The Oxygen Radical Absorbance Capacity (ORAC) test is a method developed and used to measure the antioxidant capacity and equivalence of food.[1][2]It was first developed by the clinical scientists at the National Institute on Aging in the National Institutes of Health(NIH) located in Baltimore Maryland. It has more recently become recognized by the United States Department of Agriculture (USDA) and they have listed a database of ORAC value. The role of antioxidants in the diet has received much attention recently with many studies showing protective benefits and this test was developed to address the need for a method of measuring and comparing antioxidant amounts of different foods.

The method for the test is an assay based on Glazer’s study. It measures oxidative degration of the fluorescent molecule. The fluorescent molecules are mixed with azo-initiator compounds, which generate free radicals when heated. When the mixture reacts and free radicals are initiated damages to the fluorescent molecules result in a loss of fluorescence. Since antioxidants are able to protect molecules from free radical damage protection from free radical damage to the fluorescent molecule can be measured using a fluorometer. Fluorescence is recorded for a set time period of usually 35 minutes and decay curves are recorded with and without antioxidants. Degree of antioxidant protection is quantified using the TE(trolox equivalent), which is an analogue to vitamin E.[3]

A list of hydrophilic ORAC values of 100g servings of some foods was published in 2004. More recently in 2007 a list of hydrophilic and lipophilic ORAC values of some foods was published showing higher total ORAC units of previous results because of the addition of the lipophilic measurements. Some of the highest concentrations of ORAC units have been with Freeze-Dried fruit pulp.[4]

Reference:
1.Cao G, Alessio H, Cutler R (1993). “Oxygen-radical absorbance capacity assay for antioxidants”. Free Radic Biol Med 14(3): 303-11. doi:10.1016/0891-5849(93)90027-R ( HYPERLINK "http://dx.doi.org/10.1016/0891-5849(93)90027-R)" http://dx.doi.org/10.1016/0891-5849(93)90027-R). PMID8458588( HYPERLINK "http://www.ncbi.nlm.nih.gov/pubmed/8458588" http://www.ncbi.nlm.nih.gov/pubmed/8458588).
2.Ou B, Hampsch-Woodill M, Prior R (2001). “Development and validation of an improved oxygen radical absorbance capacity assay using fluorescein as the fluorescent probe”. J Agric Food Chem 49 (10): 4619-26. doi: 10.1021/jf010586o ( HYPERLINK "http://dx.doi.org/10.1021/jf010586o" http://dx.doi.org/10.1021/jf010586o). PMID 11599998 ( HYPERLINK "http://www.ncbi.nlm.nih.gov/pubmed/11599998" http://www.ncbi.nlm.nih.gov/pubmed/11599998).
3.Huang D, Ou B, Prior R(2005). “The chemistry behind antioxidant capacity assays”. J. Agric. Food Chem. 53 (6): 1841-56. doi:10.1021/jf030723c ( HYPERLINK "http://dx.doi.org/10.1021/jf030723c" http://dx.doi.org/10.1021/jf030723c). PMID 15769103 ( HYPERLINK "http://www.ncbi.nlm.nih.gov/pubmed/15769103" http://www.ncbi.nlm.nih.gov/pubmed/15769103).
4.Schauss A, Wu X, Prior R, Ou B, Huang D, Owens J, Agarwal A, Jensen G, Hart A, Shanbrom E (2006). “Antioxidant capacity and other bioactivities of the freeze-dried Amazonian palm berry, Euterpe oleracea mart. (acai)”. J. Agric. Food Chem. 54 (22): 8604-10. doi:10.1021/jf0609779 ( HYPERLINK "http://dx.doi.org/10.1021/jf0609779" http://dx.doi.org/10.1021/jf0609779). PMID 17061840 (http://www.ncbi.nlm.nih.gov/pubmed/17061840).