.0±0.5
69.9 ± 1.3
16.8 ± 0.3f
Rapang-rapang
67.0±0.4
178.9 ± 5.2c
58.9 ± 1.7c
Sampero
77.4±0.5
70.2 ± 1.2f
15.9 ± 0.3f
Shiket
72.8±0.2
b
231.5 ± 9.8
63.0 ± 2.7b
Highland Tugui
67.3±1.7
156.5 ± 2.7d
51.2 ± 0.9d
Lowland Tugui
82.1±1.5
112.4 ± 2.3e
20.1 ± 0.4e
a-f
*
Means with different letters within the same column differed significantly
(p<0.05)
Each value is expressed as the mean ± standard deviation (n=6)
147
processes may also lead to the difference in total phenolic
content (Chung et al. 2008). For the purple yam varieties,
Daking had significantly higher (p<0.05) phenolic content
than the other varieties. Sampero and Kimabajo had the
lowest phenolic content. The synthesis and accumulation
of phenolic compounds in plants, and consequently,
antioxidant activity, is affected by genotype (Huang et
al. 2005a). Intensity of color was found to be directly
proportional to phenolic content in the purple-fleshed
varieties. Rapang-rapang and Shiket had higher phenolic
content than their light-colored counterparts. Highland
Tugui had higher phenolic content than Lowland Tugui.
DPPH Radical Scavenging Activity
One mechanism by which antioxidants inhibit oxidation
is by quenching reactive species through hydrogen or
electron donation (Singh & Rajini 2004). The DPPH
assay measures this capacity by monitoring the decrease
in absorbance of DPPH radical as it reacts with the
antioxidant, marked by the color change from purple to
yellow (Elmastas et al. 2006). DPPH radical scavenging
activity is plotted as a function of mg sample in Figure
1. DPPH radical scavenging activity was observed to
increase with sample concentration between 0.50 and
25 mg sample (equivalent to about 2-20 μg GAE).
Hsu et al. (2003) also observed the same trend for the
radical scavenging activity of freeze-dried, air-dried
and drum-dried D. alata and D. purpurea samples and
Bhandari & Kawabata (2004) for wild yam species.
However, for their studies, a plateau was observed at
concentrations of 150-200 mg mL-1 and 8-10 mg mL-1,
respectively. The plateau for the previous studies may
indicate that the DPPH radical scavenging activity of
their samples reaches saturation at higher concentrations.
This is not evident in the present study either due to
the fact that concentrations used are lower or that the
samples used in the present study have higher activity.
The range of EC50 values, the concentration at which
radical scavenging activity is 50%, of the yam varieties
analyzed is 1.7 to 14.8 mg dry sample (Table 2). For
purple yam samples, Daking, the white-fleshed variety,
had the highest radical scavenging activity. Sampero
and Kimabajo had the lowest activity among the five
varieties analyzed in the study. It was further observed
that activity is directly proportional to color intensity
for purple-fleshed varieties; hence, Rapang-rapang and
Shiket, the darker varieties, had better radical scavenging
activity than Kimabajo and Sampero. Highland Tugui
had significantly (p<0.05) higher scavenging activity
than Lowland Tugui. The radical scavenging activity of
the yam samples, on a μg analyte basis (EC50 = 6.6 to
10.7 µg GAE mL-1), is higher than that of α-tocopherol
Sample Concentration, mg/mL, dry basis
Figure 1. DPPH radical scavenging activity of methanolic yam extracts as a function of mg sample. Values are means of
six replicates.
148
Table 2. Antioxidant Activity of Yam Samples.
