Antioxidant Activity of Grains
K AFUI K WAMI A DOM †
AND
R UI H AI L IU *,†,‡
Institute of Comparative and Environmental Toxicology and Department of Food Science,Cornell
University,Stocking Hall,Ithaca,New York 14853-7201
Epidemiological studies have shown that consumption of whole grains and grain-based products is associated with reduced risk of chronic diseases.The health benefits of whole grains are attributed in part to their unique phytochemical composition.However,the phytochemical contents in grains have been commonly underestimated in the literature,because bound phytochemicals were not included.This study was designed to investigate the complete phytochemical profiles in free,soluble conjugated,and insoluble bound forms,as well as their antioxidant activities in uncooked whole grains.Corn had the highest total phenolic content (15.55(0.60µmol of gallic acid equiv/g of grain)of the grains tested,followed by wheat (7.99(0.39µmol of gallic acid equiv/g of grain),oats (6.53(0.19µmol of g
allic acid equiv/g of grain),and rice (5.56(0.17µmol of gallic acid equiv/g of grain).The major portion of phenolics in grains existed in the bound form (85%in corn,75%in oats and wheat,and 62%in rice),although free phenolics were frequently reported in the literature.Ferulic acid was the major phenolic compound in grains tested,with free,soluble-conjugated,and bound ferulic acids present in the ratio 0.1:1:100.Corn had the highest total antioxidant activity (181.42(0.86µmol of vitamin C equiv/g of grain),followed by wheat (76.70(1.38µmol of vitamin C equiv/g of grain),oats (74.67(1.49µmol of vitamin C equiv/g of grain),and rice (55.77(1.62µmol of vitamin C equiv/g of grain).Bound phytochemicals were the major contributors to the total antioxidant activity:90%in wheat,87%in corn,71%in rice,and 58%in oats.Bound phytochemicals could survive stomach and intestinal digestion to reach the colon.This may partly explain the mechanism of grain consumption in the prevention of colon cancer,other digestive cancers,breast cancer,and prostate cancer,which is supported by epidemiological studies.
KEYWORDS:Phytochemicals;phenolics;grains;ferulic acid;antioxidant activity
INTRODUCTION
Epidemiological studies have strongly suggested that diets play a crucial role in the prevention of chro
nic diseases such as heart disease,cancer,diabetes,and Alzheimers’s disease (1,2).Consumption of fruits and vegetables,as well as grains,has been associated with reduced risk of chronic diseases (1-3).This has been hypothesized to be because they contain phy-tochemicals that combat oxidative stress in the body by helping to maintain a balance between oxidants and antioxidants.An imbalance caused by overproduction of oxidants leads to oxidative stress,resulting in damage to large biomolecules such as lipids,DNA,and proteins.Oxidative damage increases the risk of degenerative diseases such as cancer and cardiovascular diseases (1,4,5).Antioxidants reduce oxidative damage to biomolecules by modulating the effects of reactive oxidants (6,7).Therefore,increased consumption of fruits and vegetables containing high levels of antioxidants has been recommended.The importance and health benefits of grain consumption in
the prevention of chronic diseases such as cancers and heart disease have also been documented (8-16).However,the attention paid to grain consumption has been little compared to that for fruits and vegetables,although nutritional guidelines put grains and grain products at the base of the food guide pyramid to emphasize their importance for optimal health (17).Recent research has shown that the complex mixture of phytochemicals in foods provides better protective health benefits than single phytochemicals through a combination of additive and/or synergistic effects (18).This has been support
ed by results from previous studies on health benefits of single antioxidants that gave inconsistent results in human clinical trials (19,20).About 5000of phytochemicals present in plants have been identified,and still a large percentage remains unknown (21).Different plants have different compositions of phy-tochemicals with different structures and thus offer different protective functions with different extents.Hence,for the maximum health benefits,sufficient amounts of phytochemicals from a variety of sources such as fruits,vegetables,and whole grain-based foods are recommended.
Grains contain unique phytochemicals that complement those in fruits and vegetables when consumed together.For instance,
*To whom correspondence should be addressed.Telephone:(607)255-6235.Fax:(607)254-4868.E-mail:RL23@cornell.edu.†Department of Food Science.
‡Institute of Comparative and Environmental Toxicology.
