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Potential of Wheat-Based Breakfast Cereals as a Source of Dietary Antioxidants

Coca Cola Company, Atlanta, Georgia (A.J.B.)
Dept. of Food Science, University of Massachusetts, Amherst, Massachusetts (C.L., F.M.C., E.A.D.)

Address reprint requests to: Eric A. Decker, Ph.D., Department of Food Science, Chenoweth Lab, University of Massachusetts, Amherst, MA 01003

	ABSTRACT

TOP ABSTRACT OXIDATIVE REACTIONS AND HEALTH DIETARY ANTIOXIDANT IN WHOLE... IMPACT OF DIGESTION ON... CONCLUSION REFERENCES

 
Whole wheat- and wheat bran-based ready-to-eat breakfast cereals could be important sources of dietary antioxidants. Of the antioxidants in wheat, free and esterified phenolic acids seem to have the greatest potential to be beneficial to health. Phenolic acids from breakfast cereals possess strong antioxidant activity in vitro at concentrations that would be obtained from a normal serving of whole wheat cereal. In addition, acid conditions and enzymic hydrolysis increase the solubility and activity of wheat phenolics suggesting that the digestive process could be important in altering the antioxidant potential of wheat-based foods. Current research on the antioxidant activity of wheat phenolics suggests that further research is warranted to determine the potential benefits of these dietary antioxidants. In addition, identification of both biological (e.g. digestion) and food processing conditions that impact the distribution, stability and activity of wheat antioxidants is needed in order to be able to produce food products with maximum health benefits.

Key words: antioxidants, wheat, whole grain, phenolics, lipid oxidation

Key teaching points:

• Wheat contains several compounds that could be important dietary antioxidants.

• Wheat phenolics are effective antioxidants in vivo at the concentrations found in ready-to-eat breakfast cereals.

• Digestive processes increase the activity of wheat antioxidants.

	OXIDATIVE REACTIONS AND HEALTH

TOP ABSTRACT OXIDATIVE REACTIONS AND HEALTH DIETARY ANTIOXIDANT IN WHOLE... IMPACT OF DIGESTION ON... CONCLUSION REFERENCES

 
Humans are subjected to many forms of oxidative stress, including pollutants, radiation, ingestion of oxidized foods, ischemia and in vivo production of free radicals [1]. In addition, oxidative damage to tissues is modulated by many factors including substrate composition (e.g., fatty acid), oxygen, prooxidants (e.g., reactive oxygen and nitrogen species, transition metals, heme-containing proteins and enzymes) and antioxidant concentrations. If unchecked, oxidation can promote a chain of chemical reactions that form free radicals, peroxides and secondary oxidation breakdown products that in turn react with and cause damage to cellular membranes, proteins and nucleic acids. Biological systems control these oxidative factors by a variety of antioxidative mechanisms that restrict the reactivity of reactive oxygen and nitrogen species and oxidation catalysts.

Since oxidation has been associated with chronic diseases such as atherosclerosis [2] and cancer [3], an extraordinary amount of attention has recently been focused on how dietary factors influence the oxidative/antioxidative balance of tissues. While the human body contains several endogenous antioxidant systems, an important niche in maintaining the oxidative/antioxidative balance is filled by dietary antioxidants. Essential nutrients that impact the oxidative stability of tissues include selenium, ß-carotene and vitamins C and E. However, foods also contain numerous nonessential compounds that possess antioxidant activity and therefore may help increase the antioxidant defense system of humans, a circumstance which, in turn, would be beneficial to health.

	DIETARY ANTIOXIDANT IN WHOLE GRAINS

TOP ABSTRACT OXIDATIVE REACTIONS AND HEALTH DIETARY ANTIOXIDANT IN WHOLE... IMPACT OF DIGESTION ON... CONCLUSION REFERENCES

 
In order to minimize the potentially damaging effects of oxidative reactions, biological systems including grains have developed multifunctional antioxidant defense systems. These defense systems control reactive oxygen species, oxidation catalysts and oxidation products in both lipid and aqueous environments by several different mechanisms, including metal chelation, free radical scavenging and peroxide inactivation [4]. Whole grains, including wheat, contain several compounds that are capable of minimizing the damaging effects of oxidation reactions. These include phytate, proteins, polysaccharides, phenolics, lignans and tocopherols.

Since grains contain significant concentrations of antioxidative compounds, it is possible that they could contribute to dietary antioxidant intake if these antioxidants are present in active forms in wheat-based foods. While the health potential of tocopherols has been well studied, much less is known about the health potential of the nonessential antioxidants in wheat. To evaluate the potential benefits of nonessential antioxidants in whole grain foods, water-soluble extracts of ready-to-eat breakfast cereal extracts were evaluated for their ability to inhibit the oxidation of phosphatidylcholine liposomes [5]. Water-soluble extracts were used so that high concentrations of tocopherols and synthetic antioxidants (e.g., BHT) would not be present in the breakfast cereal extracts.

