Antioxidant tests not suitable for complex foods: Study
The use of antioxidant on the label of food products is increasing, and some food manufactures are beginning to use values from assays like ORAC (oxygen radical absorbance capacity) to emphasise the antioxidant profile of their products.
However, according to new research from Eric Decker and his colleagues at the Department of Food Science at the University of Massachusetts, basing antioxidant activity claims on results of basic antioxidant assays such as ORAC and DPPH (2, 2'-diphenyl-l-picrylhydrazyl) could be misleading.
"Free radical scavenging assays such as ORAC and DPPH were not able to consistently predict the ability of compounds to inhibit lipid oxidation in cooked ground beef," they wrote in the Journal of Agricultural and Food Chemistry.
"While simple one-dimensional free radical scavenging assays can be helpful in evaluating the antioxidant mechanisms of a compound, the data from these assays should not be used to imply that compounds with high free radical scavenging capacities are good antioxidants in food systems."
These conclusions have implications for the growing number of food using the term antioxidant, and increasingly quoting ORAC values to back up the activity claims.
Figures from Mintel’s Global New Products Database (GNPD) indicate that more and more products are emphasising a product’s antioxidant content on the label, with 135 launches in Europe of products labelled ‘antioxidants’ in 2008, compared to 111 in 2007, and only 37 in 2006.
Similar growth of the use of the term was observed across the Atlantic, with 106 launches in the US in 2006, 131 in 2007, and an impressive 262 in 2008. These figures, which do not take into account products that label specific antioxidants like vitamins A, C, and E, and are therefore a conservative indication of the use.
New data
Decker and his co-workers evaluated the antioxidant activity of various compounds using the antioxidant assays, and then compared this to their ability to inhibit lipid oxidation in two model complex foods: Ground beef and an oil-in-water emulsion.
"Such comparisons are important because the ability of a compound to inhibit lipid oxidation in foods is thought to not only be related to its free radical scavenging activity but also its physical location (e.g., does the compound concentrate where oxidative reactions are most prevalent) and ability to participate in other oxidative pathways (e.g., metal inactivation and regeneration of endogenous food antioxidants)," they explained.
According to their findings, the ORAC results showed that, of the tested compounds, ferulic acid performed the best, followed by coumaric acid, propyl gallate, gallic acid, and vitamin C (ascorbic acid). The DPPH for non-polar compounds showed that rosmarinic acid came out on top, with higher values than butylated hydroxytoluene g tert-butylhydroquinone (TBHQ), and vitamin E (R-tocopherol).
However, when tested for their ability to prevent the oxidation of lipids in cooked ground beef, only propyl gallate and TBHQ showed any activity, while both substances, and gallic and rosmarinic acids showed any activity in the oil-in-water emulsion.
Explaining the discrepancy
"The lack of correlation between free radical scavenging and antioxidant activity in a complex food is likely due to the multitude of factors that can impact the ability of a compound to inhibit lipid oxidation," said Decker and his co-workers.
"The major drawback of the free radical scavenging assays is that they do not measure the ability of a compound to chelate metals, partition into lipids where oxidation is prevalent, or interact with other antioxidants and prooxidants (e.g., metals) in a food product," they concluded.
Source: Journal of Agricultural and Food Chemistry 2009, Volume 57, Issue 7, Pages 2969-2976, doi: 10.1021/jf803436c“Relationships between Free Radical Scavenging and Antioxidant Activity in Foods”Authors: J. Alamed, W. Chaiyasit, D.J. McClements, E.A. Decker
