chemistry

Oxygen radical absorbance capacity

Oxygen radical absorbance capacity is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Oxygen radical absorbance capacity rather than just read about it. In short: Oxygen radical absorbance capacity (ORAC) was a method of measuring antioxidant capacities in biological samples in vitro. Because no physiological proof in vivo existed in support of the free-radical theory or that ORAC provided information relevant to biological antioxidant potential, it was withdrawn in 2012.

Key takeaways

Oxygen radical absorbance capacity belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Oxygen radical absorbance capacity to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Oxygen radical absorbance capacity from memory before moving on to harder problems.

Reference excerpt

Oxygen radical absorbance capacity (ORAC) was a method of measuring antioxidant capacities in biological samples in vitro. Because no physiological proof in vivo existed in support of the free-radical theory or that ORAC provided information relevant to biological antioxidant potential, it was withdrawn in 2012. Various foods were tested using this method, with certain spices, berries and legumes rated highly in extensive tables once published by the United States Department of Agriculture (USDA). Alternative measurements include the Folin-Ciocalteu reagent, and the Trolox equivalent antioxidant capacity assay.

Method

The assay measures the oxidative degradation of the fluorescent molecule (either beta-phycoerythrin or fluorescein) after being mixed with free radical generators such as azo-initiator compounds. Azo-initiators are considered to produce the peroxyl radical by heating, which damages the fluorescent molecule, resulting in the loss of fluorescence. Antioxidants are considered to protect the fluorescent molecule from the oxidative degeneration. The degree of protection is quantified using a fluorometer. Fluorescein is currently used most as a fluorescent probe. Equipment that can automatically measure and calculate the capacity is commercially available (Biotek, Roche Diagnostics). The fluorescent intensity decreases as the oxidative degeneration proceeds, and this intensity is typically recorded for 35 minutes after the addition of the azo-initiator (free radical generator). So far, AAPH (2,2’-azobis(2-amidino-propane) dihydrochloride) is the sole free-radical generator used. The degeneration (or decomposition) of fluorescein is measured as the presence of the antioxidant slows the fluorescence decay. Decay curves (fluorescence intensity vs. time) are recorded and the area between the two decay curves (with or without antioxidant) is calculated. Subsequently, the degree of antioxidant-mediated protection is quantified using the antioxidant trolox (a vitamin E analogue) as a standard. Different concentrations of trolox are used to make a standard curve, and test samples are compared to this. Results for test samples (foods) have been published as "trolox equivalents" or TEs. A possible benefit of using the ORAC method to evaluate substances' antioxidant capacities is that it takes into account samples with and without lag phases of their antioxidant capacities. Drawbacks of this method are: 1) only antioxidant activity against particular (probably mainly peroxyl) radicals is measured; however, peroxyl radical formation has never been proven; 2) the nature of the damaging reaction is not characterized; 3) there is no evidence that free radicals are involved in this reaction; and 4) there is no evidence that ORAC values have any biological significance following consumption of any food. Moreover, the relationship between ORAC values and a health benefit has not been established. Resulting from scientific refutation of the physiological significance of ORAC, the USDA, which had been collating and publishing ORAC data for more than a decade, withdrew its web publication of ORAC values for common American foods in May 2012. Several modified ORAC methods have been proposed. Most of them employ the same principle (i.e. measurement of AAPH-radical mediated damage of fluorescein); however, ORAC-EPR, electron paramagnetic resonance-based ORAC method directly measures the decrease of AAPH-radical level by the scavenging action of the antioxidant substance.

Regulatory guidance In the following discussion, the term "antioxidant" refers mainly to non-nutrient compounds in foods, such as polyphenols, which have antioxidant capacity in vitro, so provide an artificial index of antioxidant strength—the ORAC measurement. Other than for dietary antioxidant vitamins—vitamin A, vitamin C and vitamin E—no food compounds have been proved with antioxidant efficacy in vivo. Accordingly, regulatory agencies such as the Food and Drug Administration of the United States and the European Food Safety Authority (EFSA) have published guidance forbidding food product labels to claim or imply an antioxidant benefit when no such physiological evidence exists. This guidance for the United States and European Union establishes it is illegal to imply potential health benefits on package labels of products with high ORAC.

Physiological context Although research in vitro indicates polyphenols are good antioxidants and probably influence the ORAC value, antioxidant effects in vivo are probably negligible or absent. By non-antioxidant mechanisms still undefined, flavonoids and other polyphenols may reduce the risk of cardiovascular disease and cancer. As interpreted by the Linus Pauling Institute, EFSA and the USDA, dietary polyphenols have little or no direct antioxidant food value following digestion. Not like controlled test tube conditions, the fate of polyphenols in vivo shows they are poorly conserved (less than 5%), with most of what is absorbed existing as chemically modified metabolites destined for rapid excretion. The increase in antioxidant capacity of blood seen after the consumption of polyphenol-rich (ORAC-rich) foods is not caused directly by the polyphenols, but most likely results from increased uric acid levels derived from metabolism of flavonoids. According to Frei, "we can now follow the activity of flavonoids in the body, and one thing that is clear is that the body sees them as foreign compounds and is trying to get rid of them."

Food sources Values are expressed as the sum of the lipid soluble (e.g. carotenoid) and water-soluble (e.g. phenolic) antioxidant fractions (i.e., “total ORAC”) reported as in micromoles trolox equivalents (TE) per 100 gram sample, and are compared to assessments of total polyphenol content in the samples. These values are considered biologically irrelevant by the EFSA and USDA.

With nearly all vegetables, conventional boiling can reduce the ORAC value by up to 90%, while steaming retains more of the antioxidants.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Oxygen radical absorbance capacity

Start with the simplest possible case. Write down what Oxygen radical absorbance capacity claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Oxygen radical absorbance capacity before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Oxygen radical absorbance capacity ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Oxygen radical absorbance capacity

In research: Oxygen radical absorbance capacity appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Oxygen radical absorbance capacity in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Oxygen radical absorbance capacity is common in secondary-school and first-year university syllabi. It links to neighbouring topics 2012 disestablishments, Biochemistry detection reactions, Nutrition, so understanding it makes those chapters shorter.
In everyday life: Look for Oxygen radical absorbance capacity outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Oxygen radical absorbance capacity in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Oxygen radical absorbance capacity means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Oxygen radical absorbance capacity out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Oxygen radical absorbance capacity in simple terms?

Why does Oxygen radical absorbance capacity matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Oxygen radical absorbance capacity?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Oxygen radical absorbance capacity.