Oxygen radical absorbance capacity
Oxygen Radical Absorbance Capacity (ORAC) is a method of measuring antioxidant capacities of different foods. It was developed at the National Institute on Aging in Baltimore, Maryland. A wide variety of foods have been tested using this methodology, with certain berries and legumes rated very highly. Correlation between the high antioxidant capacity of fruits and vegetables, and the positive impact of diets high in fruits and vegetables, plays an important role in the Free-radical theory of aging.

# Method
The assay measures the oxidative degradation of fluorescein after being mixed with peroxyl radical.  The reaction alone is compared to the reaction in the presence of a standard antioxidant (trolox, a vitamin E analogue) and the antioxidant sample being tested.  The fluorescent intensity of the fluorescein decreases as it gets oxidized, and measurements of this intensity are taken every minute for 35 minutes after the addition of the oxidant.  The oxidative decay of fluorescein is less rapid in the presence of antioxidants.  A graph of the decay curve (fluorescein intensity vs. time) is generated and the area under the curve is calculated.  Different concentrations of trolox are used to make a standard curve, and test samples are compared to this.  Final results for test samples are published as "trolox equivalents" or TE.
One benefit of using the ORAC method to test a substance's antioxidant capacity is that it takes into account samples with and without lag phases of their antioxidant capacities.  This is especially beneficial when measuring foods and supplements that contain complex ingredients with various slow and fast acting antioxidants, as well as ingredients with combined effects that cannot be pre-calculated.

# Applications
In 2004, scientists with the United States Department of Agriculture published an updated list of ORAC values for over 100 common foods that are commonly consumed by the U.S. population (fruits, vegetables, nuts, seeds, spices, grains, etc.). Values were reported as micromoles of Trolox equivalents (TE, vitamin E derivative) per gram both for lipid-soluble ("lipophilic" as for carotenoids) and water-soluble ("hydrophilic" as for phenolics) antioxidant chemicals in foods, thus were a sum of lipophilic and hydrophilic values or total ORAC. These values are considered to be more accurate than previously published ORAC numbers because lipophilic values were being included for the first time. These data showed that all plants have variable amounts of both lipophilic and hydrophilic phytochemicals that contribute to total ORAC.

# Comparisons of ORAC values
When comparing ORAC data, care must be taken to ensure that the units and food being compared are similar. Some evaluations will compare ORAC units per grams dry weight, others will evaluate ORAC units wet weight and still others will look at ORAC units/serving. Under each evaluation, different foods can appear to have higher ORAC values. Although a raisin has no more antioxidant potential than the grape from which it was dried, raisins will appear to have a much higher ORAC value per gram wet weight than grapes due to their reduced water content. Likewise, watermelons large water content can make it appear as though they are very low in antioxidants. To say then that chocolate has "more antioxidant" potential than blueberries is tenuous at best. While ounce per ounce chocolate may have a higher ORAC value, on the comparison of dry weight, we see blueberries have a higher ORAC value. Additionally, considering the ORAC value per calorie could be of some utility, as understanding just how much antioxidizing potential one could incorporate from a product into one's diet would determine the real utility of the product.
The range of ORAC for common fruits was around 1.40 micromoles TE per gram (watermelon) to 95 (cranberry). Lowbush blueberry (wild blueberry) was also very high at 92.6 mol/g.  For vegetables or legumes, it was 1.15 (cucumber) to 149 small red (red kidney bean); for nuts, 7.19 (cashew) to 179.4 (pecan); and for dried fruits, 23.87 (medjool dates) to 85.78 (prune). By comparison, different species of apples had ORAC values of 22.10 to 42.75 micromoles TE per gram, white potato was under 11, peanut was 31.66 and tomato about 4.00  Spices (clove, cinnamon) showed the highest ORAC values (>2500, converted to micromoles TE per gram). Cocoa has a high ORAC value, giving baking chocolate a value of 1032 and milk chocolate an average of 71.30.
A recent paper by Schauss et al. published in the Journal of Agricultural and Food Chemistry reported an extremely high total ORAC value of 1027 micromoles TE per gram for a freeze-dried fruit pulp and skin powder from the Acai berry (Euterpe oleracea).  This is the highest ORAC value ever reported for a fruit or vegetable to this date, after converting values of fresh food weights to dry weights.  This includes a hydrophilic ORAC antioxidant capacity of 997 micromoles TE per gram, and a lipophilic ORAC antioxidant capacity of 30 micromoles TE per gram.
Recently, a number of health food companies have capitalized on the ORAC rating, with dozens selling concentrated supplements that they claim to be "the number one ORAC product". Most of these values have never been published in the scientific literature so are difficult to evaluate.  It is not known whether such values are accurate or how absorbable and functional these concentrated antioxidants are in the human body.