6182J.Agric.Food Chem.2002,50,6182−6187
10.1021/jf0205099CCC:$22.00©2002American Chemical Society
Published on Web 08/31/2002
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various classes of phenolic compounds in grains include derivatives of benzoic and cinnamic acids,anthocyanidins, quinones,flavonols,chalcones,flavones,flavanones,and amino phenolic compounds(9,21,22,23).Grains contain tocotrienols and tocopherol(9),and rice contains oryzanols(23).Some of these phytochemicals such as ferulic acid and diferulates are predominantly found in grains but are not present in significant quantities in some fruits and vegetables(21,24).Phenolic compounds present in grains have antioxidant properties as-sociated with the health benefits of grains and grain products. Flavonoids have potent antioxidant and anticancer activities. Fruits and vegetables have most of their phytochemicals in free or soluble conjugate forms as glycosides(25,26).On the other hand,grain phytochemicals may exist in free,soluble conjugate, and insoluble bound forms.Most are in the insoluble bound forms,bound to cell wall materials(23,24,27).About74% and69%of the total phenolics present in rice and corn, respectively,are in the insoluble bound forms,with ferulic acid being the major phenolic compound present.Cell wall materials are difficult to digest and may survive gastrointestinal digestion to reach the colon.Colonic digestion of such materials would release the bulk of bound phytochemicals.Andreasen et al.(28) showed that human and rat colonic microflora can release diferulic acids from dietary cereal brans.Thu
s,nondigested or bound phytochemicals may have their unique health benefits in the colon and beyond after absorption.
Most previous studies in the literature reported the phenolic levels of grains using various aqueous solutions of methanol, ethanol,and acetone to extract soluble phenolics(29-33).These studies assumed long extraction times and/or use of finely powdered samples would ensure maximum extraction of phenolic compounds from grains.Therefore,the total phenolic contents of grains were underestimated in the literature without determining the content of bound phenolics.Maillard and Berset (22)reported total antioxidant activity of free and bound extracts of barley and malt,as well as bound ferulic acid and p-coumaric acid.However,they did not report free and soluble conjugate ferulic acids or free and bound total phenols and total flavonoids. Sosulski et al.(27)reported free,soluble conjugate,and bound phenolic acid contents of rice,oats,wheat,and corn flours,but antioxidant activity,total phenols,and total flavonoids were not investigated.
To our knowledge,there is still limited literature on the complete profile(free,soluble conjugate,insoluble bound)of phenolic compounds and total antioxidant activity of grains. Additionally,the total phenolic contents of grains have been underestimated in the literature.The objective of this study was to investigate the complete phytochemical profiles that exist in the free,soluble conjugate,and insoluble b
ound forms,as well as their antioxidant activity in corn,wheat,oats,and rice. MATERIALS AND METHODS
Chemicals and Reagents.Folin-Ciocalteu reagent,sodium nitrite, catechin,and gallic acid were purchased from Sigma(St.Louis,MO). Sodium hydroxide,hexane,aluminum chloride,and acetonitrile were obtained from Fisher Scientific(Pittsburgh,PA),while ethyl acetate, triflouroacetic acid,and ethanol were purchased from Mallinckrodt (Paris,KN).
Grain Samples and Sample Preparation.Samples of oats,corn, wheat,and rice were obtained from General Mills(Golden Valley,MN). Whole oats and whole brown rice were received as flours.Whole grain wheat and Sunlite whole yellow corn were received as dehulled kernels. The wheat and corn samples were milled in a coffee grinder to a fine powder.All samples were individually mixed thoroughly and divided using the quartering system.Each sample was divided into two portions and stored at-20and-80°C.The-20°C samples were used for routine analysis within2weeks.
Extraction of Free Phenolic Compounds.Free phenolic compounds in grains were extracted by blending25g of whole grain flour with50 mL of80%chilled ethanol for10min.After centrifugation at2500g for10min,the supernatant was removed and extraction was repeated one more time.Supernatants wer
e pooled,evaporated at45°C to10 mL,and reconstituted with water to a final volume of25mL.The extracts were stored at-40o C until use(18).
