The value is attained by determining the area under the curve of the sample (i.e., the quenching curve) and subtracting the area under the blank curve. The same calculation is done with the Trolox standard curve and corresponding blanks. The resulting net protection area of the sample is then divided by the Trolox net protection area. One ORAC unit is equivalent to the net protection area provided by 1 µM of Trolox. Adding the results of the hydro- and lipophilic assays provides a total ORAC score.
Prior's research on 40 food matrices showed that the water-soluble ORAC values ranged from 7.34 ±0.85 to 568.0 ±14.2 µM of Trolox equivalents per gram. The fat-soluble ORAC values ranged from 1.00 ±0.06 to 31.23 ±0.22 µM and accounted for approximately 0.59% to 39.51% of the total ORAC value. These results demonstrate that measures of total antioxidant capacity of foods need to include both the water- and lipid-soluble antioxidant in order to have an accurate total for a given food sample.
A clinical connection
While chemists and manufacturers wrestle with the validation and value of the ORAC assay in product development, the use and influence of the assay in biological research is expanding. After all, knowing the ORAC value of a compound is only part of the puzzle. Linking consumption of products with high ORAC scores to a corresponding value in human health is another story. By using the ORAC assay with plasma and other biological samples, researchers can measure the antioxidant levels in plasma and tissues from animals and humans fed diets fortified with certain foods or phytochemicals.
Although the studies are far from conclusive, results seem to show a definite connection. For example, studies on middle-aged rats fed diets with ORAC-standardized antioxidants showed evidence of an effect on the loss of long-term memory and learning capacity. Additionally, human-plasma samples from volunteers fed fortified diets of certain antioxidant foods/substances indicate an increase in the antioxidant power of the human body.
However, the interpretation and application of ORAC scores for comparative use still requires some work. For example, the origin and manufacturing of ingredients can have a tremendous effect on ORAC values. "I believe the ultimate impact on food manufacturers and an area that we have limited data on is the effect of processing on many of these antioxidant compounds," says Prior. "Processing, in some cases, seems to have very devastating effects on some of these antioxidants."
One of the goals of the USDA project is to develop a database containing ORAC values for a widespread range of representative matrices. This would allow consumers and scientists alike to make more-accurate comparisons between products and significantly enhance ongoing research.
Only the beginning
Although not yet universally accepted as a definitive method, ORAC is becoming one industry standard for measuring antioxidants. However, due to the complexity of the factors involved, no single assay will be sufficient for assessing the power of these compounds. The recent development of the NORAC and HORAC assays, to measure capability against the peroxynitrate and hydroxyl radicals respectively, represent another addition to the battery of tools at our disposal. Through an ongoing collaboration of disciplines, scientists will be able to expand our understanding of the effect of the diet on the prevention of maladies such as cancer and cardiovascular disease.
Richard Crowley is the editor of the Covance Food Science newsletter and the author of numerous articles on food analysis. He has a B.S. in Agricultural Journalism from the University of Wisconsin–Madison and is a member of the National Association of Science Writers.