philippine yam tubers phenolic content

Philippine Journal of Science140 (2): 145-152, December 2011ISSN 0031 - 7683Date Received: 04 Jun 2010

Key Words: antioxidant, phenolic content, radical scavenging activity, reducing power, yam

*Corresponding author: [email protected] [email protected]

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Djanna F. Cornago1*, Rowena Grace O. Rumbaoa2, and Inacrist M. Geronimo1

Philippine Yam (Dioscorea spp.) Tubers PhenolicContent and Antioxidant Capacity

1Institute of Chemistry, College of Science, University of the Philippines, Diliman, Quezon City

2Department 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, Rapang-rapang, 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).

INTRODUCTIONResearches 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).

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

Cornago et al.: Phil. Yam Tubers Antioxidant CapacityPhilippine Journal of ScienceVol. 140 No. 2, December 2011

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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 METHODSReagentsGallic 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, Sigma-Aldrich, 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, Sigma-Aldrich, 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.

SampleSamples 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

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 CompoundsMethanolic 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 ContentThe 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 (Perkin-Elmer, 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 ActivityHuang 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 PowerThe reducing power assay used in the study was based on the study by Singh & Rajini (2004). Sample


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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 CapacityThe 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 ActivityThe 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 AnalysisData 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 DISCUSSIONTotal Phenolic ContentThe 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 Wet Basis

Daking 69.7±0.7 421.8 ± 17.6a 127.8 ± 5.3a

Kimabajo 76.0±0.5 69.9 ± 1.3f 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±0.2 231.5 ± 9.8b 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)


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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

Figure 1. DPPH radical scavenging activity of methanolic yam extracts as a function of mg sample. Values are means of six replicates.

Sample Concentration, mg/mL, dry basis


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(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 PowerAnother 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 light-colored 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

Table 2. Antioxidant Activity of Yam Samples.

Variety

Total Antioxidant

Activityat 50 mg

sample mL-1

methanol

EC50 value (mg mL-1 dry basis)*

DPPH Scavenging

Activity

Reducing Power

Iron-chelating Capacity

Daking 95.6 ± 5.8a 1.7 ± 0.2e 6.2 ± 0.6g 27.0 ± 2.3b

Kimabajo 92.4 ± 4.6a 13.5 ± 1.3b 31.7 ± 3.4a 21.9 ± 1.2c

Rapang-rapang 94.8 ± 5.2a 4.1 ± 0.2d 14.6 ± 1.7e 34.0 ± 3.6a

Sampero 94.9 ± 2.6a 14.8 ± 1.5a 26.3 ± 3.2b 23.0 ± 2.7c

Shiket 92.7 ± 5.8a 3.3 ± 0.4d 9.5 ± 0.8f 26.9 ± 0.6b

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 Means with different letters within the same column differed significantly (p<0.05)

* Each value is expressed as the mean ± standard deviation (n=6)

Figure 2. Reducing power of methanolic yam extracts as a function of mg sample. Values are means of six replicates.


Abs

orva

nce


150

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.

Iron-chelating CapacityAntioxidants 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).

Total Antioxidant Activity The ferric thiocyanate method also measures the hydrogen-donating ability of antioxidants, which prevents the reaction of peroxides with polyunsaturated fatty acids (Lee

Figure 3. Iron chelating capacity of methanolic yam extracts as a function of mg sample. Values are means of six replicates.


%A

ctiv

ity


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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.

CONCLUSIONSLocal 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.

ACKNOWLEDGEMENTSThe 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 tubers phenolic content

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