Extracts from wheat-based ready-to-eat breakfast cereals manufactured with high bran or whole grains contain significantly more antioxidant activity than cereals produced from refined wheat [5]. Extracts from both the high bran and whole grain breakfast cereals were able to inhibit lipid oxidation catalyzed by either iron or peroxyl radicals generated from 2,2'-azobis-(2-amidopropane), indicating that metal chelation was not the major mechanism responsible for the observed antioxidant activity. Fractionation of the breakfast cereal extracts into low and high molecular weight components by ultrafiltration and dialysis, respectively, showed that the high molecular weight compounds were more effective inhibitors of iron-catalyzed oxidation of phosphatidylcholine liposomes than the low molecular weight components. Potential high molecular weight antioxidants in wheat include polysaccharides and proteins. Xanthan [6] and gum arabic [7] are examples of polysaccharides that display antioxidant activity, although the mechanism of their activity is not well understood. Cereal flours contain polysaccharides that inhibit lipoxygenase-catalyzed oxidation of linoleic acid in an emulsions model [8]. The ability of cereal flour suspensions to inhibit lipoxygenase has been proposed to be due to physical interaction between the lipid material, enzyme and/or polysaccharide, with soluble fiber showing the greatest inhibitory activity. Proteins can also inhibit lipid oxidation by acting as free radical scavengers and metal chelators [4]. Little is known about the antioxidant potential of wheat proteins. However, since these proteins contain reduced sulfhydryl groups, they would be expected to possess some free radical scavenging activity.

To determine if soluble fiber was involved in inhibition of lipid oxidation, the fiber from high bran and whole grain cereal was extracted, quantitated, redispersed in water and added back to the phosphatidylcholine liposome model system. Soluble fiber concentrations were 0.48 and 0.31 mg/mL extract in the high bran and whole grain extracts, respectively. Soluble fiber extracted from the high bran and whole grain cereals did not significantly inhibit lipid oxidation over the concentration range of 0.2–2.0 mg with 0.2 mg of soluble fiber being the approximate fiber concentrations in the amount of high fiber extract that inhibits liposome oxidation 50%. While protein and starch might also impact lipid oxidation rates in the phosphatidylcholine model system, their impact on health would likely be less important since they are hydrolyzed during digestion into amino acids and sugars that do not possess strong antioxidant activity.

Among the low molecular weight compounds in breakfast cereal extracts, phytate and phenolics are the most likely to be important dietary antioxidants. Phytic acid can protect tissues against oxidative reactions through its ability to sequester and inactivate prooxidative transition metals [9]. Phytate concentrations in whole grain and high fiber breakfast cereals are 0.14 and 0.86%, respectively. Purified phytate (Sigma Chemical Co., St. Louis, MO) at concentrations equivalent to those found in the whole grain and high fiber cereal extracts were not high enough to inhibit iron-catalyzed lipid oxidation in the phosphatidylcholine liposomes model system. In addition, it is likely that most of the endogenous phytate in the breakfast cereals and their extracts would be associated with minerals such as iron and calcium, thus making the endogenous phytate unable to chelate additional prooxidative metals in the phosphatidylcholine liposome model or in vivo. This, in combination with the observation that the cereal extracts exhibited strong antioxidant activity against nonmetallic lipid oxidation catalyst such as peroxyl radicals [5], suggests that phytate from wheat-based breakfast cereals would not be an important dietary antioxidant.

Numerous studies have evaluated the antioxidant activity of phenolics from fruits, vegetables, soybeans, herbs, teas, wines and medicinal plants [10–14]. Much less attention has been focused on the antioxidant potential of phenolics from whole grains. Wheat contains numerous phenolic compounds with ferulic, p-coumaric and vanillic acids predominating [15–17]. In wheat, these phenolics exist in several distinctly different forms, including free acids, acid esters, sugar esters and polysaccharides esters [15–16].

Phenolics inhibit lipid oxidation by scavenging free radicals resulting in the formation of a low energy phenolic radical whose energy is insufficient to promote lipid oxidation at biologically significant rates [4]. Soluble phenolics extracted from durum wheat bran extracts are effective antioxidants in dispersed soybean oil [16]. When antioxidant activity is compared at the free phenolic acid concentrations found in wheat, effectiveness is in the order of ferulic acid > vanillic acid > p-coumaric acid.

