Another Option for Antioxidant Capacity Assays

Oxidative stress is an imbalanced state in which free radicals (reactive oxygen species) exceed levels which are required for normal cell function and overwhelm endogenous antioxidant capacity and repair (Nordberg and Arner, 2001; Patel, 2002). Oxidative stress damages cell components including DNA, proteins, and lipids. In humans, it contributes to atherosclerosis, heart failure, cancer, Alzheimer’s disease, and aging.

Antioxidants found in foods, especially in plant foods, can delay or inhibit the oxidation of  oxidizable matter by scavenging free radicals and diminishing oxidative stress. In order to assay the antioxidant capacity of foods and compounds, numerous tests have been developed, including the Oxygen Radical Absorbance Capacity (ORAC), the Total Radical-Trapping Antioxidant Parameter (TRAP), the Trolox Equivalent Antioxidant Capacity (TEAC), the Ferric Reducing Antioxidant Power (FRAP), the Total Oxyradical Scavenging Capacity (TOSC), the Proxy Radical Scavenging Capacity (PSC), and so forth.

In a recent article in Food Technology (May, 2010), Neil Mermelstein outlined problems associated with these tests and quoted Rutgers University Professor Karen Schaich, who raised several concerns for each method with respect to accuracy, standardization, result interpretation, and also questioned the legitimacy of endorsing a single assay for antioxidant activity of foods.

I would like to raise another issue related to the expression of antioxidant activity in food, supplements, and nutraceuticals. The most commonly utilized assays—including TRAP, TEAC, PSC, and ORAC—use Trolox in molar units as a standard reference compound for the expression of antioxidant capacity of foods. However, since Trolox (6-hydroxy-2,5,7,8-tetra-methylchroman-2-carboxylic acid), a vitamin E analogue, is not a natural compound found in foods and is not a familiar chemical to many scientists, I question its application as a reference compound in antioxidant assays of foods and chemicals.

When the use of Trolox as a reference compound as an antioxidant assay was traced, it was found that Trolox was used in the antioxidant assay of serum (Cao et al., 1993), and in 1996, the same senior author who worked on serum modified the same method to apply to foods and conceived the automated ORAC assay (Cao et al., 1995). Since then, many scientists replicated the method with little reservation, and today the ORAC assay is the most widely utilized method in foods.

The chief problem with this method is that the expression of antioxidant capacity of dietary foods using a molar basis of Trolox equivalents is simply too obscure to the lay person.Further widening of the comprehension gap between the general population and scientists occurs due to application of the chemical term “molar units” to foods (Truswell, 1995). Current nutritional labeling on most foods and dietary supplements is expressed on a gram weight basis, certainly not as molar units as preferred by some scientists. For example, the vitamin C content in a glass of orange juice expressed as “100 mg” would be much easier to understand than “0.57 mM.” To be meaningful to consumers, antioxidant capacity of foods should be expressed in terms and units that are familiar to everyone.

Since vitamin C has been shown to scavenge reactive oxygen species and has anti-carcinogenic activities (Rock et al., 1996; Lee et al., 2002) and is, in turn, commonly recognized by the general public as a leading natural nutrient and antioxidant, using vitamin C equivalents (Kim and Lee, 2004) to describe the antioxidant capacity of dietary foods and phytochemicals seems to be a logical choice. For example, the antioxidant capacity of 100 g apple expressed as “equivalent to 150 mg vitamin C ” would be much less ambiguous than “350 ORAC value.”

Chang Y. Lee, Ph.D., a Professional Member and IFT Fellow, is Professor of Food Science, Cornell University, Geneva, NY 14456 ( [email protected] ).

References