Antioxidants and the Polar Paradox Theory

Christina Cutting

Primary Source:Decker EA, Warner K, Richards MP, Shahidi F. 2005. Measuring Antioxidant Effectiveness in Food. J Agric. Food Chem. 53:4303-4310.

Mechanisms of Action

• Scavenge ROS• Inhibit radical chain reactions• Chelate pro-oxidant metals• Inhibit oxidizing enzymes• Eliminate cofactors needed for oxidation rxns

Efficacy Depends On

• Environmental conditions– pH– Temperature

• Location, Location, Location!– Must be in close proximity to oxidizing agents– Hydrophilic vs. Hydrophobic antioxidants

The Polar Paradox Theory

• Polar AOXs more effective in nonpolar or less polar systems

• Nonpolar AOXs more effective in polar systems

• Due to “interfacial phenomenon”?

Source:Porter WL. 1993. Paradoxical Behavior of Antioxidants in Food and Biological Systems. Toxicol. Ind. Health. 9(1):93-122.

*Had to request from library. Email Christina if you would like a copy of the paper.

Interfacial Phenomenon

• Low surface to volume ratio– Bulk oils

• High surface to volume ratio– Emulsions

• Location:– Oil-air– Water-oil

Source:Frankel E, Huang SW, Kanner J, German JB. 1994. Interfactial Phenomena in the Evaluation of Antioxidants: Bulk Oils vs. Emulsions. J. Agric. Food Chem. 42:1054-1059.

Polar Antioxidants

• Most effective in nonpolar or less polar environment– Bulk oils

• Located at the oil-air interface or in reverse micelles– High amount of oxidants

present here• Oxygen

Nonpolar Antioxidants

• Most effective in polar environment– Oil-in-water emulsions

• Located at the water-oil interface– Dissolved in oil droplets of

the emulsion– Allows access to oxidizing

agents located in the water phase• Peroxides• Oxidizing metals

However…

• Polar paradox theory may not explain everything– Special case, not overall rule

• Must take into account:– Structure– Size

• Cutoff effect

– Relative antioxidant activity– Emulsifying environment – Ability of AOXs to form micelles – Concentration of AOXs– Mechanism of Action

Source: Shahidi F, Zhong Y. 2011. Revisiting the Polar Paradox Theory: A Critical Overview. J. Agric. Food Chem. Published online.

Questions?