Extraction of Bound Phenolic Compounds.One gram of whole grain flour was extracted twice with80%chilled ethanol with centrifugation at2500g for10min,and the supernatant was discarded after each extraction.The residues were then digested with2M sodium hydroxide at room temperature for1h with shaking under nitrogen gas.The mixture was neutralized with an appropriate amount of hydrochloric acid and extracted with hexane to remove lipids.The final solution was extracted five times with ethyl acetate.The ethyl acetate fraction was evaporated to dryness.Phenolic compounds were recon-stituted in10mL of water and stored at-40°C until use(27).
Extraction of Soluble Conjugated Ferulic Acid.Extracts from the free phenolic extractions above were used for soluble conjugate extractions.The extract(0.5mL)was digested with2M NaOH for1 h under nitrogen gas,and the solution was neutralized with an appropriate amount of HCl.The mixture was extracted five times with ethyl acetate,and the ethyl acetate fraction was evaporated to dryness at35°C under nitrogen gas.Phenolics were recovered for analysis in 2mL of water(22,27).
Determination of Ferulic Acid Content.Ferulic acid in sample extracts was quantified using a RP-HPLC
procedure employing a Supelcosil LC-18-DB,150mm×4.6mm,3mm column.Isocratic elution was conducted with20%acetonitrile in water adjusted to pH2 with triflouroacetic acid,at a flow rate of0.6mL/min.This was delivered using a Waters515HPLC pump(Waters Corp.,Milford, MA).A Waters2487dual wavelength absorbance detector(Waters Corp.)was used for UV detection of analytes at280nm.Data signals were acquired and processed on a PC running the Waters Millennium software,version3.2(1999)(Waters Corp.).The ferulic acid concentra-tion of sample extracts was extrapolated from the pure trans-ferulic acid standard curve.Ten microliter injections were made in each run, and peak heights were used for all calculations.The recoveries for free ferulic acid and bound ferulic acid analyses were105.13(5.23%(n )3)and89.30(1.01%(n)3),respectively.
Determination of Total Phenolic Content.The total phenolic content of each extract was determined using methods previously described by Singleton et al.(34).Briefly,the appropriate dilutions of extracts were oxidized with Folin-Ciocalteu reagent,and the reaction was neutralized with sodium carbonate.The absorbance of the resulting blue color was measured at760nm after90min.Using gallic acid as standard,total phenolic content was expressed as micromoles of gallic acid equivalent per gram of grain.Data are reported as mean(SD for at least three replications.
Determination of Total Flavonoid Content.Total flavonoid content was determined by a colorimetric met
hod described previously(35). Appropriate dilutions of sample extracts were reacted with sodium nitrite,followed by a flavonoid-aluminum complex formation using aluminum chloride.Solution absorbance at510nm was immediately measured and compared to that of catechin standards.Flavonoid content was expressed as micromoles of catechin equivalent per gram of grain. Data are reported as mean(SD for at least three replications.
Determination of Total Antioxidant Activity.A modified total oxyradical scavenging capacity(TOSC)assay(18,36)was used for determining the total antioxidant capacity of extracts.In this assay, peroxy radicals formed from2,2′-azobis-amidinopropane(ABAP) oxidize R-keto-γ-methiolbutyric acid(KMBA)to form ethylene gas, which was measured by gas chromatographic headspace analysis.The degree of inhibition of ethylene gas formation by sample extracts was used as the basis for calculating the total antioxidant capacity.The dose required to cause a50%inhibition(EC50)for each sample was used to calculate the total antioxidant activity,which was expressed as micromoles of vitamin C equivalent per gram of grain.
