SOP: Gluten Detection by ELISA (R5 Method)

Gluten-free claims live or die on testing, and the R5 ELISA is the method the Codex Alimentarius recognizes as Type I for the job. The R5 monoclonal antibody targets the QQPFP peptide sequence found in wheat gliadin and the equivalent prolamins of rye and barley, which makes it the accepted tool for verifying that foods meet gluten-free thresholds.

Sandwich ELISA formats capture the protein between two antibody layers and develop a color proportional to concentration, read at 450 nm. Because heat processing fragments proteins, the extraction cocktail step matters as much as the plate itself: without it, processed foods read falsely low and unsafe product can pass.

Purpose

To detect and quantify gluten in foods and ingredients by R5 sandwich enzyme-linked immunosorbent assay, supporting gluten-free verification and allergen control programs.

Scope

Applies to raw materials, finished foods, and surface-rinse validations where gluten control is claimed. The typical working range is 2–80 mg/kg gluten after the prescribed dilutions; samples above the top standard must be diluted further and re-analyzed. Results are reported as gluten using the gliadin × 2 conversion factor.

Equipment and Materials

• Commercial R5 gliadin ELISA kit (e.g. RIDASCREEN Gliadin or validated equivalent), stored at 2–8 °C and used within expiry
• Microplate reader measuring absorbance at 450 nm (reference 620–630 nm)
• Water bath at 50 °C; centrifuge (≥ 2500 × g); vortex mixer
• Extraction cocktail solution supplied with or specified for the kit
• 80% ethanol for the second extraction step
• Calibrated micropipettes (20–1000 µL) with disposable tips
• Timer; wash bottle or automated plate washer

Procedure

1. Grind or homogenize the laboratory sample to a uniform powder. Weigh 1.0 g into a screw-cap extraction tube.
2. Add the kit-specified volume of cocktail solution, vortex, and incubate 40 minutes at 50 °C in the water bath, mixing periodically.
3. Add 80% ethanol to the specified volume, shake vigorously at room temperature for 60 minutes, then centrifuge 10 minutes at 2500 × g.
4. Dilute the clear supernatant with the kit sample diluent as directed (commonly 1:50 for the standard measuring range).
5. Pipette 100 µL of each gliadin standard (0, 5, 10, 20, 40, 80 ng/mL), the kit controls, and the diluted samples into the antibody-coated wells in duplicate.
6. Incubate 30 minutes at room temperature, then wash the wells three times with wash buffer, tapping dry between washes.
7. Add 100 µL of enzyme conjugate per well, incubate 30 minutes at room temperature, and wash three times again.
8. Add 100 µL of TMB substrate per well, incubate 30 minutes in the dark, add 100 µL of stop solution, and read absorbance at 450 nm within 30 minutes.

Recording Results

Fit the standard curve with a four-parameter logistic model as the kit insert directs; accept the run only if the kit controls fall within their stated ranges. Convert gliadin to gluten with the factor × 2, then multiply by the total dilution factor to obtain mg/kg gluten. Report results to one decimal place, or as “below the limit of quantification” when under it (typically ~2–3 mg/kg). Acceptance: duplicate CV ≤ 15%, spiked-sample recovery 70–130%, and a reference material (when available) within its certified range. For context, the widely published Codex threshold for a “gluten-free” claim is 20 mg/kg; confirm the current requirement for the product’s market.

Troubleshooting

• High duplicate CV: inconsistent washing or pipetting; standardize the wash technique and check pipette calibration.
• Weak color in the standards: reagents past expiry or stored warm; check kit dates and storage logs before repeating.
• All samples reading high: insufficient dilution or matrix enhancement; dilute further and verify with a spiked control.
• Very high-gluten sample reading low (hook effect): antigen excess is saturating the sandwich; dilute the extract 10-fold and re-analyze.
• Poor spike recovery in a processed food: the cocktail extraction was incomplete; confirm the full 40 minutes at 50 °C with mixing.

Safety Notes

• The stop solution contains sulfuric acid and is corrosive: wear gloves and eye protection, and wipe spills immediately.
• The cocktail extraction solution may contain reducing agents: avoid skin contact and do not pipette by mouth (never pipette any reagent by mouth).
• TMB substrate is light-sensitive and a mild irritant; keep it in the dark until use and wash hands after handling.