Isolation of phenolic acids in the ready-to-eat breakfast cereals by solid-phase extraction was used to test the ability of these antioxidants to inhibit lipid oxidation in the liposome system. Total phenolic acid concentrations were determined to be 98.3, 93.6 and 34.7 µg/mL of extract in the high fiber, whole grain and refined wheat cereals respectively. In the whole grain breakfast cereal, this equates to 42 mg of water soluble phenolics per serving (based on a 30g serving size). When the antioxidant activity of the isolated phenolics from whole grain breakfast cereal were tested in the phosphatidylcholine liposome system, inhibition of lipid oxidation was observed with 74 µg of phenolics (<0.2% of the soluble phenolics in a singe serving) inhibiting lipid oxidation over 90%.

Since esterification of phenolics can impact antioxidant activity by influencing partitioning of phenolics into lipid phases [18], the antioxidant activity of esterified and nonesterified phenolics was tested. Esterified and nonesterified phenolics from wheat bran were isolated by solvent extraction and HPLC [19]. The concentrations of the major water-soluble free phenolic acids and esterfied phenolics in wheat bran is shown in Table 1. Esterified phenolic concentrations ranged from 3.4-fold to 5.3-fold greater than their free acid counterparts. To test differences in antioxidant activity between the free and esterified phenolics, the esterified phenolics were isolated by preparative HPLC, acid hydrolyzed, and the resulting free phenolics were purified by HPLC. Antioxidant activity was evaluated by addition of equal phenolic concentrations (10 µM) to 2% salmon oil emulsions stabilized with Brij 35. Esterified phenolics were more effective antioxidants than their free acid counterparts (Fig. 1) indicating that their potential as dietary antioxidants should be further evaluated.

View larger version (31K): [in this window] [in a new window]   Fig. 1. The antioxidant activity of wheat bran esterified phenolics and their hydrolyzed counterparts (10 µM) in a Brij 35-stabilized salmon oil emulsion. Emulsions were allowed to autooxidize at 25°C for 120 hours and lipid oxidation was monitored by measuring thiobarbituric acid reactive substances (TBARS).

	IMPACT OF DIGESTION ON WHEAT ANTIOXIDANTS

TOP ABSTRACT OXIDATIVE REACTIONS AND HEALTH DIETARY ANTIOXIDANT IN WHOLE... IMPACT OF DIGESTION ON... CONCLUSION REFERENCES

In order to determine if gastrointestinal pH conditions had any influence on the antioxidant activity of ready-to-eat breakfast cereals, cereal homogenates were incubated at room temperature for 30 minutes (pH 6.5–7.0) followed by decreasing the pH to 2 and incubating at 37°C for 30 minutes (to simulate transition into the stomach) and then raising pH to 6 for 30 minutes at 37°C (to simulate transition into the small intestine) [22]. Gastrointestinal pH conditions caused a dramatic increase in antioxidant activity for the high bran and whole grain cereal aqueous extracts and their LMW fractions [5] suggesting that acid conditions caused alterations in the activity, composition and/or concentration of water-soluble LMW antioxidants. It is possible that acid hydrolysis influenced soluble phenolic concentrations by causing the release of free phenolics acid from fiber in a manner similar to the increase in phenolic solubility from corn bran by alkaline hydrolysis as reported by Ohta and coworkers [23].

	CONCLUSION

TOP ABSTRACT OXIDATIVE REACTIONS AND HEALTH DIETARY ANTIOXIDANT IN WHOLE... IMPACT OF DIGESTION ON... CONCLUSION REFERENCES

 
Whole wheat- and wheat bran-based ready-to-eat breakfast cereals contain several compounds that could be important dietary antioxidants. Of these compounds, free and esterified phenolic acids seem to have the greatest potential to be beneficial to health. Phenolics from breakfast cereals possess strong antioxidant activity in vitro at concentrations that would be obtained from a normal serving of whole wheat-based cereal. In addition, simulated gastrointestinal pH treatment and enzymatic hydrolysis can increase the antioxidant activity of wheat and wheat-based breakfast cereal extracts, suggesting that the digestive process could be important in further enhancing the antioxidant potential of wheat-based foods. These factors suggest that wheat-based foods could contain important dietary antioxidants and therefore warrant further research to determine whether these dietary antioxidants could be beneficial to human health. In addition, identification of both biological (e.g., digestion) and food processing conditions that impact the distribution, stability and activity of wheat antioxidants is needed in order to be able to produce food products with maximum health benefits.

Received February 1, 2000.

	REFERENCES

TOP ABSTRACT OXIDATIVE REACTIONS AND HEALTH DIETARY ANTIOXIDANT IN WHOLE... IMPACT OF DIGESTION ON... CONCLUSION REFERENCES

 

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