Factors Influencing the Antioxidant and Pro-Oxidant Activity of Polyphenols in Oil-in-Water Emulsions

Department of Food Science, College of Agricultural Sciences, The Pennsylvania State University, University Park, Pennsylvania, United States of America.
Journal of Agricultural and Food Chemistry (Impact Factor: 2.91). 02/2012; 60(11):2906-15. DOI: 10.1021/jf204939h
Source: PubMed


The nonenzymatic oxidation of polyphenols bearing di- and trihydroxyphenol groups results in the generation of hydrogen peroxide (H₂O₂), a reactive oxygen species that can potentially compromise the oxidative stability of foods and beverages. An investigation of the factors that promote the oxidation of a model polyphenol, (-)-epigallocatechin-3-gallate (EGCG), was undertaken in a model lipid-based food system. Factors affecting oxidative stability, such as exogenous iron chelators (ethylenediaminetetraacetic acid; EDTA and 2,2-bipyridine; BPY) and pH (3 and 7) were evaluated in hexadecane and flaxseed oil-in-water (o/w) emulsions. At neutral pH, H₂O₂ levels were observed to rise rapidly in hexadecane emulsions except for EDTA-containing treatments. However, EDTA-containing samples showed the highest rate of EGCG oxidation, suggesting that H₂O₂ was rapidly reduced to hydroxyl radicals (HO•). Conversely, at pH 3, H₂O₂ concentrations were lower across all treatments. EDTA conferred the highest degree of EGCG stability, with no loss of the catechin over the course of the study. In order to assess whether or not the H₂O₂ production seen in oxidatively stable hexadecane emulsions translated to pro-oxidant activity in an oxidatively labile food lipid system, the effect of EGCG on the stability of flaxseed o/w emulsions was studied. EGCG displayed antioxidant activity at pH 7 throughout the study; however at pH 3, pro-oxidant activity was seen in EGCG-containing emulsions, with and without BPY. This study attempts to provide a mechanistic understanding of the conditions wherein polyphenols simultaneously exert pro-oxidant and antioxidant behavior in lipid dispersions.

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    ABSTRACT: Polyphenols have been observed to exert both antioxidant and pro-oxidant activity in lipid foods, and factors that influence that net effect include both polyphenol concentration and matrix pH. In this study, the effects of concentration (1-500μM) of a model polyphenol, (-)-epigallocatechin-3-gallate (EGCG), and matrix pH (2-7) on the net anti-/pro-oxidant activity of EGCG in flaxseed oil-in-water (o/w) emulsions were systematically evaluated. After 24h, EGCG (5-100μM) was observed to exhibit pro-oxidant activity in low pH (pH 2-4) emulsions, as determined by conjugated dienes (CDs) and thiobarbituric acid reactive substances (TBARSs) production. At the higher pH values studied (pH 5-7), lower CD and TBARS concentrations were detected in samples with 25-500μM EGCG at 24h. Overall, EGCG concentration and pH both played significant roles in determining net antioxidant or pro-oxidant effects, with the largest antioxidant and pro-oxidant effects observed at the higher EGCG concentrations (100-500μM) tested.
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    ABSTRACT: We investigated the mechanisms of the protection conferred by sugars to epigallocatechin-3-gallate (EGCG) against deterioration. Additionally, we present a rapid method for evaluating the deterioration rate of EGCG using absorbance spectroscopy. We found that various sugars provided different levels of protection at identical weight percentage, and the combination of sugars and β-lactoglobulin nanocomplexes provided greater protection for EGCG than each protective component alone. We suggest that the concentration-dependent protection by sugars resulted from a combination of mechanisms, including: (1) reduced aqueous O2 solubility, (2) scavenging of reactive oxygen species, and (3) chelation of traces of transition metal ions, which is suggested to be the main reason for the differences among the sugars. The observed protective effect of sugars can be easily applied by the industry in proper selection of sugars for enrichment of syrups or concentrates with EGCG and for the preparation of enriched beverages and foods for health promotion.
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    ABSTRACT: Green tea (Camellia sinensis) is a widely consumed beverage that has shown preventive effects against chronic diseases including cancer. Green tea polyphenols are believed to be responsible for this cancer preventive effect and modulation of redox activity has been suggested as a potential mechanism of action. Data from various laboratories has suggested that green tea polyphenols can act as direct antioxidants by scavenging reactive oxygen species or chelating transition metals; as potent pro-oxidants which generate reactive oxygen species via a transition metal catalyzed reaction; or indirectly to modulate redox status by up-regulating of phase II and antioxidant enzymes. Considerable data has been generated using in vitro systems, but these results must be interpreted with caution because of the relatively high oxygen concentrations available in vitro compared to in vivo. In the present review, we discuss the available data regarding the pro-oxidant and antioxidant effects of tea polyphenols and provide suggestions for future research.
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