Antioxidant Activity of Grains J.Agric.Food Chem.,Vol.50,No.21,20026183
Statistical Analysis.Data were reported as mean (SD for at least three analyses for each type of extract
ion and parameter.Results were subjected to ANOVA,and differences between means were located using Tukey’s multiple comparison test run on Minitab Release 12software (State College,PA).Correlations between various parameters were also investigated.RESULTS
Phenolic Contents of Grains.The phenolic contents of the four grains were expressed as micromoles of gallic acid equivalent per gram of grain (Figure 1).Corn had the highest free phenolic content (2.12(0.09µmol/g of grain),followed by rice (2.10(0.12µmol/g of grain)and then wheat (1.90(0.06µmol/g of grain).Oats had the lowest free phenolic content (1.77(0.12µmol/g of grain).In statistical testing,p <0.05for corn versus oats,and p >0.05for all other comparisons.The bound phenolic content was highest for corn (13.43(0.59µmol/g of grain),followed by wheat (6.10(0.39µmol/g of grain)and then oats (4.76(0.14µmol/g of grain).Rice had the lowest bound phenolic content (3.46(0.13µmol/g of grain).There were significant differences (p <0.05)in the bound phenolic contents of all grains.Also bound phenolics were significantly higher than free phenolics.The total phenolic content was highest in corn (15.55(0.60µmol/g of grain),followed by wheat (7.99(0.39µmol/g of grain)and then oats (6.53(0.19µmol/g of grain).Rice (5.56(0.17µmol/g of grain)had the lowest total phenolic content.There were significant differences (p <0.05)in total phenolic contents among the grains.
Ferulic Acid Contents of Grains.Ferulic acid contents of grains were expressed as micromoles of feruli
c acid per 100g of grain (Table 1).Free ferulic acid was highest in corn (0.92(0.02µmol/100g of grain),followed by rice (0.7(0.05µmol/100g of grain)and oats (0.65(0.04µmol/100g of grain),and was lowest in wheat (0.57(0.02µmol/100g of grain).Statistical analyses show p <0.05for corn versus other grains and for rice versus wheat,and they show p >0.05for all other comparisons.Soluble conjugated ferulic acid contents were similar (p >0.05)in corn (8.95(0.11µmol/100g of grain)and rice (9.9(0.34µmol/100g of grain),and both grains had
higher (p <0.01)conjugated ferulic acid contents than wheat (3.27(0.27µmol/100g of grain)and oats (3.4(0.56µmol/100g of grain).There were no differences in soluble conjugated ferulic acid levels between wheat and oats (p >0.05).The bound ferulic acid content was highest in corn (896.27(9.09µmol/100g of grain),followed by wheat (329.60(16.20µmol/100g of grain)and then oats (180.61(4.57µmol/100g of grain).Rice had the lowest bound ferulic acid content (142.80(8.68µmol/100g of grain).The bound ferulic acid contents in grains were all significantly different (p <0.05)from each other (Table 1).The bound ferulic acid contents were signifi-cantly higher (p <0.01)than both free and soluble conjugate ferulic acid contents in all grains tested.The total ferulic acid content was highest in corn (906.13(9.09µmol/100g of grain)followed by wheat (333.44(16.20µmol/100g of grain)and then oats (184.66(4.61µmol/100g of grain),with rice having the lowest total ferulic acid content (153.39(8.68µmol/100g of grain).
Flavonoid Contents of Grains.Flavonoid contents of grains were expressed as micromoles of catechin equivalent per gram of grain (Figure 2).The free flavonoid content was highest in oats (0.45(0.02µmol/g of grain),followed by rice (0.33(0.01µmol/g of grain)and then corn (0.16(0.004µmol/g of grain).Wheat (0.09(0.01µmol/g of grain)had the lowest flavonoid content.There were significant differences (p <0.05)in free flavonoid contents among the grains.For bound flavonoid content,the level in corn (1.52(0.17µmol/g of grain)was higher (p <0.01)than that in wheat (1.15(0.03µmol/g of grain).Both corn and wheat had higher flavonoid contents (p <0.01)than rice (0.60(0.04µmol/g of grain)and oats (0.71(0.05µmol/g of grain).There was no difference in bound flavonoid contents between rice and oats (p >0.05).The total flavonoid content of corn (1.68(0.17µmol/g)was higher (p <0.01)than those of other grains.The total flavonoid contents of wheat (1.24(0.03µmol/g of grain)and oats (1.16(0.06µmol/g of grain)were similar (p >0.05),and both had levels higher (p <0.05)than that of rice (0.92(0.04µmol/g of grain).Total Antioxidant Activities of Grains.The total antioxidant activities of grains were expressed as micromoles of vitamin C
Table 1.Ferulic Acid Contents of Grains and the Percentage Contribution of Each Fraction to the Total (µmol of ferulic acid/100g of grain)
free
soluble conjugate bound
total corn 0.92±0.02(0.1%)a 8.95±0.11(1%)896.27±9.09(98.9%)906.13±9.09wheat 0.57±0.02(0.2%)  3.27±0.27(1%)329.60±16.20(98.8%)333.44±16.20oats 0.65±0.04(0.4%)  3.4±0.56(1.84%)180.61±4.57(97.8%)184.66±4.61rice
0.7±0.05(0.5%)
9.9±0.34(6.5%)
142.80±8.68(93%)
153.39±8.68
a
Mean ±SD (%contribution to
total).
