SOP: Sugar Analysis by Refractometer (Brix)

Degrees Brix is the quickest number in beverage and fruit quality control. One drop on the prism, one reading, and you know the soluble solids. Refractometers earn their keep on receiving docks, production lines, and in QC labs alike.

Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.

Purpose

To determine soluble solids as degrees Brix by refractometry for maturity assessment, formulation control, and process monitoring.

Scope

Applies to fruit juices, syrups, jams, beverages, and sugar solutions. For pulpy or dark samples, filter or centrifuge before reading.

Equipment and Materials

• Digital refractometer with automatic temperature compensation, range 0 to 85 Brix, or Abbe refractometer
• Deionized water for zeroing
• Sucrose calibration standards (for example 10, 20, 40 Brix) or certified refractive index oils
• Lint-free tissues
• Syringe or dropper
• Filter paper or centrifuge for pulpy samples

Procedure

1. Zero the refractometer with deionized water at 20 C. The reading must be 0.0 Brix.
2. Verify with a sucrose standard near the expected sample range. The reading must be within 0.2 Brix of the certified value.
3. Mix the sample well. For pulpy juices, filter through fluted paper or centrifuge 5 minutes and use the clear serum.
4. Place 2 to 3 drops on the clean prism, avoiding air bubbles. Close the cover.
5. Wait 30 seconds for temperature equilibration, then record the reading.
6. Clean the prism with deionized water and dry with lint-free tissue between samples.
7. Recheck the water zero after every 10 samples.
8. For samples above 85 Brix, dilute by weight with deionized water and multiply the reading by the dilution factor.

Recording Results

Report degrees Brix to one decimal place with the measurement temperature. For diluted samples, record the dilution factor and the calculated original Brix. Note that Brix measures total soluble solids, not sugar alone, when acids and salts are present.

Quality Control

Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.

Troubleshooting

• Unstable or drifting readings: temperature not equilibrated or a bubble on the prism. Wait longer and reload the sample.
• Zero will not set: the prism is dirty or scratched. Clean with ethanol; replace the prism if scratched.
• Standard reads off: the standard may have evaporated or fermented. Use fresh sealed standards and store them cool.
• Shadow line unclear on Abbe models: adjust the light and the compensator, and ensure full prism contact.

Safety Notes

• No significant chemical hazards. Rinse sugary spills to avoid sticky surfaces and pests.
• Handle glass prisms and standards carefully to avoid breakage.