Variety
Total
Antioxidant
Activity
at 50 mg
sample
mL-1
methanol
EC50 value (mg mL-1 dry basis)*
DPPH
Reducing
Scavenging
Power
Activity
Daking
Kimabajo
Ironchelating
Capacity
95.6 ± 5.8a
1.7 ± 0.2e
6.2 ± 0.6g 27.0 ± 2.3b
a
b
31.7 ± 3.4a 21.9 ± 1.2c
92.4 ± 4.6
13.5 ± 1.3
Rapang-rapang 94.8 ± 5.2a
4.1 ± 0.2d 14.6 ± 1.7e 34.0 ± 3.6a
94.9 ± 2.6a
14.8 ± 1.5a 26.3 ± 3.2b 23.0 ± 2.7c
Sampero
Shiket
92.7 ± 5.8
3.3 ± 0.4d
Highland
93.5 ± 3.2a
4.2 ± 0.1d 17.1 ± 1.5d 17.5 ± 1.4d
Lowland
92.0 ± 1.8a
10.2 ± 0.7c 23.2 ± 0.7c 35.1 ± 2.3a
a-e
*
a
9.5 ± 0.8f 26.9 ± 0.6b
Means with different letters within the same column differed significantly
(p<0.05)
Each value is expressed as the mean ± standard deviation (n=6)
Absorvance
(EC50 = 23 µg mL-1). Radical scavenging activity of
the purple yam and tugui samples was found to be
significantly correlated (p<0.05, R= -0.7664) to their
phenolic content.
Reducing Power
Another mechanism of antioxidant action is through
electron donation. In the potassium ferricyanide reduction
method, antioxidants reduce the ferric ion/ferricyanide
complex to the ferrous form and activity is monitored by
measuring the absorbance of Perl’s Prussian blue complex
at 700 nm (Chou et al. 2003). Figure 2 shows the plot
of reducing power of the methanolic yam extracts as a
function of mg sample. A linear increase in reducing power
was observed over the concentration range 2.5-50 mg
sample, equivalent to 5-50 μg GAE. The results of Hsu et
al. (2003) and Bhandari & Kawabata (2004) also showed
a similar trend. EC50 values for the reducing power of the
yam samples, the concentration at which the absorbance
is 0.500, are between 6.2 and 31.7 mg mL-1 dry sample
(Table 2). The reducing power of the purple yam varieties
Daking, Kimabajo, Rapang-rapang, Sampero and Shiket
were significantly different (p<0.05). Daking had the
highest reducing power, while Kimabajo had the lowest.
The dark-colored purple-fleshed varieties (Rapang-rapang
and Shiket) also had better reducing power than the lightcolored ones (Kimabajo and Sampero). Highland Tugui
had significantly higher reducing power than Lowland
Tugui (p<0.05). The samples had better reducing power
than α-tocopherol (EC50= 94 µg mL-1), on a μg analyte
basis (EC50= 18.4 to 27.2 µg GAE mL-1). As mentioned
earlier, α-tocopherol acts through hydrogen donation and
Figure 2. Reducing power of methanolic yam extracts as a function of mg sample. Values are means of six replicates.
149
not electron donation. Significant correlation (p<0.05, R=
-0.8083) was found between reducing power and phenolic
content indicating that this antioxidant action is primarily
contributed by the phenolic substances in the sample.
Total Antioxidant Activity
The ferric thiocyanate method also measures the hydrogendonating ability of antioxidants, which prevents the
reaction of peroxides with polyunsaturated fatty acids (Lee
%Activity
Iron-chelating Capacity
Antioxidants also exhibit activity by forming insoluble
complexes with metals that catalyze lipid oxidation (Hsu
et al. 2003). The iron chelating capacity test measures this
activity as the decrease in the absorbance of the red Fe2+/
ferrozine complex as antioxidants compete with ferrozine
in chelating ferrous ion (Elmastas et al. 2006). The plot
of iron-chelating capacity as a function of mg sample is
shown in Figure 3. A sigmoidal curve was obtained over
the concentration range 10-50 mg sample, equivalent
to 7-200 μg GAE. The same trend was observed in the
study by Hsu et al. (2003) on D. alata and D. purpurea
samples and Bhandari & Kawabata (2004) on wild yam
species. Observations from these studies indicate that iron
chelating capacity tends to level off at high concentrations.
