Kitchen Polyphenol Synergy in DPPH Antioxidant Tests

ISEF Category: Biochemistry

Subcategory: General Biochemistry  ·  Difficulty: Intermediate  ·  Setup: School Lab  ·  Time: 1 to 2 Months

The Hook

Two weak antioxidants can sometimes beat one strong one. That matters because many foods and supplements contain mixed compounds, not single molecules. You can test that idea with color-changing assays and simple synergy math. The result can tell you whether the pair works like a team or just two ingredients in the same cup.

What Is It?

Antioxidants are molecules that can slow down damage caused by reactive chemicals called free radicals. A simple way to picture them is to think of sparks in a room, and antioxidants as people handing out fire blankets. DPPH and ABTS are test chemicals that start with a strong color, then fade when an antioxidant gives them an electron or hydrogen.

Synergy means the pair does better together than you would expect from adding their separate effects. If compound A and compound B each lower the color a little, the mixture might lower it much more than the sum of those two separate effects. An isobologram is a graph that shows that comparison. If your mixture points fall below the straight line for additivity, the pair may be acting synergistically.

Why This Is a Good Topic

This is a strong science fair topic because you can measure it, compare it, and repeat it with simple equipment. It connects to food chemistry, supplement design, and how mixed plant compounds behave in the body. You can learn calibration, dose-response curves, and basic statistics without needing a university lab. That makes it a real research project, not just a demo.

Research Questions

• How does the DPPH IC50 change when quercetin is mixed with curcumin at fixed ratios?
• How does the ABTS response change when EGCG is mixed with resveratrol at fixed ratios?
• What is the effect of changing the mixture ratio on the combination index for a chosen antioxidant pair?
• Does the observed mixture effect fall below the additivity line across several dose combinations?
• To what extent do DPPH and ABTS give the same synergy ranking for the same pair?
• Which pair, quercetin plus curcumin or EGCG plus resveratrol, shows the strongest synergy signal under the same assay conditions?
• How does storage time of prepared solutions change the measured synergy pattern?

Basic Materials

• DPPH assay kit or ABTS assay kit
• Quercetin, curcumin, EGCG, and resveratrol standards or food-grade sources
• Distilled water
• Ethanol or methanol approved by your lab
• Micropipettes or calibrated droppers
• Clear 96-well plate or cuvettes
• Digital scale with 0.01 g readability
• Smartphone camera or simple colorimeter
• White background or light box for photos
• Safety goggles and gloves
• Lab notebook
• Timer
• Microcentrifuge tubes

Advanced Materials

• UV-Vis spectrophotometer or microplate reader
• Analytical balance with 0.001 g readability
• Volumetric flasks
• Adjustable micropipettes and tips
• Amber vials for light-sensitive stocks
• Orbital shaker
• pH meter
• HPLC-grade solvents
• DPPH and ABTS radical reagents
• Certified reference standards for each polyphenol
• Temperature-controlled incubator or water bath
• Filtration supplies
• Plate seals
• Replicate randomization plan

Software & Tools

• Google Sheets: Organizes replicate data, calculates averages, and tracks dose-response values.
• R: Fits dose-response curves and helps draw isobolograms and confidence bands.
• Python: Automates curve fitting, mixture plots, and clean figure exports.
• ImageJ: Measures color intensity from plate photos when you do not have a plate reader.

Experiment Steps

1. Define the exact antioxidant pair, the assay readout, and the effect level you will compare across all tests.
2. Build single-compound dose-response curves so you know the baseline strength of each antioxidant on its own.
3. Choose a fixed-ratio mixture plan and map enough combination points to see whether the pair bends below the additivity line.
4. Convert raw color change into percent inhibition, then fit the dose-response curves and estimate comparable effect levels.
5. Plot the isobologram or combination index graph and compare the mixture against the expected additive response.
6. Check replicate spread and, if possible, repeat the same design with a second pair or a second assay to see whether the pattern holds.

Common Pitfalls

• Using changing room light for plate photos, which shifts the apparent color loss and distorts the dose-response curve.
• Testing only one mixture ratio, which cannot show whether the pair is truly synergistic or just strong at one point.
• Choosing a solvent that dissolves one polyphenol well and the other poorly, which can make the pair look weaker than it is.
• Skipping single-compound curves, which leaves no baseline for the isobologram or combination index.
• Calling noisy averages synergy, when a few outlier wells can move the additivity line enough to change the result.

What Makes This Competitive

A class-level version shows that two compounds reduce color. A stronger entry maps several fixed ratios, reports uncertainty, and compares at least one formal synergy model against an additivity baseline. It gets even better if you test both DPPH and ABTS, or compare purified compounds with food extracts. Careful controls and clean statistics matter more here than flashy equipment.

Project Variations

• Test whether green tea extract plus turmeric extract shows the same synergy pattern as purified compounds.
• Compare fresh and stored polyphenol solutions to see whether oxidation changes the isobologram shape.
• Use food extracts instead of pure standards, then check how natural matrix effects shift the combination index.

Learn More

• PubMed: Search review articles on antioxidant synergy, DPPH, ABTS, and isobologram analysis to find methods and caveats.
• PubMed Central: Search full-text open access papers on polyphenol interactions and antioxidant assay design.
• NIH PubChem: Look up structures, synonyms, and solubility notes for quercetin, curcumin, EGCG, and resveratrol.
• MIT OpenCourseWare: Search analytical chemistry and biochemistry courses for calibration, standard curves, and dose-response ideas.
• USDA FoodData Central: Check which foods contain relevant polyphenols and how they appear in common diets.