Figure 1.Phenolic content of
grains.
Figure 2.Flavonoid content of grains.
6184J.Agric.Food Chem.,Vol.50,No.21,2002Adom and Liu
equivalent per gram of grain (Figure 3).Free phytochemical extracts of oats had the highest antioxidant
activity (31.07(1.37µmol/g of grain),followed by corn (24.00(0.43µmol/g of grain)and then rice (16.02(0.37µmol/g of grain).Free phytochemical extracts in wheat had the lowest antioxidant activity (8.00(0.30µmol/g of grain).The antioxidant activities of free phytochemical extracts were different (p <0.05)among the grains.The antioxidant activity of bound phytochemicals in corn (157.68(0.75µmol/g of grain)was higher (p <0.01)than those in other grains.There was a significant difference (p <0.05)among the antioxidant activities of bound phy-tochemicals in wheat (68.74(1.35µmol/g of grain),oats (43.60(0.59µmol/g of grain),and rice (39.76(1.58µmol/g of grain).The total antioxidant activity of corn (free +bound)was 181.42(0.86µmol/g of grain and was the highest (p <0.01)of those of the grains tested.The total antioxidant activities of wheat (76.70(1.38µmol/g of grain)and oats (74.67(1.49µmol/g of grain)were similar (p >0.05)but higher (p <0.05)than the total antioxidant activity in rice (55.77(1.62µmol/g of grain).
There was a low correlation between parameters measured for free extracts (Table 3).For bound extractions,however,the total antioxidant activity highly correlated with phenolic content (R 2)0.991,p <0.01)and ferulic acid content (R 2)0.999,p <0.01)(Table 3).There was a high correlation between phenolics and ferulic acid content for bound extracts (R 2)0.994,p <0.01).Total values for phenolics,ferulic acid,and flavonoids were highly correlated to the total antioxidant activity.
Also total ferulic acid and flavonoids correlated with total phenolic content.
DISCUSSION
Oxidative damage to biomolecules in the body has been associated with certain disease conditions.Grains contain a wide array of phytochemicals that exert health benefits in humans through various mechanisms including antioxidant properties and mediation of hormones.Several studies involving whole grain intakes have shown a consistent protective role of whole grain consumption in reduced risk of colorectal cancer,breast cancer,coronary heart disease,and diabetes,and reduced total mortality (8,9,12-14,16,37,38).Thompson (9)suggests that lignans and phytoestrogens in grains may reduce the risk of hormone-related diseases such as prostate and breast cancers.Since most grain phenolics such as phenolic acids occur in the outer layers of grains,whole grains (compared to refined grains)are recommended for optimal health.Phytochemical contents of grains have been underestimated in the literature without determining the content of bound phytochemicals (29-32).This study was designed to determine the complete profile of phenolic phytochemicals in grains,and their relationship and contribution to the total antioxidant activities.
The total phenolic contents were analyzed using the method by Singleton et al.(34),without distinguishi
ng specific struc-tures.The free phenolic content represents contributions from free and soluble conjugated phenolics.Conjugated phenolics may still be oxidized and contribute toward total phenolic content and antioxidant activity.Our results clearly showed that most grain phenolics were in the bound fraction (Figure 1and Table 2).The percentage contributions of free and bound phenolics to the total are shown in Table 2.The free phenolic contribution ranged from 15%in corn to 38%in rice.The bound phenolic contribution ranged from 62%in rice to 85%in corn.Therefore,the total phenolic contents of grains were clearly underestimated in the literature without including the bound phenolics.The total phenolic content among grains followed the same concentration trend as the bound phenolics because of the larger contribution from bound phenolics (Figure 1).Corn had the highest total phenolic content while rice had the lowest content.Bound phenolics in grains are associated with cell wall materials that may survive upper gastrointestinal digestion conditions and may finally reach the colon.Colonic digestion of such materials by intestinal microflora may release the bulk of bound phytochemicals.On the basis of our results,most of the grain phenolic compounds may be released in the colon to exert their health benefits locally and beyond after absorption.Andreasen et al.(28)reported that both human and rat gastrointestinal esterase (from intestinal mucosa and microflora)can release ferulic acid and diferulic acids from cereal bran.These compounds have potent antioxidant properties,and their absorption into the blood plasma has been reported (28).Therefore,our results suggest that bound phyt
ochemicals in whole grains may have a more profound effect on health benefits.We believe this could partly explain the inverse association between increased whole grain and whole
grain-
Figure 3.Total antioxidant activity of grains.
