The Resazurin Reduction Test (RRT)
Resazurin Reduction Test (RRT) for Milk Quality:
The Definitive Guide
A rapid 10-minute dye test that grades raw milk through a vivid three-color shift — from blue-violet to pink to white. Everything you need: principle, redox chemistry, 6-grade visual color chart, step-by-step procedure, decision flowchart, and RRT vs MBRT comparison.
What is the Resazurin Reduction Test?
The Resazurin Reduction Test (RRT) is a rapid indirect microbiological method used to estimate the bacterial load in raw and pasteurized milk. It belongs to the same family of dye-reduction tests as the Methylene Blue Reduction Test (MBRT), but is significantly faster — delivering a quality grade in just 10 minutes rather than up to 6 hours.
The test is standardized under BIS IS 1479 Part II in India and is widely used at bulk milk collection centers, dairy reception docks, and teaching laboratories. Results are read on a 6-grade color scale using a Lovibond comparator disc.
Visual tube color guide — Grade 1 to 6
After exactly 10 minutes at 37°C, compare your tube against the reference colors below. The color grade you assign determines whether the milk batch is accepted, retested, or rejected.
Principle and redox chemistry
The RRT is based on the ability of milk bacteria to reduce dissolved oxygen through aerobic respiration, creating a progressively anaerobic (reducing) environment in the sealed test tube. Resazurin acts as an electron acceptor, changing color as it accepts electrons in two distinct steps.
The first reduction (resazurin → resorufin) is reversible — shaking reintroduces oxygen and the pink color returns. The second step (resorufin → dihydroresorufin) is irreversible. Always record the color before shaking, as agitation can artificially restore color in Grade 2–4 tubes.
Because bacteria must reduce two sequential molecular forms before the dye becomes colorless, the test produces a graded spectrum rather than a binary endpoint. This is the key advantage over MBRT, which gives only a single decolorization time.
Step-by-step procedure
Follow these steps precisely. Deviations in temperature, timing, or dye concentration will compromise the grade reading.
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Prepare resazurin solution: Dissolve 11 mg resazurin sodium salt powder (or one standard Resazurin tablet) in 44 mL sterile distilled water to make a 0.005% stock solution. Store in an amber glass bottle at 4°C and discard after 1 week. Check color before use — the reagent should be blue-violet; discard if pink or colorless.
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Collect and equilibrate the milk sample: Aseptically pipette 10 mL of fresh milk into a sterile, glass-stoppered test tube. Allow the sample to equilibrate to 37°C in a water bath for 5 minutes before adding dye to avoid temperature artifacts.
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Add dye at 10:1 ratio: Add exactly 1 mL of the resazurin stock solution to the 10 mL milk. Insert the stopper firmly and invert gently 2–3 times to mix. Do not shake vigorously — this introduces excess oxygen.
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Incubate at 37°C for exactly 10 minutes: Place the tube upright in a water bath at 37°C ± 0.5°C. Do not disturb during incubation. Use a timer — even 1–2 extra minutes can shift a Grade 5 result to Grade 4 in marginal samples.
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Read color immediately using a Lovibond comparator: Remove the tube and compare its color against the Lovibond disc within 1–2 minutes. Assign a grade from 1 (white/colorless) to 6 (blue-violet, unchanged). Record the grade before shaking the tube.
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Run controls every batch: Always include a negative control (boiled milk + 1 mL dye — should remain blue-violet throughout) and a blank control (1 mL resazurin in 10 mL distilled water — should stay blue-violet) to validate reagent quality and sterility.
Grading table and estimated bacterial counts
Color grades are read against the Lovibond comparator at the 10-minute mark and correlated to approximate Standard Plate Count (SPC) values from calibration studies.
| Lovibond grade | Color observed | Est. bacterial count / mL | Quality verdict |
|---|---|---|---|
| 6 | Blue-violet | < 5 × 10⁵ | Excellent |
| 5 | Mauve | 5×10⁵ – 1×10⁶ | Good |
| 4 | Pink-mauve | 1×10⁶ – 5×10⁶ | Acceptable |
| 3 | Pink | 5×10⁶ – 1×10⁷ | Marginal |
| 2 | Pale pink | 1×10⁷ – 5×10⁷ | Poor |
| 1 | White / colorless | > 5 × 10⁷ | Rejected |
Decision flowchart — what to do with your result
This decision guide is designed for use at bulk milk reception docks. Apply it immediately after reading the 10-minute Lovibond grade to determine the next action for each milk batch.
Advantages of RRT
Ultra-rapid — 10 minutes
Gives a quality grade before the milk tanker leaves the dock, enabling immediate rejection decisions.
6-grade color spectrum
Multi-step color progression gives finer resolution than MBRT's single decolorization endpoint.
Very low cost
Only resazurin tablets, a water bath, and a Lovibond disc. Ideal for rural collection centers.
Higher sensitivity
Detects borderline milk quality at lower bacterial counts than MBRT — fewer missed rejections.
BIS standardized
Recognized under IS 1479 Part II — applicable across Indian dairy plants and collection centers.
Minimal equipment
No specialized microbiology training required. Suitable for field labs with basic infrastructure.
Limitations of RRT
- Mastitic milk interference: Elevated somatic cells in milk from cows with mastitis have their own reducing activity, potentially lowering the grade even when bacterial count is acceptable.
- Antibiotic suppression: Antibiotic residues from recently treated animals suppress bacterial metabolism, giving falsely high grades (5–6) in actually contaminated milk.
- Psychrotrophs underestimated: Cold-storage spoilage bacteria (e.g. Pseudomonas spp.) may not reduce resazurin efficiently at 37°C, underestimating their contribution to spoilage.
- Subjective reading without comparator: Color judgment without a calibrated Lovibond disc is operator-dependent and poorly reproducible. Consistent results require trained staff and standardized equipment.
- No species identification: RRT gives a quantitative load estimate only — it cannot identify which organisms are causing spoilage or whether pathogens are present.
- Reagent stability: The resazurin solution degrades over time, especially on exposure to light. Reagent must be freshly prepared weekly and stored in amber bottles.
RRT vs MBRT — side-by-side comparison
Both tests estimate bacterial load through dye reduction, but they differ substantially in speed, sensitivity, and reading methodology. Most dairy labs use them as complementary tools rather than alternatives.
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