EC50 values, the concentration at which chelation is 50%,
for the yam samples ranged from 17.5 to 35.1 mg mL-1
dry sample and are listed in Table 2. Highland Tugui had
about twice as much chelating capacity than Lowland
Tugui. Among the purple yam varieties, Kimabajo had the
highest iron-chelating capacity while Rapang-rapang had
the least. No significant correlation (p>0.05, R= -0.2214)
was found between phenolic content and chelating
capacity when all yam samples were considered. However,
significant correlation (p<0.05, R= -0.9859) was observed
between total phenolic content and iron-chelating capacity
for the tugui varieties, again indicating contribution
of the sample phenolics in the inactivation of metallic
pro-oxidants. Non-phenolic metal chelators include
phosphoric acid, citric acid, ascorbic acid, carnosine, some
amino acids, peptides and proteins such as transferrin and
ovotransferrin (Lee et al. 2004). Another possibility is that
while there is significant amount of phenolic compounds
in the sample, the components present are inefficient metal
chelators. Among the yam samples analyzed in the study,
only Kimabajo (15.3 µg GAE mL-1), Sampero (16.2 µg
GAE mL-1) and Highland Tugui (27.3 µg GAE mL-1)
had better chelating capacity than EDTA (30.0.3 µg GAE
mL-1), on a µg analyte basis. Phenolic components must
have adjacent hydroxyl groups at the 3’ and 4’ position
in the catechol structure to exhibit metal chelation (Vaya
& Aviram 2001).
Figure 3. Iron chelating capacity of methanolic yam extracts as a function of mg sample. Values are means of six replicates.
150
et al. 2004, Huang et al. 2005b). In the assay, peroxides
are formed upon oxidation of linoleic acid and oxidized
iron to the +3 state. Activity is indirectly measured by
monitoring the relative increase in absorbance of ferric
thiocyanate complex every 24 hours until linoleic acid
is completely oxidized (Elmastas et al. 2006). Inhibition
by the methanolic yam extracts at 50 mg dry sample
is between 92.0-95.6 % (Table 2). Daking had the best
inhibitory action among the purple yam varieties while
Highland Tugui had better activity than Lowland Tugui.
However, the difference in the antioxidant activity among
purple yam varieties and between the two lesser yam
varieties were not statistically significant (p>0.05). Kaur
& Kapoor (2002) used a modified procedure involving
coupled oxidation of β-carotene and linoleic acid to
measure antioxidant activity. A 40 mg sample of D. alata
had an activity of 71.0% for the ethanolic extract and
62.8% for the aqueous extract (Kaur & Kapoor 2002).
Methanolic yam extracts had better inhibitory action
than α-tocopherol, which has an activity of 85.0 % at
101 μg. No significant correlation (p>0.05, R=0.1378)
was found between total phenolic content and total
antioxidant activity of the yam extracts suggesting that
other non-phenolic components contribute to antioxidant
activity. Also, the concentration of the sample used in the
current study may have been too high to cause a leveling
effect, i.e., the concentrations used could no longer be
discriminated by the protocol.
CONCLUSIONS
Local ube and tugui varieties contain considerable
phenolic content and significant activity against
free radicals relative to the commercial antioxidant
α-tocopherol. Significant correlation was observed among
phenolic content, DPPH radical scavenging activity and
reducing power. Only three varieties, Kimabajo, Sampero
and Highland Tugui, exhibited better chelating capacity
than EDTA. Significant correlation between iron chelating
capacity and phenolic content was observed only for tugui
samples. Moreover, there was no correlation between
inhibition of linoleic acid oxidation and phenolic content.
Identification of individual phenolic constituents as well
as other non-phenolic antioxidants will elucidate the lack
of correlation among metal chelation, oxidation inhibition
and phenolic content. The substantial amount of phenolic
compounds, as well as the significant antiradical activity,
makes utilization of yam as a source of food antioxidant
and nutraceutical commercially feasible. However, other
solvents must be investigated for commercial applications
as methanol is toxic.
ACKNOWLEDGEMENTS
The authors wish to acknowledge the Northern
Philippines Root Crop Research and Training Center for
providing the rootcrop samples and the University of the
Philippines−Office of the Vice Chancellor for Research
and Development for financial support.
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Philippine Yam (Dioscorea spp.) Tubers Phenolic Content and

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