Table 2.Percentage Contributions of Free and Bound Fractions of
Grains to Total Phenolics,Flavonoids,and Total Antioxidant Activity
phenolic content (%)flavonoid content (%)total antioxidant activity (%)free
bound free bound free bound corn 158********wheat 25757931090oats 257539614258rice
38
62
35
65
29
71
Table 3.Correlation Analysis of Phenolics and Total Antioxidant Activity
free extracts
bound extracts
totalreactive materials studies
total antioxidant activity
phenolics total antioxidant activity
phenolics total antioxidant activity
phenolics phenolics 0.076a 0.991**b 0.983**ferulic acid 0.0003c 0.695c 0.999**0.994**0.974*0.998**flavonoids
0.517
0.324
0.872
0.865
0.925*
0.933*
a
Correlation coefficient R 2.b Significantly different:*,p <0.05;**,p <0.01;all others ,p >0.05.c Total of free and soluble conjugate ferulic acid.
Antioxidant Activity of Grains J.Agric.Food Chem.,Vol.50,No.21,20026185
based products consumption and reduced incidence of colon cancer,breast cancer,prostate cancer,heart disease,and diabetes, and reduced total mortality(8,9,11-16,37-40).
The reverse phase HPLC method used for ferulic acid analysis achieved complete resolution of the ferulic acid peak(data not shown),especially for bound extracts,allowing for accurate quantification.High recoveries for both free(105.13(5.23%) and bound(89.28(1.01%)ferulic acid analysis were obtained. The results showed that bound ferulic acid was significantly higher than free and soluble conjugate ferulic acid in corn, wheat,oats,and rice(Table1).The ratio of free,soluble conjugated,and bound ferulic acid in corn and wheat was0.1: 1:100.Free and soluble conjugated ferulic acids made very small contributions(<0.6%and<7.0%,respectively),while bound ferulic acid was
the prevalent form of ferulic acid present in the grains(>93%).Thus,total ferulic acid followed the same concentration trend as bound ferulic acid(Table1).The occurrence of bound ferulic acid in such relatively high concentrations also strongly supports our hypothesis that phy-tochemicals in grains have been underestimated in the literature by excluding the bound fractions.Maillard and Berset(22) estimated that free phenolic acids in methanol extracts of barley were100-fold lower than bound phenolic acids,which was consistent with our findings.Although our extraction and analysis procedures were similar to those used by Maillard and Berset,our percentage recoveries for ferulic acid were much higher.Preliminary work in our laboratory showed that2N NaOH digestion for1-4h produced similar percentage recoveries with or without rice samples.On the basis of these findings,we used a1h alkaline digestion combined with subsequent five times ethyl acetate extractions for extracting bound phenolics.Maillard and Berset used a4h alkaline digestion followed by three times ethyl acetate extraction,giving a63%ferulic acid recovery.It is possible that phenolic acids might be partially destroyed during prolonged alkaline hydroly-sis.Our preliminary work showed that about10%loss of bound ferulic acid occurred during alkaline hydrolysis for1-4h (unpublished results).This may account for differences in the results obtained,as well as possible varietal differences between wheat samples used.The ferulic acid value reported in this work for wheat(648µg ferulic acid/g of grain)was slightly higher than that obtained(590µg ferulic acid/g of grain)by Yang et al.(41),who used the method of Maill
ard and Berset.Ferulic acid and its conjugates have antioxidant properties(15),and they are present in high concentrations in grains.
