DPPH and FRAP Explained: The Antioxidant Numbers Behind an Olive Oil Label

Two Unfamiliar Acronyms on the Label: DPPH and FRAP

Browse the quality sheet of a premium extra virgin olive oil (EVOO) and you may find two abbreviations sitting alongside the polyphenol content figure: DPPH and FRAP. Both are established analytical tools in food science, used to quantify the antioxidant capacity of an oil. Yet for most buyers, the numbers appear without explanation. This OLEA editor's guide walks through how each assay works, what it actually captures, and where its limits lie.

The DPPH Assay: Measuring Free-Radical Scavenging Capacity

DPPH is shorthand for 2,2-diphenyl-1-picrylhydrazyl, a stable free-radical compound that produces a characteristically deep-purple solution. The assay principle is straightforward: an olive oil sample is introduced into the DPPH solution, and a spectrophotometer tracks how quickly and how completely the purple color fades.

Free radicals are unstable molecules missing one electron. When an antioxidant compound donates an electron to stabilize the radical, the purple DPPH solution turns yellow. The greater the change in absorbance, the higher the sample's free-radical scavenging capacity. Results are typically converted into Trolox equivalents or ascorbic-acid equivalents and expressed as μmol TE/g.

In a 2011 opinion, the European Food Safety Authority (EFSA) reported that phenolic compounds in olive oil—including hydroxytyrosol and oleuropein—are associated with protection of blood lipids from oxidative stress. The DPPH assay is widely used in the industry precisely because it captures the electron-donating capacity of these phenolic compounds in a direct and readable way.

The FRAP Assay: Reading Antioxidant Power Through Iron Reduction

FRAP—Ferric Reducing Antioxidant Power—takes a different route to the same general question. As the name states, it measures how efficiently a sample reduces ferric iron (Fe³⁺) to ferrous iron (Fe²⁺). The FRAP reagent starts nearly colorless and turns an intense blue as the reduction reaction proceeds; the depth of that blue is read by spectrophotometry and converted to a numerical score.

If DPPH is focused on radical scavenging, FRAP is focused on electron-donating, or reducing, power. In theory the two assays provide complementary information. In practice, measurement conditions—pH, temperature, reaction time—can shift results meaningfully even for the same sample. According to multiple studies published in the Royal Society of Chemistry journal Food & Function, correlation coefficients between FRAP and DPPH scores for olive oils are generally high but not perfectly aligned.

Results are most commonly expressed as mmol FeSO₄·7H₂O/kg or μmol TE/g. Because unit conventions vary between producers, converting to a common unit before making any brand-to-brand comparison is essential for accuracy.

The Relationship to Polyphenol Content: Does a Higher Number Always Mean More?

A common point of confusion is treating DPPH scores, FRAP scores, and total polyphenol content (TPC) as interchangeable. They are related, but each captures something distinct.

Total polyphenol content is measured using the Folin-Ciocalteu reagent and reflects the overall concentration of phenolic compounds in the oil, expressed in mg/kg. DPPH and FRAP, by contrast, reflect how reactive those polyphenols are—how effectively they engage free radicals or iron ions. The same total polyphenol figure can produce different DPPH and FRAP readings depending on which phenolic compounds dominate. Oils rich in hydroxytyrosol, a phenol associated with particularly strong reducing activity, have been reported to show higher antioxidant scores at the same overall polyphenol concentration compared with oils dominated by less reactive phenols.

The International Olive Council (IOC) analytical guidelines treat polyphenol content as a primary quality criterion during certification, with DPPH and FRAP typically serving as supplementary indicators.

Limits of the Assays: Why One Number Is Never the Whole Story

DPPH and FRAP are reliable analytical methods, but several structural limitations are worth keeping in mind.

First, both assays are conducted entirely in vitro—in a test tube, under laboratory conditions. The human body's digestive, absorptive, and metabolic processes operate very differently, so a high in-vitro score cannot be read as a direct prediction of antioxidant activity inside the body. Researchers managing the USDA's nutrient-composition database have flagged the same concern and recommend against over-interpreting any single in-vitro antioxidant measurement.

Second, EVOO is an oil-based matrix, while DPPH and FRAP reagents operate in aqueous or methanol-based solvents. Before measurement, the sample must be extracted into a compatible solvent; some compounds may be lost or altered during that step, which is why standardized extraction protocols are important and why results from non-standardized procedures should be read with caution.

Third, the same oil can yield results that vary by tens of percent depending on harvest timing, milling method, and storage conditions. According to research published in the Journal of Agricultural and Food Chemistry, oils pressed from early-harvest (green-harvest) olives tend to show higher polyphenol content and higher DPPH scores than oils from fully ripe fruit.

For these reasons, specialists recommend reading DPPH and FRAP values alongside total polyphenol content, free acidity, peroxide value, and sensory evaluation rather than relying on any single figure. Antioxidant assay results are best understood as one window into olive oil quality—a useful one, but not the only one.

A Practical Guide to Reading These Numbers at the Point of Purchase

When DPPH or FRAP figures appear on a product page or quality certificate, a few checkpoints make the information more useful.

Confirm the units are comparable. Figures expressed as μmol TE/g, mmol FeSO₄/kg, and other conventions are not directly comparable out of the box. Convert to a common unit, or refer to an accredited laboratory report, before placing oils side by side.

Check whether the laboratory holds ISO 17025 accreditation. Products backed by a certificate from an ISO 17025-accredited lab carry a higher level of measurement credibility than those citing internal testing.

Note the harvest campaign year and the pressing date. Antioxidant compounds in olive oil are known to decrease gradually over time. Choosing the most recent season's oil and, once opened, storing the bottle in a cool, dark place helps preserve as much of the original activity as possible.

Antioxidant indices are a genuinely useful tool for exploring the intrinsic quality of an extra virgin olive oil. They are not, however, a direct health guarantee, and their significance is best understood within the broader context of a balanced, varied diet.