The free flavonoid contents in grains were low compared to the bound flavonoid content,and they were all significantly different(p<0.05)from each other(Figure2).Total flavonoid contents followed a similar pattern as bound flavonoid contents in all grains,because of the large contribution from bound flavonoids.The percentage contributions of free and bound flavonoids to the total are shown in Table2.The free flavonoid contribution to the total ranged from7%in wheat to39%in oats.The bound flavonoid contribution ranged from61%in oats to93%in wheat(Table2).The total flavonoid contents of wheat and oats were similar,although their free and bound flavonoid contents were different.This could be attributed to the relatively higher free flavonoids in oats.Thus,whole wheat may deliver more flavonoids to the colon compared to whole oats.Fla-vonoids have potent antioxidant and anticancer activity.
The total antioxidant activities of grains were different between grains and among different fractions(free and bound) of the same grain.Free extracts of corn,wheat,oats,and rice had significantly lower antioxidant activity(p<0.01;Figure 3)compared to those of the bound extracts,as was also observed by Maillard and Berset(22).This is attributable to higher phenolic content in bound extracts c
ompared to free extracts (Table2).Although the free and bound antioxidant activities of wheat and oats(Figure3)were significantly different(p< 0.05),their total antioxidant activities were similar(p>0.05). This could be attributed to the relatively higher antioxidant activity of free oat extracts or bound wheat extracts.The major contribution to total antioxidant activity of whole grains was from bound extracts and ranged from58%to90%.The TOSC assay measures the overall antioxidant activity of extracts including both additive and/or synergistic effects of phytochemi-cals.This gives a more accurate representation of antioxidant capacity of extracts.Free antioxidant activities represent con-tributions from both free and soluble conjugate fractions of grains.Some derivatives of phenolic compounds(conjugates) are known to have antioxidant properties.Avenanthramides are cinnamoyl conjugates that occur in oats with higher antioxidant activity(33,42).Long chain mono-and di-alcohol esters of ferulic and caffeic acids had potent antioxidant activity(43). Garcia-Conesa et al.(15)measured the antioxidant activity of 8,8′-diferulic acid and compared that to the antioxidant activities of ferulic acid and other diferulates.The Trolox equivalent antioxidant capacity(TEAC)for ferulic acid was1.96.The TEAC values for various diferulic acid conjugates ranged from 1.49to4.00.Generally,diferulic conjugates were reported to be more potent antioxidants than ferulic acid in both aqueous and lipid phases.Among the diferulates tested,8,8′-diferulic acid was the most potent antioxidant in the aqueous phase. The total phenolic content has been directly related to the tot
al antioxidant activity and may explain the high correlation observed between these parameters(Table3).Flavonoids and ferulic acid contribute to total phenolics in corn,wheat,oats, and rice.The high correlation between phenolics and ferulic acid content for bound extracts reflects the major contribution of ferulic acid to total phenolic content.The free ferulic acid contribution to free phenolics was<0.5%in all grains,implying the contributions of other compounds to total phenolics in the free extracts were more important.On the other hand,the contribution of bound ferulic acid content to bound phenolics was76%in corn,61%in wheat,43%in oats,and47%in rice. Velioglu et al.(29)reported a significant relation(R2)0.905) between total phenolics and antioxidant activity of grain products.
Our results have shown that phytochemical contents of grains have been underestimated in the literature without including the bound phytochemicals.Among the grains we tested,corn had the highest content of phenolic compounds followed by wheat and oats,while rice had the lowest phenolic content.Ferulic acid was the major phenolic compound in grains and was mainly present in the bound form.Free,soluble conjugated,and bound ferulic acids were present in the ratio0.1:1:100.Corn had the highest total antioxidant activity followed by wheat and oats, and then rice.Our results also show the major portions of phytochemicals in the grains are present in the bound form and may survive stomach and intestinal digestion to reach the colon. This may partially explain the mechanism of grain consumpt
ion in the prevention of colon cancer,other digestive cancers,breast cancer,and prostate cancer.Further studies of the unique grain phytochemicals and their mechanism of action are warranted. LITERATURE CITED
(1)Temple,N.J.Antioxidants and disease:More questions than
answers.Nutr.Res.2000,20,449-459.
6186J.Agric.Food Chem.,Vol.50,No.21,2002Adom and Liu

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